Bike Fit / Measurements

Hi, Hope you can help me a bike fit question. I currently ride a 52 cm frame C-C, with a 53.1 cm TT, a 72.5 deg head angle, and a 75 deg seat angle. My seat is pushed all the way back on an offset seatpost to get my knees in the right knee to pedal position. The head tube is 10 cm and I use a 10 cm stem. This bike fits me pretty well, even though I've got the seat back so far.
I'm considering a 51 cm Cervelo R3 which has a 53 cm TT, a 73 deg seat and head angle, and a 12 cm head tube. Considering the differences between the angles and sizes, what do you think of the 51 cm fit? The main difference is the large difference in seat angle. Anything I should watch out for? I've put all my measurements into the Competitive Cyclist fit calculator and have come up with an ok fit online. It's very difficult for me go to a shop to see this bike due to poor logistics. Thanks for any advice! :confused:

are you proposing buying the bike without trying it out?
The answer is maybe it will fit. No one here knows better than you. Good luck

Since you're already pushing your seat all the way back & using a set back seatpost, there probably really isn't a difference between the bikes in regards to seat angle... your setback seatpost & seat being pushed back is probably increasing the effective toptube of your current bike... you may not need the setback seatpost on the cervelo and then possibly you'd want a longer stem on the cervelo to even out the top tube.

But I don't know how the differences in heights of the head tube is goign to come into play.

My gut response is that it'll probably fit you and I really generally just prefer seat angles of 73 degrees. I can't get comfortable or my knees in the right spot on a 75 degree seat angle bike.

But I'm gonna caution you that I'm just some random person on the internet who doesn't know a heck of a lot about bike fit other than what I've been through to get my bikes to fit right.

Where are you ordering the cervelo frame through? a bike shop might be able to mock up a dummy/trainer bike to the same specifications as the cervelo frame so that you can try the geometry.

Thanks for your thoughts on this. I agree that since I'm using a setback seatpost and my seat is all the way back, I think it'll fit. It's difficult for me to try out the bike since I live on Maui and none of the local dealers carry Cervelo. I'm thinking of ordering one through Competitive Cyclist, where I've ordered components from them before.

Any of you frame builders here have any ideas on this? Thanks!

Sounds like you've got the seat angle roughly figured out.

The head tube is 2 cm taller though. Do you have at least 2 cm of spacers on your current bike you could leave out on the Cervelo?

Depending on her stem angle or the replacement stem... she could compensate for that on the cervelo.

Be aware that if you are set up in the same position--butt and leg-wise-- relative to the BB/cranks on the Cervelo that you are on your current bike, due to the seat angle differences, the Cervelo will actually have a reduced reach, by 1-2cm.

Thanks for your comment, Margo. I'm a bit confused though. I would think that if I'm going from a 75 deg seat angle to a 73 deg one that the net effect would be to increase the reach, not decrease it. If I'm not understanding this correctly, can you explain it to me? Many thanks, Sherry

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Originally Posted by mauisher

I would think that if I'm going from a 75 deg seat angle to a 73 deg one that the net effect would be to increase the reach, not decrease it. If I'm not understanding this correctly, can you explain it to me?

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Think of the bottom bracket as a fixed reference point. On the Cervelo, with its less steep seat tube, the top of the seat tube will be further back relative to that BB. At the same rider to pedal position, this means that the top tube has to reach further under the seat. And then, because the top tubes are nearly the same length, this means that the head tube is closer to the rider.

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Thanks for your comment, Margo. I'm a bit confused though. I would think that if I'm going from a 75 deg seat angle to a 73 deg one that the net effect would be to increase the reach, not decrease it. If I'm not understanding this correctly, can you explain it to me? Many thanks, Sherry

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On your current bike it sounds like the seat tube angle is too steep (so common on smaller frames) and you have had to push the saddle all the way back to get a good position. By doing this, you increased the reach on the bike.

With the Cervelo and the shallower seat tube, you very likely won't need to push the saddle back at all, and may even nudge it forward. This will reduce the reach relative to your old bike.

Hope that makes sense.

Got it!!! Thanks very much for your help! :D