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My brain hurts...a geometry conundrum

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My brain hurts...a geometry conundrum

Old 06-25-20, 05:39 PM
  #1  
melikebikey35
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My brain hurts...a geometry conundrum

For the life of me, I cannot figure out why the fit on my two bikes do not match.

The two bikes in question are:
Cervelo R5: Stack: 548mm, plus 10mm top cap, 10mm spacer, 100mm -6 stem. The frame reach is 380mm. HT angle is 73
Orbea Terra: Stack: 560mm, plus 15mm top cap, 0mm spacer, 100mm -12 stem. The frame reach is 378mm. HT angle is 71

I'm using the same bars (77mm reach), saddle, saddle height, and saddle setback (laser measured) on both bikes.

Based on stem comparison tools (and mental math) the two bike should be, near-as-makes-no-difference, the same. Yet, my Terra has 1.5cm less saddle-to-bar drop and 1cm short reach (saddle nose to hoods)...anyone know what's going on? I've been struggling to get comfortable on the Terra since the day I got it (back in January), and I'm to the point that I'm ready to replace it.
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Old 06-25-20, 06:12 PM
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Have you ever measured your inseam and the frame size of the bikes? By that I mean using a virtual TT to measure frame size. Just curious.
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Old 06-25-20, 07:09 PM
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I've been fitted before, and the R5 is spot on.
There is only a 5mm difference in the EFF of the two bikes (548 vs 543), due to the slightly steeper ST angle of the Terra, and is account for with saddle setback.
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Old 06-25-20, 07:34 PM
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BB drop. Maybe?
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Old 06-25-20, 07:49 PM
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It's a different bike for a different purpose- why do you want the exact same fit?

Beyond that, wouldn't it be easier to get a longer stem than to replace the bike?
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Old 06-25-20, 07:51 PM
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The BB drop is actually less on the Terra (70mm vs 72mm).
Plus, since all measurements are taken from BB center, it shouldn't have any effect...or does it?
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Old 06-25-20, 08:00 PM
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Originally Posted by woodcraft View Post
It's a different bike for a different purpose- why do you want the exact same fit?

Beyond that, wouldn't it be easier to get a longer stem than to replace the bike?
A more upright "gravel" position doesn't work for me...my road fit is what is comfortable to me. Plus, I'm not doing anything super technical so there is no/need benefit to altering my position. I ride my gravel bike, like I do my road bike, so why should the fit be different?

Yes, I could throw on a -17, 110mm stem on, but based on the geometry of the two bikes, that should not be need...thus the point of this thread
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Old 06-25-20, 08:04 PM
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Originally Posted by melikebikey35 View Post
A more upright "gravel" position doesn't work for me...my road fit is what is comfortable to me. Plus, I'm not doing anything super technical so there is no/need benefit to altering my position. I ride my gravel bike, like I do my road bike, so why should the fit be different?

Yes, I could throw on a -17, 110mm stem on, but based on the geometry of the two bikes, that should not be need...thus the point of this thread


Seems like the longer wheelbase would make a difference.

Have you tried lining them up side-by-side?
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Old 06-25-20, 08:09 PM
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The stem top cap does not enter into the equation because it is above the stem not under it. Therefor it has no effect on handlebar height.
The difference in handlebar reach may be accounted for by a difference in seat tube angle unless you measured your saddle setback from the vertical centerline of the bottom bracket.
Brent
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Old 06-25-20, 08:47 PM
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Originally Posted by obrentharris View Post
The stem top cap does not enter into the equation because it is above the stem not under it. Therefor it has no effect on handlebar height.
The difference in handlebar reach may be accounted for by a difference in seat tube angle unless you measured your saddle setback from the vertical centerline of the bottom bracket.
Brent
I'm referring to the headset (bearing) top cap, which is under the stem and spacer, not the compression plug top cap...I understand that that has no effect on stack height. Setback was set from the center of BB using a laser level, and is the same on both bikes.

Originally Posted by woodcraft View Post
Seems like the longer wheelbase would make a difference.

Have you tried lining them up side-by-side?
There is a major difference in wheelbase, 1030mm vs 976mm.

Maybe it does have something to do with the longer wheelbase/fork length(cervelo doesn't list it)/HT angle...but since everything is measured from BB centerline, it really shouldn't affect the overall stack/reach.
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Old 06-26-20, 06:48 AM
  #11  
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The reach difference has to be due to the saddle position, relative to the BB. Use a plumb Bob to check the nose of the saddle, relative to the BB.

Carefully check the saddle height on both bikes, from the center of the BB.

I measure the saddle to bar drop, vertically from the floor. If one is less than the other, just remove spacers to make them the same. Stems lengths can be measured differently and the angle also affects the horizontal length.

There could also be a mistake in the bike geometry charts.

Start with the saddle fore/aft and height and go from there.
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Old 06-26-20, 07:24 AM
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The reference point on the saddle should be something like where your seat bones sit. Saddles have different length noses and tapers.
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Old 06-26-20, 08:10 AM
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Have you tried any of the online caclulators?

Bikegeo.net is a good place to start.

And Stem Comparison Tool | yojimg.net is a great stem comparison tool.
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Old 06-26-20, 11:18 AM
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Originally Posted by DaveSSS View Post
The reach difference has to be due to the saddle position, relative to the BB. Use a plumb Bob to check the nose of the saddle, relative to the BB.

Carefully check the saddle height on both bikes, from the center of the BB.

I measure the saddle to bar drop, vertically from the floor. If one is less than the other, just remove spacers to make them the same. Stems lengths can be measured differently and the angle also affects the horizontal length.

There could also be a mistake in the bike geometry charts.

Start with the saddle fore/aft and height and go from there.
I've measured the saddle position (height/setback) many, many times, and everything checks out. And my body lets me know pretty quickly is something is off with my saddle, so I'm confident that those measurements are correct, and the same on the two bikes.
Then I've measured saddle-to-bar drop from the floor, along with a laser level along the handlebar plane...I keep getting the same number, as well. Stems were also measured, and both are 100mm. Technically speaking, the -12 stem should have a slightly longer reach, yet that is the bike that is coming up short.

I've run out of spacers to remove, and, as mentioned above, I'm more concerned with WHY the numbers aren't lining up, when the math says that they should...I shouldn't have to run a -17 stem, and a low-stack top cap to account for a 12mm difference in frame stack.

I'm going to have to assume that something is off with the geometry chart, as well

Originally Posted by grizzly59 View Post
The reference point on the saddle should be something like where your seat bones sit. Saddles have different length noses and tapers.
Same exact saddle on both bikes, so measuring from the nose works
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Old 06-26-20, 11:22 AM
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Originally Posted by topflightpro View Post
Have you tried any of the online caclulators?

Bikegeo.net is a good place to start.

And Stem Comparison Tool | yojimg.net is a great stem comparison tool.
I've used that stem comparison tool..it tells me that everything should be equal.

Then, I just ran the two bikes through Bikegeo, and this is what it gave me...
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Old 06-26-20, 11:25 AM
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Originally Posted by melikebikey35 View Post
For the life of me, I cannot figure out why the fit on my two bikes do not match.

The two bikes in question are:
Cervelo R5: Stack: 548mm, plus 10mm top cap, 10mm spacer, 100mm -6 stem. The frame reach is 380mm. HT angle is 73
Orbea Terra: Stack: 560mm, plus 15mm top cap, 0mm spacer, 100mm -12 stem. The frame reach is 378mm. HT angle is 71

I'm using the same bars (77mm reach), saddle, saddle height, and saddle setback (laser measured) on both bikes.

Based on stem comparison tools (and mental math) the two bike should be, near-as-makes-no-difference, the same. Yet, my Terra has 1.5cm less saddle-to-bar drop and 1cm short reach (saddle nose to hoods)...anyone know what's going on? I've been struggling to get comfortable on the Terra since the day I got it (back in January), and I'm to the point that I'm ready to replace it.
This link compares the two bikes. (There were multiple Orbea Terra options, so I just chose the generic one. You might need to enter a different one):

https://geometrygeeks.bike/compare/c...-terra-2018-s/
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Old 06-26-20, 01:00 PM
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I plugged the numbers into BikeGeo too. Fork length was not listed for Cervelo and Offset was not listed for the Orbea. I adjusted those numbers (386 and 52) until the tool gave me almost the exact mfg. listed stack, reach and wheelbase for the R5 and was 1 mm off on the stack/reach for the Terra.

When I did that. the stem reach was 473 and 471 and stack was 586 and 588 - which are essentially rounding errors. So, yeah, your math should be right and the fit should be the same for the two bikes.

Are you sure you have a -6 and -12 stems?
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Old 06-26-20, 01:35 PM
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Why not pick the bike that you think has the best fit, then adjust the other to match it. I still say you can't trust geometry charts. If you're sure that saddle heights and setbacks, from the BB center, are the same, then do whatever is practical to make one bike about the same as the other. Since stems mostly come in 10mm increments and angle changes the horizontal reach, you may not be able to get them the same.

I had a Colnago and LOOK with that problem, I made the reach from the saddle to the bars the same, but one had a little more saddle setback than the other. It wasn't a big deal.

From what you've posted, a 110mm -17 stem would fix most of the issues. I know that someone makes a stem that's around -21.

Last edited by DaveSSS; 06-26-20 at 02:45 PM.
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Old 06-26-20, 02:48 PM
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Why cant you just copy the fit from the Cervelo? Same seat, same handle bar. Replace seat post and stem to make it match ?
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Old 06-26-20, 02:58 PM
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Originally Posted by topflightpro View Post
I plugged the numbers into BikeGeo too. Fork length was not listed for Cervelo and Offset was not listed for the Orbea. I adjusted those numbers (386 and 52) until the tool gave me almost the exact mfg. listed stack, reach and wheelbase for the R5 and was 1 mm off on the stack/reach for the Terra.

When I did that. the stem reach was 473 and 471 and stack was 586 and 588 - which are essentially rounding errors. So, yeah, your math should be right and the fit should be the same for the two bikes.

Are you sure you have a -6 and -12 stems?
I couldn't find the R5 fork length, either so I left it at 396 that was listed. But I just measured it, and the 386 that you entered is correct. And I'm sure about the stems...I guess it forever be a mystery

Originally Posted by DaveSSS View Post
Why not pick the bike that you think has the best fit, then adjust the other to match it. I still say you can't trust geometry charts. If you're sure that saddle heights and setbacks, from the BB center, are the same, then do whatever is practical to make one bike about the same as the other. Since stems mostly come in 10mm increments and angle changes the horizontal reach, you may not be able to get them the same.

I had a Colnago and LOOK with that problem, I made the reach from the saddle to the bars the same, but one had a little more saddle setback than the other. It wasn't a big deal.
Simply increasing the setback, doesn't work for me, and it just leads to other issues. Between my sizeable leg length discrepancy, and odd proportions, getting my fit dialed in was a very long process, and it's not something that I want to mess with...another 2 months of physical therapy doesn't sound appealing.

I'm not trying to be hard headed about bike set-up, it's just that I know what works for me and what doesn't. An extra 10mm here or there, really screws with me.
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Old 06-26-20, 03:14 PM
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Any issues with the saddle on the bike that doesn't fit as well? Are you sure it's pointed dead straight?

What exactly feels off with the fit? Just the drop is different?

Last edited by ridethecliche; 06-26-20 at 03:29 PM.
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Old 06-26-20, 03:56 PM
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No issues with the saddle.
It's just the stack and reach...too high and too short, which leads to me hunching over, which leads to extra stain on my shoulders and lower back.

Last edited by melikebikey35; 06-26-20 at 04:13 PM.
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Old 06-26-20, 04:06 PM
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Throw another spacer or two under the stem of the terra and see how it shakes out. Silly reason to give up on a bike if it's too low and you have no spacers...
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Old 06-26-20, 04:12 PM
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Originally Posted by ridethecliche View Post
Throw another spacer or two under the stem of the terra and see how it shakes out. Silly reason to give up on a bike if it's too low and you have no spacers...
The Terra is the bike that is too high and too short...and I'm out of spacers to remove. I'll try to source a lower headset cover in order to get me another ~5mm of drop, and see if that helps.

There are other things about the bike that I'm not super fond of, but if it was at least comfortable, I could live with it.
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Old 06-26-20, 04:19 PM
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Originally Posted by melikebikey35 View Post
The Terra is the bike that is too high and too short...and I'm out of spacers to remove. I'll try to source a lower headset cover in order to get me another ~5mm of drop, and see if that helps.

There are other things about the bike that I'm not super fond of, but if it was at least comfortable, I could live with it.
Ah, I misunderstood. But yeah... I'd start there re:get the spacer shorter and/or try a stem with more aggressive angle.

If neither of those two simple things don't work then I'd just sell it and get something you like better.

Any more than that and you're just spending time trying to make something work that you're not happy with when you're better off just riding what does work or getting something that *actually* works.

You could also try bars with slightly more drop and/or reach to see if that makes things better.
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