Question to the bike fit gurus or to anyone with more general sense than myself. Given two road bikes of the same size, is there a specific aspect of the bikes geometry or fit that contributes to a greater sense of stability on a straight line?
I have two road bikes for slightly different purposes (commute/gravel vs normal road) and have noticed that one bike appears significantly more stable than the other, I could ride all day without touching the bars if needed whereas the other requires a lot more concentration to hold a straight line if I take pressure off the bars.
Ignoring differences like tire width, I would guess that STA and seat position are a big contributor to this with a slacker STA generally producing a more stable ride and a steeper STA / more forward seat position shifting more weight on the front wheel and generally reducing stability.
This seems to mesh up with anecdotal experience with a really slack bike like a cruiser on one extreme and a steep TT bike on the other. I also suspect though that bar height / stem length and angle play into this and am curious what more experienced people have to say on the matter.
bike weight? I’m assuming your commuter is a bit bulkier than your road bike. Just trying to give other options as my gut what you are thinking as well
The specific examples in this case are a 2008 aluminum Trek 2.3 and a 2011 Cervelo S2 with the trek being the more stable of the two. There is nothing really specific about the Trek that make it a commuter other than it’s an older, cheaper bike.
I am curious about the topic in general, but admittedly I’m also looking for direct things I can tweak on the S2 to make it closer to the stability and all day comfort of the Trek.
In all seriousness, I’ve ridden bikes with loose wheel bearings, trashed headsets, bent wheels, stiff frames, flexible frames, large frames, bikes too small… wide ranges of geometry… it’s just a matter of how you balance your weight and how you control the bike. I can;t recall even having a high speed wobble. The worst issue I had was recently on a bad headset where the bearing were basically gone. Wouldn’t stay tight and was sticking in corners when turned, rather than moving freely.
Sure some bikes have different response to inputs, but it’s not unstable until you exceed the limits of the inherent stability the overall set-up provides. Most cases, that’s still a pretty high speed, pretty aggressive riding. IF you balance you weight right for example, you can lock up the front wheel on a MTB on dry pavement… without “going over the bars”.
I think what you’re looking for is “front center”. Someone else probably mentioned rake too. You’re right that other aspects contribute to stability, but Slowman has opined fairly recently (can’t find the thread) about picking the optimal bike for handling has to do with front center. -J
Many factors contribute to the bike’s tendency to want to keep going in a straight line with little input. They can be done in concert to make a bike actually hard to lay over into a corner. Frame design is a balancing act. That said…
Increasing front-center tends to increase stability.
Increasing trail (a resultant to head tube angle, fork rake, and tire outer-diameter) tends to increase stability.
A lower BB tends to increase stability.
More weight on the back wheel tends to increase stability (also a resultant of saddle set-back, chainstay length, and front-center).
But it’s a continuum, and one’s preferences tend to color what one considers to be stable.
i have seen cable lengths
which cause bike to want to steer funny/ away from straight line
if you let go of bars
because of how the multiple cables tension as they route around the head set area of bike.
I have not been fitted on either bike and have made haphazard changes to them over the years and that is in part want I want to rectify. To understand why I am changing things and what effect it should have versus just making small adjustments and seeing how it feels.
Will post pictures of both bikes after work today, appreciate the responses.
Trail on Cervelo from their standard of 43mm rake is 58mm
Trail on the trek is advertised as 56mm but mathematically it comes out to 61 would might explain why it feels more stable. Since I don’t have fork length numbers it’s difficult to say for sure
Chain stays are 11mm longer on the Trek and the wheelbase is 34mm longer
Does one have a smaller handlebar width than the other? That would make it twitchier as smaller movements would move the wheel more making it seem like it doesn’t want to track as easily.
But trail is calculated for the frameset, so the slightly different fork lengths should already be accounted for.
The difference in the Trek’s listed and calculated trail is probably due to different assumptions about how big or small or squishy the tires are. Anyway, with similar trail, the Trek’s steeper head tube angle normally would make it less stable (all else equal).
But not all else is equal. Although the two bikes are pretty similar geometrically (similar trail), they’re likely a little different in torsional stiffness. My educated guess is the Trek might be slightly stiffer, which leans toward stability. Thus tending to cancel the Trek’s geometric lean toward less stable.
So probably something else is making the difference. Wheel perimeter weight (heavier is more stable), stem (longer is more stable), tire inflation pressure (lower is more stable), fit (higher hands are more stable), etc.
The two frames have dissimilar stack numbers (Cervelo 58, Trek 59.5), so the potentially higher handlebar on the Trek could explain a lot of the difference in feel.
I’m not a Fit Guru (or even a fit guru) but I owned an S5 from 2013 and it was a twitchy bastard. Here are the things I looked at/was told:
Stem length can affect steering response - longer = more stable. I ride a longer stem for this reason
Tyres can matter a lot. Wider/better ones ride better
Hubs & spokes need to be in good shape
Rider behaviour matters a lot - but you are the same person on both bikes so discount that…
The 3T fork which many Cervelo’s used had twitchy characteristics. A shop who were trying to see me a new S5 (with the new fork) mentioned this > Damon, don’t know if you want to comment on that?