Please help an idiot with bike geometry? Bit of a long post...

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Please help an idiot with bike geometry? Bit of a long post...

10 posts & 7 voices | Started 7 months ago by TimP | Latest reply from phutphutend

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TimP - Member

Please can someone help me on this:

I have an old Sub5. I spoke to Orange about fork lengths and they said max 120mm, and I have U-turn forks set at 120mm so all good. Except it all feels a bit unsettled and twitchy when braking hard (bit like a speed wobble) and generally light at the front but not to the point of struggling to keep the front wheel on the ground going up hill. It does feel more stable at 100mm, but would like to keep a bit more compensation up front. The stem is currently pretty long with a bit more rise than I would have liked and I have 670mm wide bars with a medium rise, and there are no spacers and I have a crank bros headset with 21mm stack height (and no more steerer left to increase any of that)

My understanding is that shortening the stem will make it more twitchy and using wider bars will not help either. Would flipping the stem thus dropping the front a bit help? Using flat bars? Or is it more to do with the head angle therefore I just need to keep dropping the suspension till it feels right. I have some lower rise bars on the way hopefully with a shorter and lower stem so will try that anyway, but just wondering if I am basically throwing money away?

Thanks

Posted 7 months ago #
z1ppy - Member

using wider bars will not help either

I thought this brings your weight forward, so might actually help add more weight over the front wheel & potentially make it more stable.

Also not all fork axle to crown length are the same, a lot of manufacturers don't quote a max fork travel, instead they give a max Axle to crown length.
What forks are you running?

Posted 7 months ago #
TimP - Member

They are Recon U-turns, but with a crank bros headset there is very little under the headtube which would help with the increased a-c would it not?

Posted 7 months ago #
PJM1974 - Member

I think you're on the right track with flipping the stem, it would lower the weight at the front and help with steering grip.

FWIW, I run an 80mm stem and low rise 710mm bars on my Enduro. I found that front end grip wasn't compromised after moving from 640mm bars and a 90mm 10 degree rise stem as the wider bars forced my shoulders a little further forward and my stance wasn't quite so upright.

Posted 7 months ago #
deviant - Member

The speed wobble you describe could be other things too, make sure you have ruled out a warped brake disc, wheel out of true, shagged hub etc before you automatically look at the geometry of the bike?

For what its worth i found the front end of my bike more stable when i switched to a shorter stem, i made the change to wider bars at the same time for the reasons somebody else above stated, it keeps some weight over the front.

Posted 7 months ago #
kudos100 - Member

Try wider bars with a zero rise stem.

Posted 7 months ago #
TimP - Member

Disc is def straight. Wheel is in pretty good nick and no obvious wobble. It mostly happens on long straight descents when I brake gently for extended periods, the whole bike just seems very unstable.

Will try flipping the stem before the new bars etc arrive and see what happens

Posted 7 months ago #
thomthumb - Member

is the fork diving through all the travel on braking? packing down on long descents?

Posted 7 months ago #
TimP - Member

Not as far as I am aware. It doesn't dive much when I brake hard on the flat so don't see why it would be much worse on a long descent. It is a coil fork, but with the u-turn it is allowing me to play around a bit. hoping eventually to swap to an air fork with a bit more tuning but fixed travel

Posted 7 months ago #
phutphutend - Member

With a single pivot, dragging the brakes will stiffen up the rear suspension. Also, although you say there's no brake dive, braking will push you weight froward and extend the rear suspension a little. Perhaps these combined effects; steepening and stiffening, are causing what you describe as wobble.

Lay off the brakes until you need to.

Posted 7 months ago #

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