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As the title really. My first foray into 29er and it seems that, thanks to Gary Fisher and G2, most forks offer a 51mm offset. I can see why it made sense back then, to try and bring the trail figure inline with the 26" bikes were were all riding at the time. However, longer offset reduces trail and stability. So, was he saying that 29ers were too stable?
And now, the marketing men are telling everyone that a shorter offset is what we want. They seem to think that all bikes need the extra stability. So who's right? More importantly, what will a 44mm fork do to a ht 29er frame with a 67.5 degree head angle?
Let the discussion commence.
be ahead of the crowd.
I have some 44mm offset forks on my Solaris. I will try some rigid forks over the winter for something to compare against.
I bought a Trek frame with G2 geometry and fitted generic Rebas which had 46mm offset and it was never right. I posted a fair few threads on here. The bike felt really unstable when leaning over in turns, like the front wheel was going to tuck under. Fitting a 51mm fork fixed it.
The thing is, that bike was designed around 51mm and the 46mm fork pushed the geometry outside what was good. Other bikes might have a suitable range that includes 46 and 51, and you could have a choice which one to use.
I think it's different offsets for different bikes, same as with every other measurement. So choose what you want but start with what the designer had in mind. Mine was dreadful with the wrong one.
I've recently had a chat with Adrian at Swarf about this very thing, ...here's what he had to say.
>> In my opinion this fork offset thing is a total load of marketing BS to try and sell new ('better'!) frames. I had a long chat to Trev Worsley (editor of Enduro mag) about this and he's got the same outlook on it as I have (he's done a load of back to back testing), yes it'll make a difference to the handling but it's so minor that you adjust to it within a few hours. He's not of the opinion that it's better, just slightly different.There is also some misunderstanding as to the actual affect is has on handling. Shortening the offset increases the trail which equates to making the steering handle like a slacker head angle, I'd say that's slowing it down not speeding it up. It'll also get more floppy in the same way slackening the head angle would. I can compare some numbers for you as I have a little spreadsheet that calculates it all.
>>> Standard contour with 51mm offset
> HA 66.5
> Trail 107.5mm
>>> Standard contour with 44mm offset
> HA 66.5
> Trail 115mm (7.5mm shorter wheelbase)
This equates to slackening the head angle on a 51mm offset fork by 1 degree without the increase in wheelbase. I rode the original proto with both the Revelation at 46mm offset and the Pike and helm at 51mm offset.... I didn't notice any difference! Having a tyre too soft or increasing the sag a bit has a similar level of change!
I'm always asking Dave Turner stuff like this
I've run a 44mm offset on my Czar for five years about to go to 51mm.
Here's his take on it to me.
<div>The only time a 51 offset is 'better' is in multiple tight turns in quick succession. Flowy trails and bridle ways don't matter as they are plenty wide and radius' large so the extra trail from the shorter offset is a bonus.</div>
<div>Wouldn't mess with it, unless you NEED another fork and/or ride in super tight, knuckle smashing woods all the time. The 44 does make the bike very stable, great for the rocky areas I see pics of from the Lakes and north..</div>
I have a bit of experience with this. I've experimented with HA and offsets with homemade forks bicycles and motorbikes.
I much prefer a steep head angle and plenty trail - which means less offset. The benefit of a slack HA is for the operation of the telescopic fork. It also creates the longer distance to the front wheel which gives rider that feeling of security IMO because their CoG is further from the pivot of the wheel.
On some telescopic forks you can reduce the offset simply by removing the lowers and turning the crown 180º, and then refitting the lowers.
However there is a lot of [s]twaddle[/s] advertorial talked on the subject because the actual geometry changes on a suspension bike being used to its limits are quite great, and riders adapt. The real technological wonder is the human brain and body.
@the00 - did you buy the Solaris from Gotama? He put a 44mm rake fork on his Solaris about 9 months ago. However, he's been quiet of late.
@molgrips - that's my worry. The manufacturer in this case suggests 51mm but doesn't give options or a range. I'm building up from a frame. Short of buying two forks, I've no way of knowing how well this will/wont work.
@dumbbot - I did see a comment in Enduro Mag from Trevor (but can't find it now) where they said it didn't make a lot of difference but they preferred short offset for tight twisty trails and long offset for wide open courses. Unless I'm remembering that incorrectly,l it seems the wrong way around to me. Unless it waas the change in wheelbase they were noticing more than the change in trail.
@rone - interesting take on things. I'm wondering if 44 would be okay yet DT is come from the angle of why do you need anything other than a 44mm offset.
@epicyclo - I've also been down a similar line of thought. Given the fork will have a tyre greater than 2" and a little over 20psi and a fork with 120mm of travel I'm sure there's more change going on that I'll feel by reducing the trail to 101mm from 108mm.
Still slightly concerned by Molgrips experience though.
Does it depend on stem length? I have 51mm forks and with a 50mm stem the set up felt great. Changed to a 35mm stem and it all went wrong. Sluggish at first followed by too quick steering. Just changed back to 50mm to compare notes. Wondering if a shorter offset would cause such a problem with shorter stems?
chestrockwell
...Wondering if a shorter offset would cause such a problem with shorter stems?
Maybe take a look at your body position with the change in stem.
If you have wide bars and bring them closer, you have to spread your arms more which could be bringing your upper body position (and CoG) forward more than shortening the stem puts it back.
I've tried and have on different bikes different offsets, all on 29ers.
If you asked me to blind test them, and tell you which was which, I couldn't tell any difference.
For reference they are 51, 46 & 44. I think its mostly bollocks. I wouldn't not buy a fork because it's offset was 51 over 44 for example.
Well to be honest it seems the wrong way around to me, I was under the impression that the shorter fork offset was to counteract the effect of the increasing reach/length as is the current fashion in bikes, but what do I know.
I'm sure a luddite like me wouldn't notice the difference anyway, so i've just bought the 29'er fork i wanted with a 51mm offset.
I have tried a few different offsets but usually it’s been combined with other differences in geometry so it’s hard to say if it does have much noticeable effect.
Would I would say is that it’s made buying forks slightly more complicated...
Could explain why I'm finding my 29" so different to 26" - my forks are 38mm offset 🤔
On which? That's quite normal for a 26" fork.
If you want to check trail figures etc for various combinations, then http://yojimg.net/bike/web_tools/trailcalc.php
Cheers. I have been playing about with that very calculator. Throws up some interesting results, it's just interpreting them that's tricky. After all, who knows what a flop of 38mm feels like?
Well to be honest it seems the wrong way around to me, I was under the impression that the shorter fork offset was to counteract the effect of the increasing reach/length as is the current fashion in bikes, but what do I know.
I’m sure a luddite like me wouldn’t notice the difference anyway, so i’ve just bought the 29’er fork i wanted with a 51mm offset.
I believe that is what Transition say/said in some material for their latest geometry - slacker head angle and shorter offset brings the front wheel back to where it was. Which is one consideration, but completely ignores that you will be running even more trail thereby and experiencing even more flop. Change one dimension, you change several parameters.
There are so many compromises here. If you want long geometry but to keep the handlebars within reach, then the easiest way to achieve that is with a slack head angle. You could go for a steeper head angle and less or even negative offset* to achieve the desired trail and stabilty. You would then have low flop, but you would need a negative stem length* to get the bars within reach. Stem length also affects steering feel and stability. I think the reason modern bikes get away with really short stems is because of the wide bars, which counteract the effect of the short stem - a 35 mm stem and old-school narrow straight bars might feel quite weird. I don't think a steep head angle, loads of trail (ie zero or negative offset) and a stem on backwards would feel nice either, but I could be wrong.
* I guess this is what you were experimenting with @epicyclo. How did you deal with the reach issue? I believe Tony Foale experimented with near-vertical head tubes and linkage-connected steering (on motorbikes) to bring the bars nearer the rider. I can't find a link to that now. That would be interesting on an mtb, but probably not worth the hassle.
that’s my worry. The manufacturer in this case suggests 51mm but doesn’t give options or a range. I’m building up from a frame. Short of buying two forks, I’ve no way of knowing how well this will/wont work.
I'd start with the manufacturer's recommendations. They've designed the thing after all and whilst a different offset might not make a difference, it might. It's unlikely to be 'wrong' imo.
I put shorter offset forks on my el-mariachi as they were in the sale (this was before it was fashionable). It just made the bike feel slacker without actually making it any taller/slacker.
Whether it's better or worse would depend on whether you wanted that or not. I think there's some maths (on the cotic site?) that predicts the point at which a wheel will 'flop' in a corner, which gives a clue as to how likely it is to tuck in when it loses grip too (rather than drift out). So a short offset and steep angle, and slack angle long offset will feel similar initially, but will behave differently, and I can't remember which one was supposed to be mathematically better.
True, and that's the thought I keep going back to. But those short offset forks are a bargain. It was reading that old mbr article of three offsets back to back and then realising they used the same wheel size and head angle that I'm planning that set the cogs turning.
greyspoke
* I guess this is what you were experimenting with @epicyclo. How did you deal with the reach issue? I believe Tony Foale experimented with near-vertical head tubes and linkage-connected steering (on motorbikes) to bring the bars nearer the rider. I can’t find a link to that now. That would be interesting on an mtb, but probably not worth the hassle.
The problem with a vertical HA on a bike is that to get the front wheel in the same place as normal, you need a very long toptube, and the downtube needs to be curved or otherwise compromised so it does not interfere with a turned wheel. That leads to either stiffness issues or weight issues - however composite construction has probably advanced to the stage where that is not a problem.
I don't think negative stems are a problem. I regularly ride a bike on rough stuff where my hands are behind the steerer. 🙂
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I agree with Tony Foale's finding that a steep HA handled ruts better, which I think is particularly important in the case of fatbikes where you can be riding in frozen ruts, but also useful in general mtb riding if you strike a rock with the side of your tyre at speed.
Vertical HA would be probably too many frame compromises, but I reckon a fatbike with about 80º HA and a heap of trail would make riding in deep snow or mud much easier.
I got my HA to that on a fatbike by using a homemade adjustable fork, and the bike felt pinned on the corners, but because there had been no frame mods, the BB was too low. If I was building custom, that's the way I'd go.
In the meantime I suggest fatbike riders snap up old school fat bike frames because the new stuff is coming out slack to conform to fashion - and while perfectly ok for general trail riding, I wouldn't want one in deep snow or ruts.
EDIT: this might be the link you're looking for https://motochassis.com/articles/experiments-with-steering-geometry/
Interesting @epicyclo. I guess the only way to find out is to build one. But if you want a suspension fork, that would mean making your own lowers. Or perhaps using backwards uppers. Or a diy linkage fork...
If I didn't know I was pretty crap at fabricating things, I would be tempted to have a go.
Yes that's the link, thanks.
There used to be a thread on how I built a set of adjustable forks on the UK Fatbike forum, but unfortunately that's defunct now.
However if you click on this pic and go back and fore through the other pics, you'll see how I made one.
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All done with scrap forks scavenged from the lbs bin. The trick is finding a set of lowers that are a snug fit with the uppers because you'll be doing this without the bushes. It's all clamped together with seat clamps, and I didn't die. 🙂
There's an easier method now. Just get a set of these forks and try setting the legs at different heights which will give you effective HA changes. You can change the offset by reversing the crowns.
https://classic-cycle.com/all/642/double-crown-fork-73-chrome?c=459
Of course, that's all rigid, but it may help if you want to experiment.
hi guys wondering if you can help me out just been reading the forum I currently have a merida big trail but it came with different spec forks than listed, so I am looking at upgrading them the current ones are 27.5+/29" 51mm offset boost forks, I am running 27.5 boost wheels with a 2.6 front tyre. should I be looking at replacing the forks with exactly the same as above or will I be ok looking at 27.5 boost forks with a 44mm offset I cant see my self ever running a 29" wheel in the frame any help would be great cheers guys
I couldn’t explain it but having swapped forks around, my Pivot Les is much happier with a 46 off set fork, while my On One Parkwood is happier with a 51 off set. Neither bike feels right the other way round.