I just had a bike fit done, and they have recomended moving my pads back 20mm which would mean useing a 80mm stem, but will this make the handeling a bit twitchy?

It would do the opposite, it would make it react slower.

_________________
Slam your stem.

In my experience, changing stem lengths noticeably changes the handling--for about five kilometers, and then I adjust and everything feels normal. Perhaps at extremes it would become a persistent negative factor, but I wouldn't think 80 mm counts as extreme.

yes it may make it a bit more twitchy but 80mm is not that rare on TT bikes so you may adapt to it after a while.
no way it makes it handle slower.

shorter stem = twitchier

Its all about angular velocity. the shorter the stem the less distance required to change the steering angle by one (or many degrees) the longer the stem the greater the distance. Thus, more distance = slower and potentially more stable handling on the aerobars.

Can you not off-set the pads backwards any more??

80mm might be too twitchy, but try it and see!

Yeah, an 80mm stem isn't really uncommon on TT bikes. Combined with the longer extensions (compared to the reach of road bars+levers), it's not really a drastic change in overall length (about 5-6% in overall cockpit length?). Also consider that the trail is usually quite a bit higher on a TT bike, which will affect the handling in a way that's different from what you'd expect when going to a very short stem on a road bike.

Does the fitter want the pads back 20mm, or do they want the tips of the bars back 20mm, or both?

For some reason I read that as longer... My bad LOL

_________________
Slam your stem.

thanks for the replies, the fitter wants the back of the pads back 20mm. I use Zipp Vukar bars which dont have any option to move the pads back, just wider.

now all I have to do is find a 80mm/ 26mm diameter stem

Might be worth looking at MTB stems? not a lot of roadie 26mm stems out there anymore! Deda Logos are one of the few i can think of at the moment.

Moving the pads backwards (if possible) would have the same effect on handling as using a shorter stem.

Oval make 26.0 stems which are quite nice

putting the pads on backwards gets quite a bit more setback on those bars. My notes from the last time I did a set up with them don't specify whether it's enough to achieve what you need but from what I recall it should be close.

_________________
http://www.speedtheory.co.nz

Technically it might feel more twitchy but its nothing to do with the angular velocity. Its to do with a shorter stem (radius) giving you a larger angle of movement for the same linear distance moved at the tip. If that makes sense.
I doubt it will take long to get used to and i doubt it will be that bad anyway. In my experience a shorter stem makes it a little easier to control your bars because your arms are tucked in a bit tighter. This was explained to me by a world kilo champion when i was a teenager and its true.

There should be plenty of 26mm stems floating around, look second hand if you need to.

_________________
You can't be a real country unless you have a beer and an airline. It helps if you have some kind of a football team, or some nuclear weapons, but at the very least you need a beer.
-- Frank Zappa

Isn't Angular velocity describing the responsiveness/ twitchyness? as for a given lateral or steering force, the rate of change of angle will be dependant on the length of the stem. As the vector of the steering force will be large in comparison to the the other lengths in the case of a shorter stem and vice versa for a longer one.



I may be wrong, its a while since physics at school.

well I guess we are both being a bit technical and picky but I thought you angular velocity point was based on steering by pushing the handlebars, which doesn't really happen unless you're doing a u-turn or something like on an out-and-back course. When steering at any real speed, you're steering by leaning so the stem length makes even less difference.

Physics arguements aside, I still think it will be absolutely fine.

_________________
You can't be a real country unless you have a beer and an airline. It helps if you have some kind of a football team, or some nuclear weapons, but at the very least you need a beer.
-- Frank Zappa