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 Post subject: 80mm stem on a TT bike
PostPosted: Mon May 28, 2012 9:07 am 
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Joined: Sun Jan 27, 2008 10:20 pm
Posts: 65
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?


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PostPosted: Mon May 28, 2012 9:12 am 
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Location: Melbourne, Australia
It would do the opposite, it would make it react slower.

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PostPosted: Mon May 28, 2012 9:51 am 
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Posts: 43
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.


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PostPosted: Mon May 28, 2012 9:54 am 
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Location: UK/Singapore
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. :roll:


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PostPosted: Mon May 28, 2012 9:57 am 
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Joined: Wed Jun 22, 2011 9:39 pm
Posts: 147
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!


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PostPosted: Mon May 28, 2012 10:07 am 
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Location: Bay Area, CA
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?


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PostPosted: Mon May 28, 2012 10:10 am 
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Location: Melbourne, Australia
For some reason I read that as longer... My bad LOL :lol:

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PostPosted: Mon May 28, 2012 11:02 am 
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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 :noidea:


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PostPosted: Mon May 28, 2012 1:22 pm 
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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.


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PostPosted: Mon May 28, 2012 9:51 pm 
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Moving the pads backwards (if possible) would have the same effect on handling as using a shorter stem.


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PostPosted: Mon May 28, 2012 10:31 pm 
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Oval make 26.0 stems which are quite nice


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PostPosted: Mon May 28, 2012 11:04 pm 
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Location: Cambridge, New Zealand
Jamiemcp wrote:
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.


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.

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PostPosted: Tue May 29, 2012 11:34 am 
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Location: Sydney, Australia
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.

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PostPosted: Tue May 29, 2012 12:21 pm 
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kman wrote:
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.


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.

Image

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


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PostPosted: Tue May 29, 2012 10:18 pm 
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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.

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