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PostPosted: Thu Nov 09, 2017 3:37 am 
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Posts: 477
Location: RVA,USA
I'm about 183cm and have a 56 caad12 with a 110 stem. It's about 40mm behind my axle. One of the guys in my shop said he wants me to try a 58 one day instead of a 56, I'm thinking about it.

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Posted: Thu Nov 09, 2017 3:37 am 


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PostPosted: Thu Nov 09, 2017 5:50 am 
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Joined: Fri Mar 18, 2005 4:12 pm
Posts: 2928
Location: eh?
Hexsense wrote:
youngs_modulus wrote:
Most bike road racers (including myself) will dip a knee to the inside just like motorcycle racers do. But we don't have any mechanical bits that we'll grind if we don't, and motorcycle racers do. Yet we do it anyway. I mean, I know it's useless, but that's what I learned and I feel slightly more secure in corners when I do it.

Interesting, i tend to do opposite. I push the bike lean low to the corner but keep my body relatively straight up. It effectively move inside knee closer toward top tube and outside knee away from the frame as the result. Not sure if it affect anything it just my instinct to do it this way.


I've been waiting for someone to make this point. Hanging the knee to the inside to keep the bike more upright is all wrong IMO. Doing so interferes with the riders ability to control the radius of the arc and manage pressure on the outside foot. And it's hard to do without a lot of weight on the saddle which makes everything about cornering worse. I have watched many elite cyclists hang the knee and thought that none of them had ever skied. And then I watched Cancellara descend. That looked better. Still a bit of knee but also angulation and counter-rotation, that makes more sense to me. It's not like we are on GP motorbikes that weigh hundreds of pounds that want to stand up and go straight so we have to hang off the side just to keep them leaned over.

Sorry for the total thread hi-jack. Couldn't resist.

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PostPosted: Thu Nov 09, 2017 9:06 am 
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Joined: Mon Jul 24, 2017 12:02 pm
Posts: 529
rothwem wrote:
I measured my old bar position, and I was ~8cm behind the front wheel axle on that bike. My new position (math is fun, right?) is 11cm behind. It seems excessive, but maybe it’s normal?

Anyone here on a ~56-58cm bike? How long is your stem? And for everyone else, how far behind your front axle is your stem clamp?

Your stats are pretty normal.


Slammed wrote:
How far is my stem behind my axle? It's not. My stem's a good inch or two ahead of my front hub. I also have freakishly long arms and run a 15cm stem on a medium frame though.

Photo please. The math does not add up. A slammed 150mm -17deg stem would be in the ballpark of 40mm behind the front axle of the typical medium-sized race bike.


evan326 wrote:
I'm about 183cm and have a 56 caad12 with a 110 stem. It's about 40mm behind my axle. One of the guys in my shop said he wants me to try a 58 one day instead of a 56, I'm thinking about it.

Doing the math based on listed geometry, the center of my bars is approximately 105mm behind my front axle. This is a 54cm Emonda SLR Disc with 110mm/-6 degree stem and 24mm in spacers/bearing-cap. However using pixel-mm conversion, it seems like it's closer to 96mm. I tried using a tape measure and plum bob to measure... about 98.5mm from the center of the tops to the center of the front axle. Not sure how you got 40mm unless you're slammed and have a -17deg stem, and that still wouldn't account for the difference.

Image


Last edited by TobinHatesYou on Thu Nov 09, 2017 10:16 am, edited 6 times in total.

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PostPosted: Thu Nov 09, 2017 9:47 am 
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Joined: Mon Jul 24, 2017 12:02 pm
Posts: 529
Yeah, uh, looking at your CAAD12. That's not 40mm...not even close. That looks like 120-125mm to me.

He wants you on a 58cm frame because you have a spacer stack of at least 4cm on the 56.


Last edited by TobinHatesYou on Thu Nov 09, 2017 2:01 pm, edited 1 time in total.

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PostPosted: Thu Nov 09, 2017 12:33 pm 
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Joined: Fri Nov 03, 2017 8:44 am
Posts: 9
Hi,

I'm 1m74 and I ride a BMC Roadmachine size 51. These are my bike's setup measurements:

Image

As you can see, the center of my bars is 126,2 mm behind my front axle. Hope this helps!


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PostPosted: Thu Nov 09, 2017 1:43 pm 
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Joined: Wed Jul 09, 2014 4:45 pm
Posts: 40
youngs_modulus wrote:
I also have a motorcycle background; I used to pore over Keith Code's book. I feel compelled to suggest that your position over the front wheel doesn't matter as much as you suspect. It matters on motorcycles partly because they have way more suspension compliance than a 700x23 (or 700x42) bike tire. Bikes weigh so little that turn-in is effectively instant, and you don't need to weight the front wheel any extra to make it happen faster. If you want to do that, I'd recommend trail-braking a bit with your rear brake.

The change in CG based on your new fit is tiny; it doesn't change how the front wheel is weighted to any significant degree. The fit is just different and you seem to feel the that difference more when cornering. It's not a big deal.

Bernard Hinault used to promote a descending technique that involved making many "micro-turns" through sweeping corners. It was silly, but Hinault was a good descender, and he told a story about it that made sense to him.

Most bike road racers (including myself) will dip a knee to the inside just like motorcycle racers do. But we don't have any mechanical bits that we'll grind if we don't, and motorcycle racers do. Yet we do it anyway. I mean, I know it's useless, but that's what I learned and I feel slightly more secure in corners when I do it. I guess my point is that shifting your saddle back 10mm is not going to have a measurable effect on your cornering ability. But you can kiss your brifter all you want if it makes you feel more confident.


Hey, its great to hear from someone else that really learned to corner on a motorcycle. As far as the CG shift is concerned though, wouldn't the effect of moving you body be amplified on a bicycle since its so light? My R6 was ~400 pounds, a 200 pound rider would be 33% of the system weight, while a 20 pound bicycle and a 200 pound rider would have 91% percent of the system weight be due to the rider.


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PostPosted: Thu Nov 09, 2017 1:51 pm 
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Joined: Tue Apr 08, 2014 6:43 pm
Posts: 118
I must be missing something. Your new stem is 2cm shorter than the old, and the fitter moved your saddle back by 10cm? So for a guy with supposedly short arms he's stretched you out by 8cm. If you are worried about the turning feel then why not go back to the old stem and only move the saddle back 8cm? Pretty sure that would still be more muscle recruitment than the old position.

Does no-one else find the 10cm saddle change odd?

rothwem wrote:
kidrob wrote:
rothwem wrote:
Its a CAAD10, 58cm. The old stem is a 120, -6°, and the new stem is a 100, 0°.

What changed other than that?


He also scooted the seat back ~10 cm to allow for more recruitment of my posterior chain muscles while riding

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PostPosted: Thu Nov 09, 2017 2:04 pm 
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Joined: Mon Sep 25, 2017 1:36 pm
Posts: 23
Ettore wrote:
Hi,

I'm 1m74 and I ride a BMC Roadmachine size 51. These are my bike's setup measurements:

Image

As you can see, the center of my bars is 126,2 mm behind my front axle. Hope this helps!


lol someone isn't messing around here! What application did you use to make this?


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PostPosted: Thu Nov 09, 2017 2:11 pm 
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Joined: Wed Sep 20, 2006 1:03 am
Posts: 507
Location: Madison, WI USA
That’s almost certainly Solidworks. A sketch like that is pretty straightforward.


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PostPosted: Thu Nov 09, 2017 2:25 pm 
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Joined: Wed Jul 09, 2014 4:45 pm
Posts: 40
Rudi wrote:
I must be missing something. Your new stem is 2cm shorter than the old, and the fitter moved your saddle back by 10cm? So for a guy with supposedly short arms he's stretched you out by 8cm. If you are worried about the turning feel then why not go back to the old stem and only move the saddle back 8cm? Pretty sure that would still be more muscle recruitment than the old position.

Does no-one else find the 10cm saddle change odd?

rothwem wrote:
kidrob wrote:
rothwem wrote:
Its a CAAD10, 58cm. The old stem is a 120, -6°, and the new stem is a 100, 0°.

What changed other than that?


He also scooted the seat back ~10 cm to allow for more recruitment of my posterior chain muscles while riding


Ha, oops, it was 10mm, not cm.


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PostPosted: Thu Nov 09, 2017 2:36 pm 
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Joined: Wed Mar 17, 2004 1:38 pm
Posts: 1738
DMF wrote:
Yes, you're numbers may be correct as far as contact points go, but if that leaves you with a bike that doesn't fit with regards to your weight distribution over the wheels and your centre of gravity with regards to the wheel axles (not to talk about how HTA and trail figures go into this), then maybe you've just stumbled into one of the many reasons why bike fitting purely on a trainer is a poor idea.



Indeed. Coming up with an optimum fit for a rider does not automatically lead to a well handling bicycle based on that fit. It takes a different skill set to design a well riding bike and, in my experience at least, very few "fitters" have that skill set.


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PostPosted: Thu Nov 09, 2017 2:50 pm 
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Joined: Wed Jul 09, 2014 4:45 pm
Posts: 40
BdaGhisallo wrote:
DMF wrote:
Yes, you're numbers may be correct as far as contact points go, but if that leaves you with a bike that doesn't fit with regards to your weight distribution over the wheels and your centre of gravity with regards to the wheel axles (not to talk about how HTA and trail figures go into this), then maybe you've just stumbled into one of the many reasons why bike fitting purely on a trainer is a poor idea.



Indeed. Coming up with an optimum fit for a rider does not automatically lead to a well handling bicycle based on that fit. It takes a different skill set to design a well riding bike and, in my experience at least, very few "fitters" have that skill set.


Well, in defense of the fitter, bicycles are designed to fit the middle of a bell curve of people. Ideally, the fitter would find a position, then a bicycle could be constructed around those dimensions.

In my case though, I didn’t even pick my bike, it was a warranty replacement for a CAAD8 from 2006 that was a team deal from my college cycling club. If I had been fit for a bicycle earlier, I probably would’ve gone for something different, although maybe I wouldn’t have. It’s tough to say.


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PostPosted: Thu Nov 09, 2017 3:00 pm 
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Joined: Wed Mar 17, 2004 1:38 pm
Posts: 1738
rothwem wrote:
BdaGhisallo wrote:
DMF wrote:
Yes, you're numbers may be correct as far as contact points go, but if that leaves you with a bike that doesn't fit with regards to your weight distribution over the wheels and your centre of gravity with regards to the wheel axles (not to talk about how HTA and trail figures go into this), then maybe you've just stumbled into one of the many reasons why bike fitting purely on a trainer is a poor idea.



Indeed. Coming up with an optimum fit for a rider does not automatically lead to a well handling bicycle based on that fit. It takes a different skill set to design a well riding bike and, in my experience at least, very few "fitters" have that skill set.


Well, in defense of the fitter, bicycles are designed to fit the middle of a bell curve of people. Ideally, the fitter would find a position, then a bicycle could be constructed around those dimensions.

In my case though, I didn’t even pick my bike, it was a warranty replacement for a CAAD8 from 2006 that was a team deal from my college cycling club. If I had been fit for a bicycle earlier, I probably would’ve gone for something different, although maybe I wouldn’t have. It’s tough to say.


I once had a fit from the guy who set up the fitting school for a very prominent US builder of the 1990s and 2000s. He somehow came up with a fit that was 4.5 cm shorter in reach than anything I ever comfortably rode before then or since and simply copied over the angles from the bike I was riding then onto a new "bike design". Needless to say the result would have been a sh*t show if my builder hadn't taken one look at the "bike design" and refused to build it as it was saying the bike would be virtually un-rideable and unsafe. My confidence in the bike designing acumen of fitters has been slight ever since. I find that builders know their stuff though since they design and build the actual frames and learn quite quickly what works and doesn't, and them having to be on the financial line for bad designs leads them to sort things out or disappear from the market.


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PostPosted: Tue Nov 14, 2017 8:50 pm 
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Joined: Wed Sep 14, 2016 8:51 pm
Posts: 477
Location: RVA,USA
TobinHatesYou wrote:
Yeah, uh, looking at your CAAD12. That's not 40mm...not even close. That looks like 120-125mm to me.

He wants you on a 58cm frame because you have a spacer stack of at least 4cm on the 56.

The photos of my bike aren't current, but yea I'm still working my way down the spacer stack. I'm down to 45mm, so good guess.
In those pictures I had 55mm of spacers, 100mm stem, and 42mm bars.
Unless I'm completely misunderstanding what to measure, I'm going from the center of my bars at the stem with a plumb, and measuring from the front axle to that. If I had a third hand or friends I'd take a photo.

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PostPosted: Wed Nov 15, 2017 2:07 am 
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Joined: Mon Jul 24, 2017 12:02 pm
Posts: 529
evan326 wrote:
Unless I'm completely misunderstanding what to measure, I'm going from the center of my bars at the stem with a plumb, and measuring from the front axle to that. If I had a third hand or friends I'd take a photo.


I'd have to see how you're setting everything up. How are you leveling out your bike? I use two Feedback Sports bike stands for example.

Anyway, I did the math on my H1 Emonda with a shorter front-center and a longer 110mm stem...and the distance behind the stem was 105mm based on advertised geometry.

Ettore also did the math, with proof, on his bike geometry and his bar is 126mm behind the front axle (thanks to a slack head tube angle and bigger rake than normal)

So do you trust the math or do you trust your eyeballs?


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Posted: Wed Nov 15, 2017 2:07 am 


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