Frame Geometry Characteristics, Rear Tire Jump/Slip

Discuss light weight issues concerning road bikes & parts.
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Joined: Sun Jan 11, 2009 2:43 am

by robd1000

Hey Guys, thanks to all the posters, so much good info out here on WW. I wanted to ask a question about frame geometry characteristics. More specifically what could cause one bike to have a rear wheel that jumps around/slips a lot more than another during out of the saddle efforts? Below is a Bike Comparator with both measurements entered.

The first bike a 2008 56 Cervelo SLC-SL (red) jumps around like crazy. The second (green), a 2008 56 Cannondale six13, is much more stable.

The first thing I notice in the Comparator is the top tube and seat stay angles, much much different.

Maybe it’s not the geometry (stiffness?, weight?) or frame at all (bars?).
Any thoughts would be appreciated. ... 3%2056.xls" onclick=";return false;" onclick=";return false;

by Weenie

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by kulivontot

Is guess it has to do with technique more than anything. A stiffer frame of course will do that more.

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by XCProMD

Same wheels? Most probably stiffness but it's hard to tell. A bicycle is an hyperstatic system, thus any variation of the characteristics of any component varies the forces other componets are submitted to.

A house with solid foundations, for instance is an isostatic system, hence is easier to trace what does what.

That's why bicycle manufacturers can fill thosand of catalogues with marketing bullshit and still (almost) everything is possible. :)

Can you elaborate on yur description?

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by bricky21

The longer chainstays on the Cannondale are giving you more traction. High cadence(low torque) out of saddle sprints will help your technique, so you can keep the rear wheel down on the Cervelo.

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by boots2000

Chainstay length, bb height, and amount of weight on front end.

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by Qman

I`ve noticed the same thing on my old R900 Cannondale. Uphill out-of-saddle sprinting causes the rear wheel to skip off the ground. On my SuperSix HM it doesn`t skip. It isn`t wheel related since I`ve run the same wheels on both.
I`m thinking it is related to weight distribution while sprinting, which would be due to frame geometry as well as bar positioning. It`s a combination of putting your weight on the handlebars while pulling up on one of the pedals.

While writing this I just noticed that boots2000 has just said essentially the same thing.

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legs 11
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by legs 11

I used to be big on the old CAAD 4 and 5 Dales. I raced on them for quite a few years.
It was a characteristic of that frame to have lots and lots of 'tail wag'. My team mates used to laugh and joke about it and we all noticed the same effect even with different sizes and fits etc.
They would step out and bounce around all over the place, but always came back and never caused anything more than a few raised eyebrows in the sprints.
We always put it down to the huge amount of lateral stiffness in the rear triangle (them chainstays were by far the meatiest around at the time)
Interestingly, I ride a BB30 CAAD 9 now, and don't notice it to the same extent, It seems much more stable. This would indicate it isn't probably material related, or indeed the chainstays as the CAAD 9 has enormous tubes and bags of stiffness. Probably just a quirk of the geometry.
Could be worth you pulling up a geometry chart and overlaying them to look for similarities if it really bothers you.
Alternatively, just keep riding it and get used to it, alter your technique a little bit to compensate.
What I'm trying to say is......some frames just do that, not much you can do but learn to live with it. :thumbup:
Pedalling Law Student.

by Weenie

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