One of the recent developements in the history of 29″ers that has caused more confusion and questions than probably the existence of 29″ers themselves is the change in fork offset by prominent suspension manufacturers to “tune” the handling of 29″ers. Why was this done? Is there an “old 29″er geometry” and a “new 29″er geometry”? Can you mix and match forks and frames from different design philosophies? And the questions go on. In these posts I will try to briefly touch on some facts and try to dispel some myths about the current state of affairs as it relates to handling characteristics of 29″ers.
Fact: A 29 inch front wheel will have more trail because of its size. This is the reason for everything that follows and the first thing one needs to understand to be able to follow along. Fork trail is the measure that helps one determine the relative stability/instability of an inline wheeled vehicle. Simply picture a shopping cart wheel or a dolly wheel. The “pivot” that attaches to the cart/dolly is the steering axis. The contact patch of the cart/dolly wheel follows this axis as you push the cart/dolly along. The distance from the centerline of the steering axis to the wheel contact patch is a measure called “trail” or in the case of a bicycle, “fork trail”.
This “trail” measurement increased when modern 29″ers were first developed as a direct result of using a larger diameter wheel. (I won’t get into the details of this, but trust me, it is true) While most of the other critical front end measurements stayed the same, this increase in fork trail dialed more stability into the front end of the earliest 29″ers. The early adopters liked this trait, and so they did not try to change the front end geometry much if at all.
Fact: Many judged the 29″er against what they knew before- the 26 inch wheeled mountain bike. Some early critics of 29″ers were quick to point out this new 29″er geometry as being “not quick and snappy, like a 26″er”. Thus they panned the new wheelsize for this, (and several other) reasons. Others were interested in seeing if something could be done to “improve upon” the matter at hand, so the first of many “fixes” were applied to 29″er bikes. These were sometimes used alone or in combination with each other. The most popular being to increase the head angle from the 26″ers 71 degree angle to something around either side of 72 degrees head angle. This had the effect of decreasing the trail measurement back closer to a 26″ers and made the front end steer with less effort and lost a bit of stability in the process. Of course most applauded the effort to more closely approximate 26″er handling, but the critics still said, “Too sluggish!”
Okay, that should set the stage for what has happened recently. The big problem was always that you were pretty much stuck with using the fork offset on suspension forks that was developed for 26″ers on 29″ers. This made getting a suspended hardtail or full suspension 29″er to steer with a snappy, quick feel hard to obtain. The idea was that if the slate could be wiped clean, a new offset(s) that would work to make 29″ers more like a 26″er in turn in feel and not be unstable could be obtained. Either that or use the same offset as always, (38mm) and keep steepening the head angle to achieve a more snappy, quick front end. And finally, some companies have elected to keep doing the same type of front end geometry that 29″ers have been using since about 2003, which is a slightly steeper head angle than a 26″er and keeping the offset at 38mm or so. Some would call the first two examples “new” 29″er geometry, and some would call the last example “old school geometry” for 29″ers.
Myth: There are “old” geometry 29″ers and “new” geometry 29″ers This is simply false. The fact of the matter is that there are several types of geometry solutions all being worked on at the same time. This is a time that is marked by ongoing experimentation and research into just what a 29″er should steer like; and quite frankly, it is not going to shake out anytime soon. Nor should it. The situation now days is a blessing and a curse. For one thing, it is a blessing in that we can now “tune our rides”. It is a curse in that now you have to think about this stuff. It is not a moot point like it was back in the earlier days of mountain biking when every hardtail had “NORBA geometry” and you just assumed it would handle like a 26 inch wheeled mountain bike “should” handle. No, now there are options to consider, and the wise 29″er shopper will be aware of these options going in.
Next post: Can you swap around forks and frames to acheive a desired result?, and other 29″er handling questions. Stay tuned!
I just installed an F29 on my 2007 Gary Fisher Cobia and I can’t believe the difference in steering performance. In my opinion, no stability was lost (though I’ve only had one good ride on it), but the steering was quickened quite a bit. Can’t wait to slice some switchbacks on it.
I personally believe, that after a few more years, we’ll find a 29er standard offset (my guess would be about 44mm) that builders will start designing frames around. Slightly slacker HAs make fore a great all mountain bike, while not impacting climbing too much.
One thing is for sure though, I agree that people have to stop judging 29″ bikes by their experiences on 26″ bikes. My Cobia was my first mountain bike and every time I hop on a 26″ bike, I feel like I’m on a kiddie cycle.
Well said Jared.
Here’s some angle/offset/trail dimensions for reference:
26″ wheel, 71 h/a, 38mm offset = 75.2mm trail.
29″ wheel, 71 h/a, 38mm offset = 89.3mm trail
29″ wheel, 72 h/a, 38mm offset = 82.2mm trail
29″ wheel, 73 h/a, 38mm offset = 75.2mm trail
29″ wheel, 71 h/a, 44mm offset = 82.9mm trail
29″ wheel, 72 h/a, 44mm offset = 75.9mm trail
So, what does all them numbers say? I don’t know. My personal preference is for a steeper head angle with more offset ( 72 degrees w/ 44mm offset).
To address Jared’s comment about all-mountain bikes, slightly slacker head angles are necessary because of the copious amounts of suspension travel (front and rear).
I think when most people talk 29er geometry, they are talking about hardtails. Rear suspension throws a whole different variable into the mix. And longer travel ads further variables. The more travel in the fork, the slacker the head angle as trail decreases with suspension fork compression. Conversely, if the bikes is a full suspension, trail will increase with rear compression. Combining front and rear compression and ……. ow, my head hurts thinking about that one.
But, all that being said, the human body is a remarkably adaptable creature. We can get used to almost anything given enough time on it. To me, it’s not so much the exact nature of how something rides and performs, but just the fact that I’m able to get out on the trail.
Mike: Thanks for taking the time to crunch out all of those numbers. I know a lot of folks really put a lot of weight on that information. However; you also mention something in the last paragraph of your comment that gets lost on a lot of folks. I think it is the most important factor of all.
This is why I’m focusing more on the “tune your ride” aspect than the hard numbers in my posts. I feel that like yourself, many people will find a geometry package that suits them and their trails to a “T”. (T = trail, if you want. Heh heh!
) Anyway, The mere fact that we as 29″er riders even have the option to do this is itself an amazing thing in my mind.
As I said, and also as you alluded to, this can require a bit of a “brain tease” to figure out, especially, as you so astutely put, to figure it out for a full suspension, long travel bike.
Thanks again for chiming in. I appreciated your comments.
Ah Mike I’m not sure it’s that simple . Surely a 490mm fork with a 72 angle and 47mm offset is going to have a different trail number from a 400mm fork with a 72 angle and a 47 mm offset?
the offset’s the same but the angle won’t be.
Singular’s website quotes 485mm fork, 72 head angle, 48mm offset = 70mm trail.
Does your brain hurt now?
jeremy uk:
You’re right. The HA will change as the Axle-to-Crown (AC) length changes. My research into 29er geometries has shown that most 29er geometry for front sprung bikes (80mm) is designed around a 480mm-490mm AC lengths.
Altering that AC length (i.e. my Fox forks 501.8mm AC length @ 100mm travel) changes the geometry. Not just the HA either. Seat Tube Angle, BB height, BB Drop, and other angles are also altered. I *think* that raising the AC length by 20mm, effectively gives you about 1/2 a degree decrease in HA. Then, as Mike mentioned, fork sag (not to mention rear sag) changes this again.
I think this issue has been beat to death though. Most often, these numbers are just that; numbers. I thought I could predict the ride of my bike when I changed the fork, but alas, I was wrong. As so many others have said before, you’ve just GOT to ride it before you really know.
http://www.papkegroup.com/superbikeengco/superbikesupensionkinematics1.html
a motorcycle trail calculator
Yeh I’d known that altering sus fork lengths alter head angles, by about a degree an inch, but I hadn’t sussed that the rigid length of the fork also is a variable as to trail, independant of head angle.
If the bike has rear suspension and that compresses the same amount at the same time the trail figure still alters independantly of head angle [which would stay the same]. Whether that’s dependant on the offset being engineered at the drop out or more at the crown I couldn’t say.
I haven’t worked out which holds the offset at a more even gap
I’d have to do some more fag packet calculations to check that
[ I did draw some lines to check variable rigid fork lengths real world as my brain hurt just on that simple problem].
I think that’s maybe why the new full sus G2 Fisher geometry is so steep, but that’s only from memory
I’m just playing devil’s advocate, having a little fun
All this is precisely why Ted is steering clear of the numbers game!
Well, I expected the numbers to get flaunted and the trail calculators to be proffered, but remember, those are only guide posts and not a hard and fast way to identify how a bike will handle for you and your riding style. I like this part of Jared’s comment, which I think sums up the issue succintly:
“I think this issue has been beat to death though. Most often, these numbers are just that; numbers. I thought I could predict the ride of my bike when I changed the fork, but alas, I was wrong. As so many others have said before, you’ve just GOT to ride it before you really know.”
This has been my experience as well. The “human factor” overides all of these numbers and theories. It is vastly more complex, unpredictable, and adapative. It cannot be neatly packaged in a trail calculator from an online source.
I’m not saying that those numbers are worthless, or that we should ignore them, but that they are only a hint, ( a good hint perhaps) at what you might expect from any certain sort of geometry for you and your trails. Again as Jared says, “…you’ve just GOT to ride it before you really know.” It is pretty much exactly what I was trying to say in the post. There is no “easy button” to push when it comes to the handling characteristics of 29″ers these days.
Heck, back in the early 90′s print magazines made a living out of dissecting the handling traits of trail bikes that had EXACTLY the same geometry, and yet they were able to winnow out the differences between the different models. Interesting if you consider it.
I don’t pretend to understand the trail calculator, just proferred it as evidence that fork length is a variable.
Trail is not a function of fork length (at least the A-C). It’s a function of wheel diameter, fork offset, and head angle. This applies only to rigid forks. Does anyone actually use suspension forks
? The only reason the “relative” trail changes on a suspension fork as it “shortens” is because the compression of the fork changes the head angle
Those numbers are really reference.
Upon further thought, if you took a bike with a suspension fork, fixed the head tube so it’s angle did not change and compressed the fork up from below, the trail would not change.
Luckily, the bike industry is chock full of very talented designers who have, for the most part, dialed in geometry very nicely.
My head hurts now
Mike: That was the point of that first part of my last post. Trail IS changed IF A-C changes because the altered A-C length changes HA. The originally spec’d HA on a frame is ONLY applicable if the A-C the frame was designed around remains intact post fork change (as you eventually alluded to). A-C is pertinent in determining HA, therefore has effect on trail (though it is not calculated for in trail formulas because the math has already been done in determining HA). I think the horse is beginning to stink.
Anyway, I for one look forward to GT’s next post as I have changed the handling of my bike dramatically for the better with a fork swap.
Sorry for that confusion. I’ve been spending all my time lately working with (and riding) rigid forks that I sometimes forget folks are talking suspension forks.
Hehe. No worries. I always assume everyone rides sprung, not rigid.
You’re correct Mike. I redid my geometry with a little more care. Apologies.
Well at least my head’s a little clearer. Anyway tomorrow my 16″ On-One should arrive [71/47] and I can go out and compare it with what I remember of my old Fisher [clue christened 'the Lorry', er that's english for truck]
That is if the projectile vomiting bug has finally done with me.
This is highly subjective, but while I loved my 26er 5-Spot, it had a kind of slash and burn feel to cornering. Not bad at all, but that was the feel. Switching to a 29er Sultan, I didn’t find the handling any slower or faster, but definitely a smoother “carving” kind of feel to cornering. I love it. I treated myself to a new Superfly race bike, and with only a few winter rides so far, it kind of hits the middle ground between the two (I know, HT to rear sus comparo, but it’s all I’ve got). I like it, and suspect I’d absolutely love it WITHOUT the different offset F29 fork.
A long-winded way towards saying that I hope the whole 29er cornering experience isn’t screwed up by trying to chase the same feel of a 26er!
i ride 3 different bikes, a curtlo fully rigid 24hr, a haro 29 ss, and a 05 sworks stumpjumper. first go to bike is almost always the stumpy. full susp, comfort, handling…. easy on the back (43yr old)
second is the haro and lastly the curtlo. i have played with the travle on the stumpy (talas fork), swapped the 26 fr. wheel for the 29 on the curtlo to make a 69r.
i,like the relaxed angles i guess because when i put the 29 fr. wheel on the curtlo it made it much more comfortable, and better handling. fork on the curtlo is a vicious cycles 100mm corrected, so plenty long enough for the 29 wheel. came to the conclusion that it is closer to the stumpy in layout with a larger fr. tire. ie., its what i’m used to. if you have the chance, swap some parts around, you may be suprised that you like not what you thought you would. now, next i wanna try a 29r full susp. that has a little laid back geometry, sound like the ticket for me…… maybe not you.
I’ve been reading up on all the head tube angles, trailing, corrected suspension etc., etc…. I’ve come to the conclusion that one must build it and ride it to really see what happens. I’m getting ready to 69er my Trance. Putting a Reba 29 on the front end. The head tube angle is going to be very slack. I’m kind of concerned, but it is the only way to find out. I’ve got a Mary SS that is a real 29er. Love that lay out in full rigid. We’ll see what happens with the Trance. I’ve tried calculating this and that and I get to a point where my brain explodes. Intersting posts and commentary on the 29er steering. Thanks a bunch! I enjoy the thought it generates for me.