Frame size, new bike fitting, questions

[Sjoerd]

I appreciate the comments, but my only question is: how can two bikes, with the same HT/ST angles, and a difference of 11 mm in top tube length, have nearly identical reach numbers?

[Fiery]

Stack is not independent from reach. Reach is measured from a fixed vertical line bisecting the bottom bracket to the center of the headtube; since the headtube is angled back toward the line you're measuring from, the higher the stack, the shorter the measured reach will be, with everything else being the same. Using trigonometry, we can calculate approximately how much the reach changes with changes in stack or head tube length - with a head tube angle of 73 degrees, for every 10 mm increase of tube length, measured reach decreases by 3 mm.

In case of the Cervélo and the Rose, the difference in head tube lengths is 36 mm. If we use the above approximation to normalize the stack measurements, we get that the Rose would have a reach of around 384 mm if it had the same stack as the Cervélo - so the effective difference in reach is about what one would expect looking at the horizontal top tube lengths. There is error in the calculation because the seat tube and head tube angles of the two bikes are not the same, but this should still give you an idea of how stack and reach affect each other when comparing geometries.

[ultimobici]

Stack is not independent from reach. Reach is measured from a fixed vertical line bisecting the bottom bracket to the center of the headtube; since the headtube is angled back toward the line you're measuring from, the higher the stack, the shorter the measured reach will be, with everything else being the same. Using trigonometry, we can calculate approximately how much the reach changes with changes in stack or head tube length - with a head tube angle of 73 degrees, for every 10 mm increase of tube length, measured reach decreases by 3 mm.

In case of the Cervélo and the Rose, the difference in head tube lengths is 36 mm. If we use the above approximation to normalize the stack measurements, we get that the Rose would have a reach of around 384 mm if it had the same stack as the Cervélo - so the effective difference in reach is about what one would expect looking at the horizontal top tube lengths. There is error in the calculation because the seat tube and head tube angles of the two bikes are not the same, but this should still give you an idea of how stack and reach affect each other when comparing geometries.

Have you checked the correct sizes? The measurements are as follows

Top Tube 581 v 570
STA Both 73
HTA Both 73.5
Headtube Length 199 v 190

[stella-azzurra]

Because the slope of the top tube is different between the two.

BTW you posted the link to the R5 not the R3 but the geometries are the same.

[Sjoerd]

Thanks for pointing that out Stella, I'll fix the link.

@ultimobici: the head tube on the Rose bike is 180 mm.

I can see that a taller head tube contributes to a smaller reach. It's angled so more ht-length brings the top of the head tube closer to the seat tube. But I'm still having a bit of a hard time interpreting the numbers correctly.

[Fiery]

My mistake, I was looking at the size 57 instead of 59.

For the head angle of 73.5 and head tube length difference of 19 mm we get 5.6 mm of normalization needed for reach. So, it'd 389 vs. 396 if both bikes had the taller head tube.

[ultimobici]

Thanks for pointing that out Stella, I'll fix the link.

@ultimobici: the head tube on the Rose bike is 180 mm.

I can see that a taller head tube contributes to a smaller reach. It's angled so more ht-length brings the top of the head tube closer to the seat tube. But I'm still having a bit of a hard time interpreting the numbers correctly.

True. Even so, why are you considering a frame that size if the Cervelo is too big. Seems pointless to me. Get a proper fit without telling the fitter what you are considering. Then see if the Rose is suitable.

[audiojan]

Get a bike fit... the best investment you can make

[GT56]

how tall are you, inseam ?

[Sjoerd]

187 cm tall
91 / 92 cm inseam
I ride my saddle at 82 cm with 9,5 cm of setback
I have 10,5 cm of drop between saddle and bars, with a slammed stem and an extra thin headset-cover
The stem is 6 degrees angle and 120 mm long

So, long legs and short torso.

@ultimobici: the Cervelo is tall enough but feels too long. It's more a question of weight distribution than being unable to reach the bars. I'm quite light for my length, in the summer I go down to 60 kg and judging by feeling, most of that weight is on the rear wheel. A longer bike makes cornering harder for me. If you look at the size of the Rose, the bike looks shorter than the Cervelo with a relatively long seat tube, so I thought that it would fit me better. I need the height because of the saddle height, but I don't need the long top tube that ususally comes with it. I've had a 57 cm BMC Pro Machine (57,5 cm toptube, 18,7 cm headtube) which fit me much better than my current bike, but unfortunately I broke two frames and couldn't get a warranty replacement because they weren't being made anymore.

[wingguy]

What length stem and bar reach?

[Sjoerd]

12 cm stem, reach from the tip of the saddle to the middle of the stem (where it holds the steerer) is approximately 60,5 cm.

[edit] I ride with 3T Rotundo bars, reach is 83 mm [edit]

[ultimobici]

@ultimobici: the Cervelo is tall enough but feels too long. It's more a question of weight distribution than being unable to reach the bars. I'm quite light for my length, in the summer I go down to 60 kg and judging by feeling, most of that weight is on the rear wheel. A longer bike makes cornering harder for me. If you look at the size of the Rose, the bike looks shorter than the Cervelo with a relatively long seat tube, so I thought that it would fit me better. I need the height because of the saddle height, but I don't need the long top tube that ususally comes with it. I've had a 57 cm BMC Pro Machine (57,5 cm toptube, 18,7 cm headtube) which fit me much better than my current bike, but unfortunately I broke two frames and couldn't get a warranty replacement because they weren't being made anymore.

A frame no longer being in production shouldn't preclude the warranty being honoured. Or did you break the frame in a crash?

As for the weight distribution, that may be a product of the Cervelo's HT being longer, so perhaps you aren't as low as you used to be on the BMC.

Bottom line is I think you're coming at this from the wrong direction. Start with a clean slate and figure out your ideal position. Then see which frame will allow you to achieve that. All too often I've seen customers come in with a particular bike in mind, only to have to tell them that it isn't the optimum fit. Problem is their mind is already set on it.

Couple of years ago a friend wanted a Look 695 and I was in a position to do one at a killer price for him. On looking at the geometry of his then current bike and the 695 it looked like he fell between sizes. So I told him I'd only get it for him after he'd been professionally fitted. Lo and behold my fears were confirmed. The 695 was wrong for him. He didn't find anything the fitter's shop did appealing so they suggested a few alternatives they were able to source that would fit the bill. The one he went for was a C59, something he'd never considered. It fits him perfectly, and for the first time he reckons he is part of the bike rather than a passenger.

[5 8 5]

I've had a 57 cm BMC Pro Machine (57,5 cm toptube, 18,7 cm headtube) which fit me much better than my current bike

Ignore the Cervelo, use the BMC geo as the basis for comparison.

[Sjoerd]

A frame no longer being in production shouldn't preclude the warranty being honoured. Or did you break the frame in a crash?

The frame was warrantied, but the Pro Machine was no longer available and I wasn't very keen on the new BMC design, so the importer offered me a choice between the different brands he imports, hence the choice for the Cervelo. I've had the feeling I could've gotten away with a 56, but I can't change that now.

Bottom line is I think you're coming at this from the wrong direction. Start with a clean slate and figure out your ideal position. Then see which frame will allow you to achieve that. All too often I've seen customers come in with a particular bike in mind, only to have to tell them that it isn't the optimum fit. Problem is their mind is already set on it.

I understand what you're saying and I thank you for your concern

That is exactly the mistake I don't want to make. As I said in my opening post: I'm trying buy the bike that fits me best (and ticks a number of other boxes of course). I felt pretty confident I could make that call based on my current position, my position on the BMC and on previous bikes. That's why I've been comparing a lot of bikes and looking at geo-charts so much. I also had contact with a various number of members regarding their bikes. I just got confused when I started comparing stack and reach numbers, because I thought they'd give an exact comparison, no matter what. And that was a mistake, I learned through the replies I got.

Thanks all for taking the time to help me out!