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Steering geometry

by Spangly Shiny
Sat, Oct 26, 2013 14:26
3

I have a 2008 Giant TCR Advanced that I just cannot ride no hands because it is so twitchy. For the same reason I cannot ride it on the rollers either so I intend to fix it.
I understand that the standard fork on the size M/L is a 45 degree rake. If I replace it with a 43 degree rake am I right in thinking that the following will pertain?

1. The trail will be increased.
2. The steering will slow making the bike more stable.
3. The toe overlap will increase with the reduced wheelbase.

Most importantly with such a small change, will I notice?

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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3 comments

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Paul Hewitt | 10 years ago
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The trail will be increased if you fit a fork with a less offset / rake, this will not really affect the slowness of the steering this is affected more by the head angle.

The toe overlap, if you have some, will be increased by the 2mm, you shouldn't really notice this though. This site is well worth a look- http://yojimg.net/bike/web_tools/trailcalc.php

Having said all the above, I would be surprised if the twitchiness is being caused by the frame geometry, as most manufacturers will have this element of the design pretty much sorted and tested, and would generally err on the safer (more trail) side of the line.

So I would be inclined to check a few things first, all of which will be a lot cheaper than a new pair of forks.
a). Check the headset bearings, worn bearings are probably the main cause of twitchy unpredictable steering, has this issue always been there? or has is recently started and gradually got worse? if this is the case I would be checking the following.
b). Check the tyre sizes are the same size, a smaller tyre on the front would reduce the trail. You could fit larger tyres.
c). Has the bike ever been involved in a crash? could it be out of track?
d). Your position being sorted could help things, it would be worth getting this looked at for a number of reasons, but I think the problems you have re. the steering are more likely to be caused by a). b). or c). above.

www.hewittcycles.co.uk

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Spangly Shiny replied to Paul Hewitt | 10 years ago
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Thanks for the in depth reply Paul. The bike has never been stuffed, I have had it from new. I had a bike fit done at Race Scene, so the basics are all pretty well sorted as regards positioning.
The bike has always been twitchy at speed, to such an extent that speeds in excess of 35MPH are cheek clench inducing. It is very susceptible to side winds and even in still conditions will wander alarmingly at speed.
Yesterday I fitted some new tyres going from 23 to 25, giving me an extra 1mm of trail and today, although blustery it felt a touch more stable.
There might be something in looking at the headset though, cheaper than a new fork and worth eliminating first.

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mrkeith119 | 10 years ago
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A few of my friends who have complained about not being able to ride no handed on road bikes, but can on other bikes, have had their saddle set to high. This might not be your problem but it's worth checking as it's cheaper than new forks.

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