TT length affect on handling.

comsense

1. On my road bikes I get a sensation that the front is very light - hardly any feedback from the front wheel. This translates into nervous cornering and very slow descending.
I have had several episodes of front shimmy and one or two of these nearly put me off cycling completely ( I have read extensively about shimmy and believe in my case its that the front is too light.)
2. with a cycling inseam of 90.7 CM I always landed up with a long TT - shortest I have is 56.5CM BUT I once had bikefitting.com fit that said I should be on 54.6 - despite that I need my saddle 79cm from BB. My trunk is VERY short relative to my heigth

My question is: Does anyone believe that a 2 cm too long TT could affect handling balance in such a significant manner or am I grasping at straws?

geoff_ss

I think you're grasping at straws. Just think how long a tandem top-tube is and ours handles perfectly well at speed.

Steering of a conditionally stable vehicle like a bicycle is something of a black art but the main factors seem to be steering head angle and fork rake which both affect the trail. A lot of trail makes the the bike feel very stable and less trail makes it feel lighter, quicker and less stable.

We bought a secondhand Jack Taylor tandem some years ago which had very light steering. It was quite a contrast to the old Sun Wasp it replaced but it was a bit too light for my taste. I got Mercian's to straighten the forks a bit to reduce the rake and increase the trail and it transformed the handling (for the better)

Geoff

neeb

Just an idea - are you compensating for the perceived long top tube by using a very short stem? That could affect front end handling...

comsense

thanks for the replies. Geoff - I see your point but would have thought that tandems were designed to place weight proportionatly front/rear in the same way a bike is. My problem ( I think ) is there is too much weight too high and far back - If I had a shorter top tube I wonder would it help?
Neeb - the stem is 110mm and I have several stems I've tried.

neeb

If anything I thought that having too long a reach tended to put more weight on the front of the bike, as it forces you to shift your weight forward more to reach the bars. Lowering the bar height in relation to the saddle certainly shifts your weight more towards the front end of the bike.

despite that I need my saddle 79cm from BB

This is saddle height obviously, i.e. vertical distance, but what is the horizontal distance of the tip of the saddle from the BB (setback)? If you have an extreme amount of saddle setback (and if you have long legs in relation to torso you would need it to be more than average) could this contribute to what you are experiencing?

wildmoustache

I think you'd be better with a 54.6 tt (or thereabouts) and a 13cm to achieve a similar overall TT + stem length (there is also a head tube difference, so you have compensate with spacers).

In anycase I once had a 56.5 frame with an 11cm stem and switched it for a 54.8 frame with a 13cm stem and the handling was greatly improved ... slower and more stable. they were similar specialized frames so it wasn't down to the frame construction i don't think.

dennisn

You say road "bikes" so I've got to assume that all of them have this front end shimmy
most of the time. I sure it's possible to have your body so far back over the rear wheel that the front end gets really light. How else do you explain "wheelies"? If this is the case
you are almost surely in the wrong position or have bikes that are too small for you. Key
word being "if". Will 2cm make a difference? I doubt it. Discounting frame, fork, and wheel
problems you need to take a good look at your position on the bike. Better yet have
someone else take a look.

Dennis Noward

wildmoustache

dennisn wrote:

You say road "bikes" so I've got to assume that all of them have this front end shimmy
most of the time. I sure it's possible to have your body so far back over the rear wheel that the front end gets really light. How else do you explain "wheelies"? If this is the case
you are almost surely in the wrong position or have bikes that are too small for you. Key
word being "if". Will 2cm make a difference? I doubt it. Discounting frame, fork, and wheel
problems you need to take a good look at your position on the bike. Better yet have
someone else take a look.

Dennis Noward

dennis, i don't disagree with much of your comment, but 2cm can make a really big difference on a road bike. even going up or down 1cm in stem length can make a very noticeable difference

dennisn

wildmoustache wrote:

dennisn wrote:

You say road "bikes" so I've got to assume that all of them have this front end shimmy
most of the time. I sure it's possible to have your body so far back over the rear wheel that the front end gets really light. How else do you explain "wheelies"? If this is the case
you are almost surely in the wrong position or have bikes that are too small for you. Key
word being "if". Will 2cm make a difference? I doubt it. Discounting frame, fork, and wheel
problems you need to take a good look at your position on the bike. Better yet have
someone else take a look.

Dennis Noward

dennis, i don't disagree with much of your comment, but 2cm can make a really big difference on a road bike. even going up or down 1cm in stem length can make a very noticeable difference

You're right. 2 cm will make a difference. It just seems that he has other "issues" that
need to be addressed and the 2 cm is IMO not one of them.

Dennis Noward

comsense

Neeb - thanks, your comments got me thinking and I decided to ignore my setback temporarily and moved the saddle forward by 1.5cm. This appears to help greatly and the thing that surprised me is that my knee is just slightly behind the spindle - so it looks like I have been riding with the saddle too far back.

wildmoustache wrote:

I think you'd be better with a 54.6 tt (or thereabouts) and a 13cm to achieve a similar overall TT + stem length (there is also a head tube difference, so you have compensate with spacers).
In anycase I once had a 56.5 frame with an 11cm stem and switched it for a 54.8 frame with a 13cm stem and the handling was greatly improved ... slower and more stable. they were similar specialized frames so it wasn't down to the frame construction i don't think.

Thanks for this wildmoustache - its great to get this kind of relevant experience and I am seriously considering a smaller frame. I did get a loan of a bike for a day and it really felt like I was sitting on it instead of perched on a great heigth.

Dennisn - I agree about having someone qualified look at this. As I wrote earlier I did have a bikefit done but I don't trust the results. I also used this program http://www.competitivecyclist.com/za/CC ... ATOR_INTRO
The results from both of these vary wildly. I am now thinking that the problem is that my setback was too great (was 86mm) Reduced now to 72mm It definitely feels a lot better.
THANKS A MILLION FOR ALL YOUR HELP.