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give us some more quality comment then
OK, I'll have a go then.
There seems to be two factors being discussed here. Pedal bob and overall suspension performance.
I bought a VPP frame solely to eliminate pedal bob and was impressed with how well it worked. Is there any other linkage that does the same job equally well.
It looks to me like the only linkage that uses the chain tension to counteract the weight on the pedals.
Have I missed something ?
My point is that "feels fast" =/= "fast"
I agree. At last. There is a god.
which seems to be the basis of your position
aaarrrghhh. I haven't said that, this is the first time I've mentioned it, and we, amazingly, agree.
No.
I bought a VPP frame solely to eliminate pedal bob and was impressed with how well it worked. Is there any other linkage that does the same job equally well.
The technical paper on DW link says it achieves the same result without using chain tension, through pivot and linkage configuration. This apparently (and it would) has the added advantage of reducing chain snatch which certainly used to be a problem on VPP v1.
what's chainsnatch then?
what's chainsnatch then?
I've probably used the wrong term, that's what it's called in motorcyling, I think. pedal feedback?
Hold on a minute.
I wasn't even sure what a DW link was, so I found this.
http://www.dw-link.com/physics.html
." When a bicycle accelerates forward, the rider's mass is transferred rearward. Without something to counteract this mass transfer, the rear suspension on most bicycles will compress under acceleration. This mass transfer as a reaction to acceleration is what riders have come to know as "bob."
That's not pedal bob, that's just weight transfer.
If I pedal up hill at a constant speed on a full sus, the bike bounces up and down with each pedal stroke.
It's got nothing to do with acceleration, it's all about the varying weight on the pedals.
The acceleration of the bike and rider is proportional to chain tension, which varies with crank rotation, so your acceleration varies like a sine wave, and so does the resulting bobbing of your suspension, caused by this variation in acceleration. This is why you get less bob with spds - there is less difference in power transfer to the bike around the full crank revolution. At least that's how I understand it.
What I can't work out though is whether Dave Weagle means a real mass transfer as in your torso moving back and forth as you pedal, or a theoretical one due to the acceleration.
I am not sure where pushing on the pedals gets factored in. but you're right, the vertical force must have a reaction and that, ultimately mus come through the wheels and suspension....
isnt the rearward movement of the mass actualy a rotation about the rear wheel axle
http://www.bikechecker.com/ follow the link to get your own version of the linkage software,,
Linkage is a powerful and easy-to-use while still well-featured 2D suspension analysis software for Windows systems, focusing on mountain bikes, but also capable of simulating other linkages.
Linkage also provides a user-extensible web library of bike models to help bike-enthusiasts compare different designs or publish their own ones. Linkage comes with data files for many bike designs already on the market (or past models).
Main features:
simulation of all popular full-suspension bike frame systems from single-pivot swingarm to four-bar linkages with additional shock driving linkage ("6-bar linkage")
calculation of linkage forces by a kinematic model, allowing different shock-setups
front fork movement simulation for examining different scenarios
comparisons allowed on diagrams to easily recognize differences between different designs
multi-window working environment with numeric or mouse input
creating new designs quickly using photos
export all results in high-resolution graphical or text formats
calculation of chain related changes, as well as "pedal-kickback"
more than 60 existing designs included
I have used this a lot to compare designs and to design from scratch much quicker than the old string models i used to use back in the late 90's
the pedal feedback is especialy interesting as you can click and drag to move pivot points as well as changing chainring sizes etc
so there you go,,, i will have just ruined your families christmas as you will be glued to your computer screen for the rest of the night comparing vpp bikes with single pivot bikes lots of rubbing chins and going hmmmm
fab 😀 nice one Ade
The acceleration of the bike and rider is proportional to chain tension...
Not if you're riding up an increasing gradient, in to an increasing headwind or through deepening mud it isn't.
Chain tension will increase while speed decreases.
I just read http://www.dw-link.com/reasons.html#
There is so much nonsense on that page I don't know where to begin.
Do people really spend £1000+ on a frame based on this pseudo-science ?
hmmmm...you're right [edit]...hang on...
what I was referring to was the variation of chain tension in one rotation of the pedals, not overall working harder
... so your acceleration varies like a sine wave, and so does the resulting bobbing of your suspension, caused by this variation in acceleration.
I understand what you're saying there, I just feel that the vertical force of the rider lunging on the pedals with each stroke is going to have far more effect on the suspension than the variations in the horizontal force of acceleration.
hmmmm...you're right.
You're not supposed to say that. You're supposed to start arguing. This is STW remember.
it's ok, I editted... 🙂
I just feel that the vertical force of the rider lunging on the pedals with each stroke is going to have far more effect on the suspension than the variations in the horizontal force of acceleration.
yes i think that's the elephant in the room here and probably what cynic-al was referring to in his bit about smoothing out pedalling technique. you can't do much about that in suspension design I don't think.
Is it not the case that pivot placement is the most important thing rather than the system used?
i.e. pick any system, vpp/horst/sp, and design it well and it will work but put the pivots in the wrong place and it will be terrible.
have you ever noticed that when you get to a certain cadence you can make a hardtail bob,, i dont think it's speed related but cadence related
but you can use chain tension to inhibit this
on a single pivot bike it's basicly the height of the pivot if the pivot is above the point where the chain meets the front ring then the rear suspension will tend to extend
if it is below the point where the chain meets the front ring then the suspension will tend to compress
now a vpp bike can be summed up as a single pivot bike with a pivot position that moves around so the virtual pivot position moves depending on the swinging arm position and the postion of the linkages supporting it
"on a single pivot bike it 's basicly the height of
the pivot if the pivot is above the point where
the chain meets the front ring then the rear
suspension will tend to extend
if it is below the point where the chain meets
the front ring then the suspension will tend to
compress"
sorry mate this is just incorrect!
family visiting shortly but I'll try to explain if I get a chance...bet you can't wait!
TT...happy to agree!
sorry mate this is just incorrect!
family visiting shortly but I'll try to explain if I get a chance...bet you can't wait!
you right i cant wait to see your explaination
oh jesus...this could get messy 😯
if the pivot is above the point where the chain meets the front ring then the rear suspension will tend to extendif it is below the point where the chain meets the front ring then the suspension will tend to compress
I get this and sort of agree with it.
If you imagine the points where the chain meets the chainring and sprocket and the pivot point as the three corners of a triangle, then the tension on the chain in isolation will tend to raise or lower the suspension as described. Shortening one side of a triangle affects the angles.
I still think this force is negligible in relation to the rider's weight on the pedals though, so it makes no difference..
chain tension is proprtional to load on the pedals and depends on which gear you are in ,,
It only depends on which gear you are in with derailleurs, which is why I mentioned earlier that I used a Rohloff on a VPP Blur.
Hub gears are an advantage in that situation.
It only depends on which gear you are in with derailleurs
this has got me thinking,, i have an alfine on my whyte preston fs bike,,
as the front and rear rings are constant with hub gears and hammersmit then the amount of feedback are the same in respect to the position of the chainpull and pivot position
but the amount of feedback is also a result of chain tension and wouldnt this depend on which gear you are in
I was going to say that chain tension is entirely dependant on pedal pressure, but a Hammerschmit would alter that.
As quoted earlier:
[quote
http://www.dw-link.com/physics.html
." When a bicycle accelerates forward, the rider's mass is transferred rearward. Without something to counteract this mass transfer, the rear suspension on most bicycles will compress under acceleration. This mass transfer as a reaction to acceleration is what riders have come to know as "bob."
It's badly worded IMO because mass doesn't really shift rearward if until the suspension moves (usually and then only by a small amount) but really he's talking about how momentum is important to consider, not just simple chain/braking forces.
The rider and everything bar the rear suspension have the majority of the mass and therefore momentum. What DW is talking about is that if you drive the rear wheel even on a design that has no chain forces having ANY effect on the suspension (eg main pivot concentric about the bb) you'll still get bob on most designs because the rear axle will drive the frame forward and this will activate the suspension one way or another. Imagine fixing a frame solidly fixed at the main frame so it can't move. Remove the shock and then push the dropout horizontally - almost any design will either compress or extend the suspension and even if it was a single pivot in line with the rear axle or four bar it'd still only be completely unaffected in one position through it's travel.
To eliminate the effect of chain tension,the pivot would need to be at the point where the chain meets the chainring, not at the centre of the BB.
At least with a hub gear the difference between the two is constant.
Any linkage that is designed to take chain tension in to account can't work properly on all three rings of a derailleur.
I see your point about the rear axle driving the mass forward, but under load the majority of the mass will be concentrated on the forward pedal travelling downwards in an arc, first forward and then back. I still think this is by far the greatest force on the suspension and will not be affected by the driving force its self.
Mtqg nope bb concentric will do it and work the same in all rings which is why I used that example rather than in line with pivot examples.
I still think this is by far the greatest force on the suspension and will not be affected by the driving force its self.
But that force is also driving the rear wheel isn't it... You're suggesting that pedalling forces are much bigger than drive forces which can only bs true if the efficiency is very low. Bike drivechains aren't very inefficient so it can't be true...
pivot concentric to bb gives zero chain growth so should have no effect on suspension
pivot concentric to bb gives zero chain growth so should have no effect on suspension
I'm sure you have done the trials, but for a chainring & cassette setup with a swingarm pivot at the bb and the chain big ring/small sprocket for example, dropping the rear axle will tension the chain, they form a triangle. Or am I getting terms confused?
I see your point about the rear axle driving the mass forward, but under load the majority of the mass will be concentrated on the forward pedal travelling downwards in an arc, first forward and then back. I still think this is by far the greatest force on the suspension and will not be affected by the driving force its self.
The majority of the mass is normally on the saddle. However, if you stand on the pedals what you say is true. Needs to be borne in mind though that it's change in force that causes bobbing, and the suspension is already supporting your body weight, so simply standing on the pedals does not immediately apply more force to the suspension. It might cause that force to vary more as you lunge about, but that's a different and problem. You can only push down on the pedal buy your own body weight plus how hard you can pull up on the bars and up on the other pedal after all.
A BB concentric pivot means the suspension has no effect on chain growth.
Chain [i]tension[/i] will still have an effect on suspension though.
The line of force through the chain is above the pivot, so it will pull the swinging arm in that direction.
You're suggesting that pedalling forces are much bigger than drive forces which can only bs true if the efficiency is very low
You've almost got me admitting I'm wrong here, but I'm not ready to give in yet. 😛
Gears multiply force [i]or[/i] speed.
In any gear higher than 1:1 the downward force on the pedals will be greater than the forward force on the frame.
I'm sure you have done the trails, but for a chainring & cassette setup with a swingarm pivot at the bb and the chain big ring/small sprocket for example, dropping the rear axle will tension the chain, they form a triangle.
No it won't! How could it? (ok, other than a tiny change due to the mech jockey not being concentric too)
MidlandTrailquestsGraham - Member
A BB concentric pivot means the suspension has no effect on chain growth.
Chain tension will still have an effect on suspension though.
The line of force through the chain is above the pivot, so it will pull the swinging arm in that direction.
Nope. You need chain growth for chain tension to matter...
They form a triangle mate. The chain goes from the top of the chainring back to a point very near the rear axle, the chainstay goes from the alxle to the bb and then add a line from the bb to the top of the chainring. you get a triangle. drop the axle and you tension the chain. no?
You know what concentric means, right. I don't mean a pivot near the bb, I mean around the bb. Again, concentric means no chain growth so chain tension is irrelevant.
trailer - no.
the wheel is free to rotate independently of the swing arm oh forget it i give up.
The majority of the mass is normally on the saddle
If we're talking about situations where pedal bob is a problem, then there will be more mass on the pedals, even when seated.
The mass may be constant, but the weight isn't.
Stand on your bathroom scales, then bend and straighten your legs quickly.
The dial will jump about all over the place.
That's what bike suspension has to deal with
I think there's some confusion over the use of the phrase "chain tension".
A concentric BB pivot won't affect tension as regards slack in the chain.
I'm referring to the tension, or force, in the top run of the chain.
You know what concentric means, right. I don't mean a pivot near the bb, I mean around the bb. Again, concentric means no chain growth so chain tension is irrelevant.
I do, and I don't agree I'm afraid. No being awkward. If you draw my diagram you will see what I mean. if you put in a fixed length of chain and fixed the chainring in position you could pick the bike up and the suspension would not extend. so that pivot position will cause pedal feedback because of chain growth as the suspension extends. that's how I understand it. rolling along is more complex than this obviously but...
I guess I don't understand what you mean by chain growth in that case...
No awkwardness inferred though you are wrong 🙂
If you had that swingarm free to move with no shock/etc to stop it extending too far and you picked the bike up or got air, the rear wheel would end up verically under the bb. That would still be true if he pedals were being turned the whole time. To put it another way, what would stop the rear wheel dropping under gravity? There is no force to resist it other than friction in the pivot maybe.
...and the chain would have had to grow by approximately one quarter of the outside circumference of the chainring?
Chain growth = bb to rear axle measurement increases through suspension travel.
...and the chain would have had to grow by approximately one quarter of the outside diameter of the chainring?
No, think about it 🙂
As the swingarm falls the chain also unwraps from the underside of the chainring...
got you. you are quite correct. thanks for bearing with me 🙂 bit of a misleading term though as it doesn't actually involve the chain 😀
As the swingarm falls the chain also unwraps from the underside of the chainring...
no clubber, the definition of chain growth is a feature of a suspension system in which the distance from the bb axle to the rear axle varies with compression/extension - it doesn't involve the chain at all. my error.
and actually in all this consideration you can ignore the loose bit of the chain, it has no relevance at all. the only bit that matters is the taut bit from the chainring to the back wheel. I'm getting there.
I'm not quite clear if you agree or not, now 🙂
😀 I agree that there is no chain growth with a concentric bb but i think that if you tension the chain (the bit between the chainring and the rear wheel) on concentric bb pivot bike, that tension will cause the suspension to compress. its the same effect as for a vpp bike with the pivots below the bb, both times the pivots are below the line of force being applied to the rear axle, so the axle is pulled up.
we actually cam into this via a misunderstanding re:chain growth. I don't think there is any disagreement here 😀
Sorry, it's just not. I'm trying to think of a good way to explain without just repeating myself. Anyone else?
I'm not sure either,but I think I'm with trailertrash.
Ignore the chainring for now.
If you've got a concentric pivot and you imagine a chain wrapped round a fixed sprocket with the brake on.
With the chain held vertical, it takes very little effort to lift the swingarm.
Bring the chain down to almost horizontal, but still above the pivot, and it takes a far greater effort.
Far more effort than is actually available to have a significant effect on the suspension when you take the riders weight in to account.
Besides I guess it's OT as I was only using it as an example of why chain tension/growth isn't the only thing to consider which is what DW is all about - in his marketing at least 🙂
My attempt at explaining it.
[img] 
[/img]
The jib is the singing arm.
The rope is the chain.
Pull on the rope,the jib rises.
Put tension on the chain, the suspension compresses.
Will you chaps please slow dooooowwwwn, I'm attempting to catch up but I read slow you post fast!
I think these are two different topics - chain growth, where the action of the suspension (some systems only) causes instantaneous changes in taut chain length due to bb and rear axle moving together or apart (e.g. while coasting over rocks), and chain tension applied by the rider, causing the suspension to compress due to what Midland has posted above.... ❓ these can both happen at the same time...and so interact..and it gets very complex...
I've just checked, and STW currently doesn't have a smashing my head against a brick-wall smiley 🙂
Here you go 😉
[img] 
[/img]
blah blah blah. How does the bike feel? How does it ride? How easy is it to maintain?
sorry if i caused unnecessary frustration 🙁
Lovely debate and it has really got me thinking. I agree with this
I agree that there is no chain growth with a concentric bb but i think that if you tension the chain (the bit between the chainring and the rear wheel) on concentric bb pivot bike, that tension will cause the suspension to compress.
Obviously with a pivot concentric to the bottom bracket the position of the swing arm doesn't cause any chamge in the relative lenths of the chain top and bottom.
If you pick up the back of the bike and turn the pedals to accelerate the rear wheel this is what happens (we are ignoring bearing friction here)
The top chain now has more tension than the bottom chain (this applies a torque to the rear wheel causing its angular velocity to increase)
However it also applies a turning moment to the swing arm as the line of action of the force through the chain doesn't run through the pivot. This will cause the swing arm to rotate upwards. This rotation in the swing arm will compress the shock until the moment of the shock balances the moment of the tension in the chain. When you stop pedalling the swing arm will rotate downwards.
If the pivot is located higher up so the line of the chain (at the top) passes through the pivot then pedalling will not cause the swing arm to rotate.
All of this of course ingnores two other effects
The movement of the riders mass (legs going up and down)
The resulting acceleration of the bike if the wheel is on the ground. As has been mentioned this also effects suspension. Its not helpful to talk aabout mass transfer. When the bike acclerates the force is applied at ground level, but the bike and riders centre of gravity is much higher. So the force applied at ground level tends to make the bike rotate (front wheel up rear wheel down). This causes the reaction force at the rear wheel to increase and decrease at the front wheel.
I believe it is possible to combine these to effects for a given chain position so that pedalling has no effect on the position of the swing arm (the chain tries to rotate the swing arm down and this balances the increased reaction at the rear wheel)
But of course this only works for one chain line and ignores your legs going up and down
I dunnno, women go to all sorts of lengths to try to prevent it, but I find the sight of a faint outline quite a turn on actually.
So ade if you imagine a sp suss bike with a single chainring and the pivot a few inches behind the seat tube in line with the again in the middle gear (yes the pivots would be on the chainstay).
your theory means that in a low gear the suss contracts as the again line is above it, and conversely that in a high gear it extends because the chain is below it.
that doesn't make sense!
of course there is tension in the chain but it ALWAYS pulls the bb TOWARDS the hub axle because that tension is balanced or counteracted by traction of the wheel on the ground.
there used to be a Spanish website that explained it all but I can't find it.
I'm not reading all that, so
a: what's the verdict?
b: if you cobbled the belt on a treadmill.and put each bike on, would they take off?
Enjoyed reading this, hope it reaches some sort of consensus that I can understand!
Elfinsafety - Member
I dunnno, women go to all sorts of lengths to try to prevent it, but I find the sight of a faint outline quite a turn on actually.
😆
So ade if you imagine a sp suss bike with a single chainring and the pivot a few inches behind the seat tube in line with the again in the middle gear (yes the pivots would be on the chainstay).your theory means that in a low gear the suss contracts as the again line is above it, and conversely that in a high gear it extends because the chain is below it.
that doesn't make sense!
of course there is tension in the chain but it ALWAYS pulls the bb TOWARDS the hub axle because that tension is balanced or counteracted by traction of the wheel on the ground.
there used to be a Spanish website that explained it all but I can't find it.
Hi Cynic-al, could you please expand on your explanation? You say the chain always pulls the bb towards the rear axle but that statement doesn't disprove what Ade was saying, unless I'm missing something?
Ade says the suss extends when the chain line is below the pivot.
That would mean the axle-bb distance increasing.
cynic-al - Member
Ade says the suss extends when the chain line is below the pivot.
That would mean the axle-bb distance increasing.
POSTED 1 HOUR AGO # REPORT-POST
Suspension can extend under chain tension without the axle/bb distance increasing depending on pivot point location.
Though Ade may have over simplified with his chainline description.
I would have to make a jib crane type thing with string and lolly sticks to test.
Take a trail bike like the Orange 5, a Single Pivot bike with a pivot roughly level with the 32T ring. This is the optimal position according to Orange for the best trade off between Axle Path, Pedal Kick Back and whatever else they are considering in their design. As most people spend most of their time in the middle ring, the 32T PP is pretty good, PKB may be a problem for some people on larger hits and the AP generally arcs forward after the rider's sag point, draw a circle with the Pivot Point as the centre to see this.
Where the chain pulls on the rear wheel in relation to the PP can cause the suspension to extend/compress when pedalling. In simple terms Chain line above PP will mean compression and vice versa. There is also the Mass Transfer that DW talks about, ie accelerate in your car and weight moves rearward due to momentum etc.
The Virtual Pivot Point, as the name suggests allows suspension to pivot around a point on/around a frame which physically does not exist. So now a manufacturer can have an initial pivot point which appears to be mid wheelbase or 3 feet in front of the bars or wherever they want really. If you look at the 'bars' of the Four Bar system the Instant Pivot Point can be seen by drawing lines through the supporting bars, for better explanation go the DW website and draw a line forward between the bearings on the lower link and the bearings on the upper link, the intersection of these lines is the VPP (ignore the SC patent for now!) However this point is constantly changing throughout the travel and is normally set up for pedalling efficiency around the sag point, general 32T level but often forward of the down tube. This should help with pedalling.
Now as the suspension moves through its travel, the VPP will change, here the designer may decide that on a big compression, the rider is not pedalling and move the VPP in a way to limit Chain Growth or for whatever effect they want. This is part of the reason for the extra price, a 1mm difference in actual pivot placement may drift the VPP out by a lot more and upset the system, coupled with the countless options to whittle down and a bit of marketing to lure you.
As with any system the designer has to make some assumptions on what a rider wants and design around that. So having the high forward PP should counter 'bob' and keep the press happy! So VPP/DW/FSR will be around for a while but as with SP they all compromise something in the end.
However it comes down to personal preference in the end and usually geometry plays a bigger part in getting a fun bike.
Where the chain pulls on the rear wheel in relation to the PP can cause the suspension to extend/compress when pedalling.
I'm still waiting for someone to convince me that this really happens.
An elite cyclist should be able to generate over 1kw.
That's a little over 1HP.
If bicycle suspension designers need to take 1HP in to account, how do motorcycles work with a simple single pivot swinging arm and a 150HP engine ?
A motorcycle engine isn't rotating at 60rpm with pistons that weigh 2 stone each.
Which is where we get back to my original point that the weight of the rider lunging on the pedals is all that matters when it comes to pedal bob.
The comparatively small force of the tension on the top run of the chain is insignificant.
MTQG trust me it matters...ride any high pivot bike like the 1st cannondale suss and you'll know all about it.
If you have read up to this point, is there a form you can fill in and submit to get the time lost added back on to your lifespan?
Think of a roller system on a rail and imagine a rope pulling on it at right angles and the roller would not move. Now move the rope up a few cm and pull again, the roller would want to follow and move up also. The swingarm and pivots control the path the axle will take on the bike but can still cause the axle path to be moved by chain tension.
Some systems take advantage of this and drive the wheel down a little, extending suspension but is cancelled out by your body momentum moving back in relation to the bike so there is little bobbing.
Standing up and can your whole body mass go up and down with each stroke will cause the suspension to react as will jumping up and down on the pedals or landing etc. The designer will generally assume the rider is seated for the majority of their pedalling and will design accordingly.
If you fitted an engine to a mountain bike, the bike may squat but as the power is constant you would not see bob. Plus the fact if you're providing the power to make you go forward you want to be as efficient as you can, if Yamaha is providing the power who cares!!
no idea what your roller analogy is about!
I'm unlikely to ever get the chance to ride an early Cannondale, what's the problem with them ?
Imagine a drag bike doing a static burn out, so weight transfer due to acceleration doesn't come in to it.
The suspension is designed for a total combined weight of bike and rider of 300kg.
The engine is putting out over 100hp to overcome the friction of the tyre and spin the back wheel.
The suspension will squat a little bit due to the tension on the top run of the chain.
Imagine a cyclist pedalling hard to maintain a constant speed in to a strong head wind, so weight transfer due to acceleration doesn't come in to it.
The suspension is designed for a total combined weight of bike and rider of 100kg.
The "engine" is putting out 1hp to drive the bike forwards.
Are you really saying that 1/100th of the power through a suspension system that is designed for 1/3rd of the weight will be noticeable ?
The rail travels vertically on the left hand side of your screen. There is a roller on it which is free to move up and down, almost frictionless if you will and it is in the middle of the rail.
Now a piece of string is attached to the roller and you bring it out to the middle of the right hand side of your screen and pull really hard, the roller doesn't move. Now relax for a minute, move your hand and string to the top right of the screen and pull, the roller will start to move up the rail.
I [s]think[/s] hope that might help!
A static burn out.
No cyclist could ever spin a tyre that wide on dry tarmac, therefore the bike is putting far more tension on the top run of the chain than any bicycle suspension is designed to deal with.
It's a simple single pivot swinging arm.
It's not squatting.