Frame size, new bike fitting, questions

Discuss light weight issues concerning road bikes & parts.
Tubby531
Posts: 3
Joined: Fri Nov 04, 2011 9:22 pm

by Tubby531

Recently bought a Scott Foil and I'm starting the build. It's a size small and has a zero setback seat post. To replicate my position on my previous road bikes I need a 25mm setback seat post. As replacement seatposts with 25mm setback don't seem to be available I have a bike fit question.

With the zero setback post my saddle is as far back as it can go, and is forwards of the range marked on the saddle rail, easily by 25mm. If i push the saddle forwards 2cm this is within the centre of the rails and if I now fit a 2cm longer stem I've recreated my position on my previous road bike, albeit I've reduced the saddle set back to bottom bracket measurement by 2cm.

Will this reduction in setback and increase in stem length have any effect on handling, ride etc?

by Weenie


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DMF
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Location: Sweden

by DMF

It will change the way your knees are aligned in relation to the pedals and how your hip is angled. It's not just a question about reach to the bars. You could potentially suffer serious injury, and loose power with the change in hip angle. Assuming your previous position was the best for you ofcourse...
Last edited by DMF on Sun Jan 27, 2013 10:10 pm, edited 1 time in total.

em3
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by em3

Tubby531 wrote: If i push the saddle forwards 2cm this is within the centre of the rails and if I now fit a 2cm longer stem I've recreated my position on my previous road bike, albeit I've reduced the saddle set back to bottom bracket measurement by 2cm.


Actually, you haven't recreated your position, as you have significantly changed your setback, compared to your original position. Your saddle setback is in the most important touchpoint of your fit as it determines the fore/aft distance from the BB.. The workaround you suggest is a poor solution. Contact Scott and get the correct setback seatpost for your fit...apparently Ritchey is producing the Foil seatposts in three offset options. See the review here which explains such:
http://www.cyclingtips.com.au/2012/05/s ... il-review/" onclick="window.open(this.href);return false;
______________

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fa63
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Location: Atlanta, GA, US

by fa63

^ Agreed. You never want to mess with saddle setback to compensate for reach.

nathanong87
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by nathanong87

em3 wrote:Actually, you haven't recreated your position, as you have significantly changed your setback, compared to your original position.


x a million.

Tubby531
Posts: 3
Joined: Fri Nov 04, 2011 9:22 pm

by Tubby531

Ok - valuable confirmation. Your feedback is appreciated.

The challenge now is to locate a 25mm setback Ritchey pro Foil aero seat post..

system787
Posts: 138
Joined: Fri Mar 16, 2012 1:49 pm
Location: LA/OC, California

by system787

I just acquired a Scott Foil 20 in size 56, and the seatpost is a 25mm offset one. I need a zero offset post, so would you be willing to trade?

neeb
Posts: 370
Joined: Tue Jan 27, 2009 8:19 pm

by neeb

I'm actually considering getting a Foil frameset largely because it is one of the few high-end bikes with an integrated ISP that /doesn't/ have a large setback on the seatpost in a size medium, and my fit needs a relatively small BB/saddle distance.

The current trend for ISPs and dedicated seatposts is actually a big problem for getting correct fit, as most often there is only one setback option available. Also, most manufacturers won't actually tell you what the seatpost setback is in the geometry charts.. If the seatpost is supplied with the frame and there are no replacements available, then the setback is an absolutely critical part of the geometry and it is unforgivable if manufacturers won't supply this information.

The Italian brands seem to be the worst. Despite several emails to Wilier and various distributors and retailers, no-one has actually been able to tell me what the setback is on the new Cento Uno SR seatmast and if any replacements with different setbacks are available. The Bianchi Oltre in a size 55 has a 73.5 degree seat tube angle and (so I eventually found out) a 25mm seatback on the dedicated seatpost, which means I could effectively never ride one. Good that Scott at least make a range of different setbacks on their seatmasts (if they won't actually supply you with one I would make a BIG fuss about it), I think this has to become the required standard for frames with ISPs etc.

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djconnel
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by djconnel

First, everyone's assuming the last bike had the best position.

It seems to me if you bring the saddle forward if you want to keep the hip angle the same you don't want to just push the bars out. Rather you want to rotate your hips and bars about the bottom bracket axis. That means raising the saddle slightly to preserve saddle-to-BB, moving your bars out, but then dropping them slightly.

I did some calculations here.

Basically you want to povot this diagram about the bottom bracket:
Image

The net result:

ΔyS = ‒Δx (xS / yS)
ΔyH = ‒Δx xH / yS.
ΔxH = Δx yH / yS,

Check the link for details.

But this is just simple geometry. Honestly I know nothing about proper fitting.

system787
Posts: 138
Joined: Fri Mar 16, 2012 1:49 pm
Location: LA/OC, California

by system787

I don't think the Foil comes in an ISP, but rather uses a proprietary seatpost made by Ritchey. They come in different setback options, but afaik, all frames 54cm and under come with a zero setback post, and 56 and above comes in setback. Which sucks for me, because I have short femurs and have my saddle slammed almost all the way forward on a zero setback 73.5 STA

neeb
Posts: 370
Joined: Tue Jan 27, 2009 8:19 pm

by neeb

djconnel wrote:First, everyone's assuming the last bike had the best position.

It seems to me if you bring the saddle forward if you want to keep the hip angle the same you don't want to just push the bars out. Rather you want to rotate your hips and bars about the bottom bracket axis. That means raising the saddle slightly to preserve saddle-to-BB, moving your bars out, but then dropping them slightly.

I did some calculations here.

Basically you want to povot this diagram about the bottom bracket:

The net result:

ΔyS = ‒Δx (xS / yS)
ΔyH = ‒Δx xH / yS.
ΔxH = Δx yH / yS,

Check the link for details.

But this is just simple geometry. Honestly I know nothing about proper fitting.

Yes, you are right - if you bring the saddle forward and maintain the same saddle height measured to the BB, you need to raise the saddle a little, so you are indeed effectively rotating around the BB.

My last bike fit moved me forward a bit amongst other things, and it seems to work quite well for me. As mentioned above, it changes the angle the hips make with the torso when pedalling. The fitter seemed to think this would give me more power (it certainly hasn't reduced it), but it's also commonly said that sitting further back can give more power (especially for climbing) by activating the glutes more.

I just sit a bit further back on the rear edge of the saddle on the rare occasions I feel I need to, but you will often see people do the opposite, i.e. have the saddle set further back and then move forward to be "on the rivet" when they want more power on the flat. Me, I find it more comfortable very occasionally sitting further back on the saddle than frequently sitting on the nose without my sitbones supported...

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Sjoerd
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Location: Les Pays Bas

by Sjoerd

I am in the process of replacing my Cervelo R3 (size 58) with something new. Because I've owned a number of bikes that have been too big for me, including the Cervelo, I want to make sure I get the size completely right. I will not buy the brand I like most but the brand that will fit me best.

I was comparing the size of my current bike with a Rose Xeon RS 5000 (size 59). Both bikes have the same headtube-angles and seat-tube angles. The Cervelo has a (horizontal) top tube of 581 mm and the Rose has a (horizontal) top tube of 570 mm. I got confused when I was comparing reach-figures. While the Rose looks a lot shorter on paper, the reach is 394 as opposed to 396 on the Cervelo. Almost no difference! The headtubes on the bikes aren't the same length and altough I suspect that could make some sort of a difference I cannot get it straight in my head.

For the record...I was and still am terrible at math, but I guess you have already noticed that :oops:

Guys, please enlighten me! What am I overlooking here?

Here's a link to the Cervelo R3 geometry-chart:

http://www.cervelo.com/media/docs/R3-f4f2ba9b-e376-4b93-9a11-9ce57652f024-0.pdf

Rose's geo-chart can be found here:

http://farm9.staticflickr.com/8372/8585734728_a235892c12_k.jpg

Any help will be greatly appreciated!
Last edited by Sjoerd on Sun Mar 24, 2013 2:09 pm, edited 1 time in total.

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ultimobici
in the industry
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by ultimobici

If the Cervelo is, as you admit, too big, why base your sizing question on it?

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Sjoerd
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by Sjoerd

Because I need something as a reference point.

Rodrego Hernandez
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by Rodrego Hernandez

Get a bike fit. In a proper shop. By someone who knows what they are doing.

by Weenie


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