[Closed] Linkage Suspension Fork Mock-Up

Very nice!

Now I want to know how it rides!

Good work.

woody74 - Member
Good work

Wood work

Nice. I always have a soft spot for linkage forks. I designed a pair that looked almost exactly the same back in 1991 at university. But we had just got a carbon fibre winding machine thing so rather than a shock it used a carbon fibre spring. Love a linkage fork 😀

I would like to try a good linkage fork though.

Vorsprung did a Tuesday tune about the telescopic vs linkage forks - always good for a watch
https://m.pinkbike.com/news/tuesday-tune-ep-8-why-were-riding-telescopic-forks-2016.html

I rode (and still have the remains of) some Quasar linkage forks, and Mrs was sponsored to ride the last generation of Girvin forks (Noleen coil shock).

Very supple over bumpy fields but slightly disconcerting over drop offs (Girvin axle path).

Main issue with the Quasars was constantly chasing play in the bushes and grease splurging everywhere.

A common problem was the handlebars having to be very high to avoid clashing with the top link - I guess current trend for short stems will help that issue.

I've toyed with making some using long rubber torsion bushes for the pivots (like car suspension arms) to avoid bearing play but don't know if they could ever be made laterally stiff enough or maintain vertical wheel alignment (Greeves motorbike used them).

I predict Epicyclo will be along shortly to join in 🙂

Kilo still make some rather nice looking linkage forks. Light too.

[img] [/img]

[img] [/img]

2017 linkage genius/madness*

*Delete as applicable.

decking and fence need some work

Nerd Alert

🙂

and no rim brake option i'm oot!

Look Fournales fork

I don't remember those Look forks. I did lust after AMP and Girvin Vector forks, though, they made a lot of sense once you'd spent time riding an early gen set of Manitou elastomer forks.

Love it, mentioned in another thread I'm a bit surprised they aren't as common as they used to be.

This is my fave at the moment [url= https://www.singletrackworld.com/2017/06/wtf-the-funny-fork/amp/ ]https://www.singletrackworld.com/2017/06/wtf-the-funny-fork/amp/[/url]

I had a set of AMPs on an F2? back in the day. They were very nice in a straight line as the axle path ate up the rough stuff. Not so good under braking or in corners as the trail reduced and everything got a bit frantic.

They're a great idea and ought to be able to be built light and stiff with great damping characteristics, only trouble is that telescopic forks are the norm on all forms of 2 wheeled transport and have a lot of development so they really work well these days.

http://teamcow.ca/archives/872

The guy who helped set up Niner, then left, said he'd like to develop some linkage forks but everyone accepts telescopic are the norm so it's not worth the time and money commercially

https://books.google.co.uk/books?id=84hF-qoR5I8C&pg=SA7-PA1&lpg=SA7-PA1&dq=bmw+link+fork&source=bl&ots=F-w2xEPbOg&sig=10k9_rRgp-ll9RhTB8ZzPDeSSDo&hl=en&sa=X&ved=0ahUKEwiE2q2O1rbYAhVHJsAKHdhJDScQ6AEIcTAO#v=onepage&q=bmw%20link%20fork&f=false

mick_r - Member
...I predict Epicyclo will be along shortly to join in

Good to see someone experimenting with linkage forks.

I grew up on motorbikes with girder forks, plus others with leading links, and trailing links, and being a young idiot at the time I rode them flat out. I always preferred them to telescopic because of their superior ability to iron out road vibrations - the sort of little irregularities that a telescopic has too much "stiction" to respond to.

An advantage of a properly designed linkage fork is that braking dive can be controlled or eliminated, thus a 3" linkage fork can do the job of a longer travel telescopic because less travel is wasted on brake dive.

Modern telescopic forks though are very good, but to get that performance is expensive.

I have quite a few different linkage forks for bikes in my collection. Unfortunately most do not take advantage of the body of knowledge that existed with motorbikes so they make the basic errors of an axle path that was ok for a short arc but then rapidly changed the offset of the fork to the detriment of the handling in extremis.

The best girder fork made was the Webb as used on Velocettes, and the Vincent Girdraulic (IMO). These would be regarded a very short travel these days. The Vincent is capable of 150mph with the right tuning so needs a good fork.

Incidentally there is a salutary lesson to be learned from the Vincent. Racers looking for more travel when racing against much more modern bikes started extending the travel of the fork by using longer shocks. This then lead to a series of very expensive VIncents stotting off into the scenery at hard braking s-bends. The problem was coming off one corner, then hitting the throttle raised the front end, only to be followed by extremely hard braking initiated when the fork was fully extended. The weight transfer tended to lock the fork up because the links were at an extreme angle. It took quite some time for the cure to be realised because otherwise the fork functioned very well.

That's a longwinded way of saying use long links, and make sure they are always in a position where they can't get locked from braking, ie put travel stops in the design.

If you can get hold of a copy of Phil E Irving's book "Motorcycle Engineering" he's got a chapter on various linkage forks that's well worth reading. It's old (1950s) but the fundamentals haven't changed.

OP, Chris Porter disagrees with all of your points about body position, and he's all about having fun down hill as well.

Hi mick_r - I think that the move to shorter stems is one of the reasons why this type of fork has a better chance of success now than it did back in the 90s. As you say, trying to get the top link to clear something like an 80mm stem is a challenge that becomes much easier to overcome if the stem is only 35mm long.

Hi epicyclo - I agree, I think that the longer you can make the links (within reason!) the better. I went with links that were 120mm long combined with a shock with 50mm of travel giving just over 100mm of wheel travel. The longer links help to minimise the change in fork offset, helping to keep the handling more consistent through the forks travel. It also reduces the angle that the links have to rotate through.

Hi nickc - I've got a lot of time for Chris Porter, but I certainly don't agree with everything he has to say! I find it a bit odd that, as someone who has previously referred to Motocross bikes when talking about bike geometry, he appears to have chosen to take his bikes in the opposite direction. More generally, for me the biggest issue with modern bike design is the lack of differentiation between bike-stability and rider-stability. Longer front-centres and steeper seat angles are resulting in longer, more stable bikes. And if you derive your fun from going as quickly as possible then this move if possibly a good one. But these same changes are actually putting the rider in a less stable position, tipping the rider forwards and requiring their arms to support more of their weight more of the time. I get most of my fun from threading a bike down a challenging piece of singletrack. My speed is (almost) irrelevant - it's the fun of confidently playing with a bikes limits across technical terrain that I get my kicks from. I believe that this is more likely to happen if the rider can adopt a more stable body position, with the rider being able to support almost all of their weight through their legs most of the time. This is actually more like the position that a Motocross bike places the rider in, rather than the stretched out position of something like the Geometron, which Chris Porter had helped to develop. But, as I've said, I think that he is trying to achieve something else with his bikes.

Few bike companies as divisive, I know, but linkage fork fans should all quite like Moulton’s forks. Not the coil sprung leading link types, but the ones with flexitors. The pivots, springing, damping and bearings all done in one. Lovely satisfying bits of kit.

😀
great stuff.

Interesting thread!
🙄

Some of it: quite some engineering madness.
But nice that people still tinkering around with it.
😉

The other good thing about linkage forks is that you don't have to run floppy slack head angles to disguise the forks deficiencies.

See, where I disagree with about stability is this; you suggest that a crouched position is more stable, and it is, when compared to standing fully upright, but more stable even than crouched, is on all fours, with the weight spread something like 70/30 legs and arms. Where I think most riders fail, is that they don't get low enough, because 1. they haven't learned to ride that way, 2. a lot of riders just aren't fit enough to maintain that body position for any length of time.

I agree with you, in that I think a lot of development has gone into making bikes stable on descents mostly it seems because lots of riders can't physically get in that position (I'm not saying that many riders could do with losing a few pounds, but...) or don't seem to want to learn how to ride off road in way that will make them more stable, so if you want to make a bike that is fast/fun but with good rider control, make the bike more stable is probably your best bet.

What's most fun of all (in my opinion of course), is 1. being on a stable a predicable bike, and 2. getting in the right shape on the bike. Having a stable rider [i]and[/i] bike.

You haven't discussed (in your very interesting blog) your thoughts on telescopic seatposts, and they way they've influenced bike design. I'd be interested in your thoughts.

I'd be very keen to have a go on your bike though, it looks super interesting. Will you keep us updated as you build it?

these same changes are actually putting the rider in a less stable position, tipping the rider forwards and requiring their arms to support more of their weight more of the time.

Like this thread very much and like if people do such great work at home.
Compliment!
based on above post from nickc - bloke I checked:

https://www.daveypushbikes.com/full-moto-frame.html

Interesting.
My impression - without adding any deeper know how of mountain bike and suspension design:

From my "feeling" I doubt this position thing.

A rider in one of the world’s largest motorcycle races, the Enduropale du Touquet in France

Mountain biking, going downhill:
I have at least the feeling that - in the case I'am scared to go over the bar and shift my weight too much to the back I'am slow.
Not enough grip on the front tire.
Getting the center of gravity LOW and MORE TO THE FRONT: this makes me fast... In this situation very little pressure on the back tyre. Which is good.

A rider in one of the world’s largest motorcycle races, the Enduropale du Touquet in France

Mountain biking: center of gravity of the biker has the biggest influence... It has to be low. And in fact - when going fast I have the impression that I move my bike with my body position and less with steering.

I might be wrong. But this came to my mind when reading this thread and when reading> https://www.daveypushbikes.com/full-moto-frame.html

The position thing you describe I doubt.

Additional, linkage forks: beside advantages I would fear a lack of stiffness - or even play - in the yaw / steering direction. Isn't this design in this direction very soft? Problem of instability around this axis?

But I like the whole project very much - and I'am not an expert. Means very possible you are right on your path and above only noise.

Great learning project.
Very, very neat!
Good luck!
😉

Hi nick_c - the position that I believe I should be trying to achieve on a bike is the cycling equivalent of an 'Athletic Stance'. It's primarily an American term, but usefully helps to describe the starting position that a person adopts in many different sports (a tennis player waiting to receive a serve, a boxer preparing to take or deliver a punch or a golfer reading to tee-off, as examples). Obviously, it varies from sport to sport, but it's an incredibly stable position from which a person can react effectively to external influences (I've written about it in more detail throughout my blog, but specifically here www.daveypushbikes.com/blog/balancing-act-part-3).

Bike stability is an interesting one - I'm not sure anyone who rides a mountain bike for fun would actually want a 'really' stable bike. One that could steam-roller its way from the top to the bottom of a hill with minimal input from the rider. However, for those who race, stability takes on a whole new level of importance so long as the bike can still accelerate and turn effectively. I know it's easy to bash the marketing departments of the big bike companies, but I would suggest that its easier to market a fast bike / race bike (using this speed as a proxy for fun) rather than a bike that, from the outset, has been designed to maximise fun.

As for dropper-posts, I believe that these can only have a positive influence on riding (so long as you can find a reliable one!) The position and support that a rider requires when pedalling up hill when compared with attacking a downhill trail (for example) is so different that it must make sense to better optimise the bike (all else being equal) for these situations.

I will be building my new frame in February and I already have all of the other parts for it so will hopefully have it in the woods soon after. I'll be putting updates on my blog, but can also post here on the forum if you're interested?

Hi singlespeedstu - I must state that I have never ridden a Geometron, so I am about to adopt the very dubious position of the armchair critic! With that rather large caveat in place, I would suggest that the need to weight the front wheel more on this bike than one with more traditional geometry is primarily the result of the slacker head angle (64.2 degrees on the G13 in its 'low' setting). Try riding this bike with a 68 degree head angle and I suspect it would start to feel very 'nose-heavy'. However, as I said, this is all just theorising from my sofa whilst watching Spectre, as I have never ridden the standard bike, let alone one with a more conventional trail bike head angle.

Thanks for MX sidecar suggestion. I suppose a big part of the reason for building the mock-up fork was to start exploring whether this set-up really does have merit for contemporary mountain biking.

Hi andreashoen - I'm certainly not disputing that a motorcycle is heavier, faster and has very different power delivery(!) But when I look at the evolution of mountain bike geometry, I believe that we started with something reasonably good (Klunkers, which were actually not that far off Motocross bikes in terms of their standing rider position), but then became overly preoccupied with racing in the 1980s, adopting a lightly tweaked version of road bike geometry. In many ways I think that we've been trying to perfect this overly compromised arrangement ever since. I've written more about my views here www.daveypushbikes.com/blog/assume-the-position. I suppose I'll find out shortly whether or not my theory is correct when I build my new frame!

really looking forward to seeing your bike design OP.

oliverdavey80 - Member
...Bike stability is an interesting one - I'm not sure anyone who rides a mountain bike for fun would actually want a 'really' stable bike. One that could steam-roller its way from the top to the bottom of a hill with ...

I've done similar but cruder experiments in the mid 70s and agree with his conclusions.

Hi epicyclo - yes, Tony Foale's work is pretty comprehensive! There's also a book by Gaetano Cocco that has some helpful/relevant things to say about motorcycle geometry and suspension design. I've summarised some of what I understand about this in relation to bottom bracket height and centre of gravity here www.daveypushbikes.com/blog/bottom-bracket-height-the-most-misunderstood-dimension

Like the whole project.
But doubt the theories.
read:
http://www.daveypushbikes.com/blog/bottom-bracket-height-the-most-misunderstood-dimension

Mmmm. We talk about mountain bikes? And going downhill? The rider has to be able in msec time frame to move and push the bike in such a way to avoid crashing.
Part of the mountain bike frame design is being able to push the saddle very much down. Means the biker is able to get very low and is able to push his body "freely" around.

mountain bike for fun would actually want a 'really' stable bike.

No fun, no fast. Same as with sea kayaks (the ones for really good paddlers and really bad weather are UNSTABLE) and same with aerobatic airplanes. These airplanes are unstable as well...

Going on a rough trail downhill is very much like aerobatic flying...?

Great thread. As someone who rode an AMP fork a lot in the mid-90’s, I find the idea of non-telescopic forks very interesting and the work you have put in OP, is excellent.

I don't think there's much to gained from trying to compare the unstable characteristics of aerobatic planes to bikes TBH.

If I might be so bold, OP has some odd dimensions, I can imagine that trying to find the bike he wants that performs well on his specific terrain would be hard to impossible to get via an industry that is set up to shift large numbers of bikes to a formula, building a custom bike seems like a great idea, I imagine it'd be a great way to experiment. Good luck to him, I hope it works.

Interesting blog - I shall read more of it!

Regarding BB height, this is something I’ve experimented with by adjusting the geometry on my current full-sus and hardtail (with movable dropouts, different fork lengths, anglesets and sag). Although it’s just anecdotal evidence based on my messing about I can say with a fair degree of certainty that as soon as a bike slides laterally a lower BB height makes it easier to keep the bike balanced. It almost feels like cheating!

I don't think there's much to gained from trying to compare the unstable characteristics of aerobatic planes to bikes

aerobatic airplane: designed to be not stable. Airplane follows every control input right away.
This is very much what I want from an mountain bike...
💡
How reacts the machine to the control input? That's an important question for the design of a machine. No matter if airplane, seakayak or bike.

Touches this question right away:

I'm not sure anyone who rides a mountain bike for fun would actually want a 'really' stable bike.

Linkage Suspension Fork Mock-Up

lets say, small cruising airplane: designed to be stable. Makes hands off flying possible.
I would compare this airplane to: road race bike for example
aerobatic airplane: designed to be not stable. Airplane follows every control input right away.
This is very much what I want from an mountain bike...

It's what you might want from a trials bike or a BMX, but I can't see it being much use for the majority of mountain biking. I agree there might be a place for less extreme variations, though, but still limited.

Airplane follows every control input right away.
This is very much what I want from an mountain bike...

That only works if you’re very strong and very skilled with excellent balance etc. A mountain bike being ridden on gnarly trails is constantly subjected to near-random destabilising forces, which not only cause the bike to deflect from its path but also unbalance the rider causing them to apply unwanted inputs to the bike.

That only works if you’re very strong and very skilled with excellent balance etc. A mountain bike being ridden on gnarly trails is constantly subjected to near-random destabilising forces, which not only cause the bike to deflect from its path but also unbalance the rider causing them to apply unwanted inputs to the bike.

Don't have the know how or expertise.
But very possible as well:
Above is different for each of the 6 degrees of freedom (6dof).
In certain degrees of freedom the mountin bike should be "more" stable in others not?

In my opinion funny as well, history:
Oliver and wilbur wright had both bike background. Knowing about the 6dof controls issue from BIKES.

They used this know how to design the first powered airplane. 1904 or so?
And their goal was, for this aiplane: to be very stable in all 6 dof.

At this time in history: this was extremely smart.
But indicates as well: airplane controls and bike controls might have something in common.

Above question:

I'm not sure anyone who rides a mountain bike for fun would actually want a 'really' stable bike.

Maybe helpful to answer this - for mountain bikes - for each of the 6 dofs?

No idea how to do this.
Just an idea.
💡

Hi andreasrhoen - I think that we are possibly both arguing for the same thing? In my first response to nick_c I made the distinction between a stable [i]bike[/i] and a stable [i]rider[/i]. While these things are rarely if ever binary, they are often discussed in such ways. I believe that the current trend in mountain biking (longer, lower, slacker) is creating more stable [i]bikes[/i] at the expense of [i]rider[/i] stability. I also believe that, within reason, rider stability (i.e. being able to control a bike from a position of stability) is central to having fun on a bike.

First and foremost, what I would like to create is a frame that places the rider in a more stable position than I have been able to achieve with previous bikes. The stability of the bike is almost a separate (but very much interrelated) issue.

Hi nick_c - you're right, I do have some pretty odd dimensions - I have very long legs for my height. But while this is almost certainly the main reason why I have played around so much with handlebar position and, ultimately, decided to build my own frame, I still believe that many riders might benefit from the thinking that I have already described above and discussed in my blog. This is particularly true as a riders height increases (regardless of physical proportions) as many manufacturers fail to increase a bikes stack in proportion to its reach. For example, Trek maintain exactly the same stack height across the first four frame sizes of its 2017 Fuel EX trail bike (something that I've never seen mentioned in a review of this bike). Essentially, the taller you are, the more you are expected to reach down to the 'bars (placing the rider in a less stable position). I've plotted some similar numbers for a range of bikes here www.daveypushbikes.com/blog/balancing-act-part-2 and its a similar story for all of them (Rolo did something similar a while back for road bikes). Therefore, I suspect I am not alone in experiencing this issue (even if in my case it is more extreme than most).

Essentially, the taller you are, the more you are expected to reach down to the 'bars (placing the rider in a less stable position).

Is that really the case or are the shorter riders expected to use flat bars and the taller riders riser bars with more spacers under the stem?

@oliverdavey80

I believe that the current trend in mountain biking (longer, lower, slacker) is creating more stable bikes at the expense of rider stability. I also believe that, within reason, rider stability (i.e. being able to control a bike from a position of stability) is central to having fun on a bike.

First and foremost, what I would like to create is a frame that places the rider in a more stable position than I have been able to achieve with previous bikes. The stability of the bike is almost a separate (but very much interrelated) issue.

Interesting approach. You seperate rider and bike stability.
Mmmhhh
Have to admit: I was wrong.

possibly both arguing for the same thing?

Funny enough, answer is: yes
😉

like this thread! Makes me thinking!
Neat.

Hi chiefgrooveguru - using spacers and riser bars is certainly one way to get around this issue, but it's hardly an optimal solution. I would suggest that this sort of thing should be kept for fine tuning rather than something as fundamental as rider height. From what I can workout, the four different Trek frame sizes that I mentioned are supposed to cover everyone from 5'1'' to 6'4''. That's quite some range.

I don't think that its unreasonable to expect that each frame size should be optimised for the height and proportions of Mr or Mrs average who sits in the middle of each size range. But really this issue is just one part of the bigger challenge of achieving a more stable riding position.

I do agree that head tubes should get a little longer as bikes get bigger but not a lot. I think there are multiple aspects which affect bike fit:

1. Height and proportions (leg length, back length, arm length).
2. Weight distribution (ie build).
3. Flexibility
4. Strenth
5. Natural riding style (I suspect this is usually a product of the previous four aspects).

I don't think it's unreasonable to suggest that a rider with a longer torso and shorter limbs, plus more weight higher up, will benefit from a longer reach bike (both for fit and stability reasons) but could need a relatively short head tube to get the handlebars in the right place.

Conversely a rider with long legs and short torso will want less reach, especially if more of their weight is in their legs (lower CoG) and if they have short arms.

Another thing to bear in mind that is a bicycle (engined or not) is most stable if the weight is concentrated low down and forwards. So even if the rider is in a less natural attack position, that could be outweighed by the increased stability of the rider+bike system.

All very interesting stuff (in a massively geeky way!)

I had a set of Quasar Link forks back in the day. Looked trick but were utterly awful in use. All the damping was friction based; tighten up the bolts holding the spindles in place for more. Naturally the damping worked equally badly / well on either compression or extension - and with a sodding great elastomer shock in place, it usually never worked. The whole enterprise was a disaster - worse even than the original "stop working at all when it rains" RC36's. An interesting historic diversion, and it would take a lot to convince me to ever bother again.

Not a [i]fork[/i] as such, but I've just seen this bike mentioned on a Bike Radar vid
http://structure.bike/

Main reason it won't take off is because it's ugly as sin. But it's all about the linkage!

[img] [/img]

Went for a ride with a bloke from Middleburn back in the day who had an AMP - those forks were even more flexy than our Pace RC36s. He kept crashing 😆

I believe that the current trend in mountain biking (longer, lower, slacker) is creating more stable bikes at the expense of rider stability.

That's the bit that I disagree with, I don't think they are [i]generally[/i] I can see why these bikes might not be optimal for you specifically, but for me (Mr average at 5'10" with 32" inseams), I am much more stable on them than older style geometry secondly I sadly think rider awareness, confidence, ability, and understanding is never going to match the capability of bikes, and in some ways that's fair comment from an industry aware that folks want to go fast, and are largely providing the tool for most folks with average capability to achieve that. That might not be what you (or I) think of as fun, but that's not for us to judge, is it? and thirdly dropper posts, which most people use these days which has a massive effect on CofG that you haven't mentioned in your blog, and I'd be keen to hear what effect they have on your ideas for you bike

I love your experimentation though, I think more cyclists should question the accepted view as you've done.

andreasrhoen, I think the aeroplane analogy works up to a point, after all I think you're right that everyone would prefer something perhaps "flicky" and "fun" and so on. but the difference is of course the thing that mostly effects the airplane is input from the pilot, that's NOT true of mountain bikes, where loads of the input is out of the riders control.

I’m interested to understand the biomechanic differences/similarities between foot “input” forces being applied via
1. Feet on solid ground (Tennis Athletic stance)
2. Feet on fixed foot pegs (Motocross Athletic stance)
3. Feet on a pivot point located in line with point of contact. (Push Bike)

Hi chiefgrooveguru - don't get me wrong, I completely agree that if your proportions sit someway outside of the middle of the bell-curve (like mine do) then custom geometry may be the best bet.

Characteristics like strength and flexibility are an interesting one. For a discipline like time trialling where there are competing and conflicting demands on body position (in this case, for example, to generate power and to be aerodynamic at the same time) then they definitely come into play. As for a mountain bike designed for having fun (rather than for racing) I'm not so sure where something like flexibility or even strength comes into the mix? I must admit, I haven't spent a great deal of time thinking about this (because I'm only designing for myself with a fixed/declining (!) level of strength and flexibility), but it's an interesting question. If greater strength means higher speeds then there is an argument for geometry that is more stable. But when it comes to the riders position on the bike, I would still argue that it should still be governed primarily by stability, which will be very similar, if not the same, regardless of rider strength (all else being equal).

As for CoG, low is more stable when braking or accelerating, but (unlike four wheeled vehicles) this doesn't hold true when turning. A bike can be thought of as an inverted pendulum (the analogy of balancing a mop handle vertically in the palm of your hand is often used). Rather counter intuitively, the higher the centre of gravity, the more stable a two-wheeled vehicle (or mop!) will be when turning or riding over rough terrain, because more force is required to knock it off course by a fixed amount. Equally, more energy is then required to correct the bike by the same amount.

andreasrhoen, I think the aeroplane analogy works up to a point, after all I think you're right that everyone would prefer something perhaps "flicky" and "fun" and so on. but the difference is of course the thing that mostly effects the airplane is input from the pilot, that's NOT true of mountain bikes, where loads of the input is out of the riders control.

And: airplanes with too much stability tend to be very bad in turbulence!
Certain airplanes are just great in calm weather conditions and behave nasty when it get's "wild". 😯

But overall: the airplane thing might guide us onto the wrong path...
😥
Thanks for thinking about it so!
😉

As for a mountain bike designed for having fun (rather than for racing) I'm not so sure where something like flexibility or even strength comes into the mix?

Flexibility and mobility are hugely important for riding a MTB well. When my hips are moving well I can flick the bike about better, balance it on the limit of grip and flow with more style and fun. And when I’m feeling strong I have more spring when jumping and can hit technical or rough sections with more speed and confidence.

Often the biggest difference you’ll notice between two riders of differing descending ability is the better rider is moving their hips more.

Rather counter intuitively, the higher the centre of gravity, the more stable a two-wheeled vehicle (or mop!) will be when turning or riding over rough terrain, because more force is required to knock it off course by a fixed amount.

I read this on Geoff Apps’ site some years ago and I didn’t believe it was true then and I still don’t now. I don’t know if it’s the case of applying an incorrect analogy or incorrectly applying an analogy - I wonder if it could be that however hard you try a MTB will be destabilised by external forces and the bike with the higher CoG will be harder to restabilise?

If you don’t believe me, borrow a bike with adjustable geometry (like my Banshee Spitfire). The increased cornering stability (and increased reluctance to flick slaloms turns) when in the lowest BB setting is very obvious. It takes about 5 minutes to change the geometry so it’s easy to AB test.

Hi chiefgrooveguru - sorry, I've just re-read my last post and I should have been more explicit / reiterated that the point I was trying to make was specifically in regard to rider position. I completely agree with you that flexibility and strength are important to riding a bike well. I would certainly love to be more flexible and stronger on the bike! My point is that I'm not certain how flexibility or strength would significantly affect the placement of a bikes contact points (and therefore a riders body position) if, like me, achieving rider stability is the main goal? Whether a rider is strong or weak, flexible or inflexible, all else being equal I suspect that a stable body position will be the same.

As for a bikes centre of gravity, this is my best attempt at trying to explain what's going on www.daveypushbikes.com/blog/bottom-bracket-height-the-most-misunderstood-dimension

Whether a rider is strong or weak, flexible or inflexible, all else being equal I suspect that a stable body position will be the same.

But it isn’t because the stable body position is part of a dynamic series of positions and the greater the rider’s strength and flexibility the better the bike geometry can be optimised for bike handling rather than rider stability. It would be different if we rode one hour plus descents but we don’t - a few minutes is normal and twenty minutes is pretty much the max - so we don’t need our riding position to be the easiest to hold stably, we can compromise to get better handling.

The easier you find it to hip hinge, the lower the bars can be for your height.

As for a bikes centre of gravity, this is my best attempt at trying to explain what's going on http://www.daveypushbikes.com/blog/bottom-bracket-height-the-most-misunderstood-dimension

That was the first bit of your blog I read and the only bit I really disagreed with. The cited Wikipedia article doesn’t help either. And Geoff made similar claims with his Cleland which again don’t stack up.

I wouldn’t bother dropping the BB on my Spitfire for uplift days if it didn’t make a difference to cornering stability - it isn’t like I need to slacken the head angle, it’s already at 64 deg, I do it to make the bike lower. I’ve yet to see any convincing maths to counteract my own experience.

I’m sure the BB height does relate to both the axle heights and the height from the ground, and to the crank lengths and riding style.

Also, before my Spitfire and Zero AM I had a Soul with 140mm forks. The BB height on the Soul was 40mm higher than on the Zero. Corners in mud when both wheels are drifting are so much easier on the latter - you could say that’s because of the longer wheelbase which is why my original examples were smaller adjustments (up to 15mm) on the same frame but it was a BIG difference between those bikes.

I am really unsure of the relevance of bb height above/below axle height as a separate issue (separate from the other issues of wheel size and bb height, with which it is obviously intimately related).

Really, there is no substitute for mechanical analysis of this with maths and stuff, everything else is just words.

Is stability the same thing as handling, or is it something different?
http://cycleseven.org/bicycle-stability-and-centre-of-gravity-or-mass

This is all very interesting, but any chance we can get back to the original subject? 🙂

You've got some time to do stuff before the framebuild course - I want to see the CAD development of a tubular steel spaceframe version of the fork. This list has all the cheap and easily obtained Columbus cro-mo plain gauge tubes (8-10-12.7-14-16-19-22mm etc).

http://www.framebuilding.com/Spare%20Tubes.htm

Have you also seen the Nukeproof Reactor frame / fork from the 90's?

Cool project, be interested to see how it pans out.

I just realised why the balancing the broomstick analogy does not hold up. Once the limit of grip has been exceeded we are not moving the contact patches of the bike to balance it, we’re moving our own mass.

So a better illustration would be to get a set of stilts and see how balancing on them is easier or harder as you vary the height of the footrests. And then repeat the experiment on a low friction surface.

Hi paton - we're drifting into semantics here, which is always dangerous territory! I will simply say that the definition of rider stability that I subscribe to is the ability for a rider to remain in a steady state (i.e. maintain balance) when subject to external forces. This is not the same as handling.

Hi mick-r - You're right, and apologies for veering off on such a tangent.

I'll see what I can do about the CAD development! I have produced some simple 2D drawings, which are included in my blog, but my next steps are likely to be some form of structural analysis in parallel with the pivot designs. Unfortunately, right now I'm not quite sure when I'm going to get the time to do this, but I'd love to see it become a reality.

Back on Linkage forks, I've seen most of the designs everyone else will have and one thing struck me on the point about axle paths;

Pretty much all of the designs I can remember seeing use either a parallel or trapezoidal linkage (USE are an exception in using a sort of oleo link as pointed out in the lefty thread the other day) and therefore they all result in some degree of curved axle path...
But has any one ever attempted to use a scissor type link to produce a linear axle path? You'd inevitably need more linkage members, but you could reproduce that linear axle path that all of us Tele' riding MTBers are now used to.

I do like a linkage fork, and sort of remember my old girvin with a retro fondness

Love the new wraith bikes mantis stuff, totally bonkers

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But has any one ever attempted to use a scissor type link to produce a linear axle path? You'd inevitably need more linkage members, but you could reproduce that linear axle path that all of us Tele' riding MTBers are now used to.

There'd be a boatload of extra moving parts, to replace the one moving part in a tele fork. That'd cost a fortune and be heavy.

Wonder what that mantis rides like.

That mantis is glorious in a "two fingers up to the establishment" sort of way 😆

The axle path on a telescopic fork is pretty close to idea from a bump absorption perspective, being heavily rearwards. The downside is the steepening of head angle and shortening of front centre and wheelbase.

Really you want the front centre to stay constant rather than shortening, the trail to remain constant and the axle path to be rearward (especially between about 20 and 80% travel). Something that resists brake drive (without losing grip under braking would be nice too. But they're mutually exclusive. So which to prioritise?

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Full Mantis madness, gearbox, 5" travel fs, variable offset linkage fork, bonkers...totally. bonkers.... and apparently rides really well.

That looks a lot of fun!

They're missing a trick not making the fork single leg 🙂

Edit: Oops, accidental thread resurrection