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Easy Bike Fit Question(9 posts)

Easy Bike Fit QuestionKeebs64
Aug 17, 2001 10:50 AM
My question is about the manufacturers numbers for bike sizing.

I'm looking into purchasing a Trek 2200 or a Cannondale R800. My inseam measurement is 34" which translates to a 56cm seat tube.

When I look at 56cm frames from both of these manufacturers there is a significant difference in the numbers for what is supposed to be a similar frame size.

Cannondale 56cm = 58.7cm seat tube & 31.9" standover
Trek 56cm = 56cm seat tube & 31" standover

In order to get close to a 56cm seat tube on a Cannondale I would have to go down to a 53cm frame:
Cannondale 53cm = 55.7cm seat tube & 30.8" standover

Am I reading these numbers correctly? Would a 53cm frame from Cannondale actually fit the same as a 56cm frame from Trek?

Thanks, confused.
These questions aren't easy at alljtolleson
Aug 17, 2001 11:04 AM
Your fit will be driven by a lot more than your inseam size. The other big factor is reach/top tube length and its importance simply cannot be emphasized enough.
Cannondale and Trek measure their frames differently. Cannondale measures center-to-center and Trek measures center-to-top.

As far as "fitting the same," there are too many variables to count. Seat tube length, seat tube angle, top tube length, stem length, crankarm length, handlebar width.

You need to get thee to a bike shop and ride, ride, ride both bikes.
Easy Bike Fit Question - is there such thing?Mike K
Aug 17, 2001 11:09 AM
The problem you are running into is that different frame manufacturers measure frames differently.
C-Dale measures C to C - meaning that the frame is measured from the center of the bottom bracket to the center of top tube.
Trek measures C to T - meaning that the frame is measured from the center of the bottom bracket to the top of the top tube.
This will effectively make any Trek frame fit "smaller" in a given size than the C-Dale (you have to add half the diamiter of the top tube to convert a C to C to a C to T measurement).
Not to confuse the hell out of things but the measurement you are looking at is not the really important one anyway. What matters most is the length of top tube because it controls your actual position and reach on the bike.
If you look at the numbers from both Trek and C-Dale the Trek will have a longer "effective" top tube per frame size than the C-Dale which should make the bikes fit and feel differently.
To sum up - use the numbers you got via your inseam measurement as a starting point only. Then go out and ride both bikes in different sizes (and not just around the parking lot) to find what fits best.
For reference I have a 32" inseam (5'10") and one of my bikes is a 56cm C-Dale. I've never really gotten comfortable on a Trek (this is just me and may be exactly the opposite for you) but the closest I've come to getting a good fit was on a 54cm. Hope that helps.
re: Go to your LBSMike Prince
Aug 17, 2001 11:11 AM
Get fit by the shop for the right size frame. They will help you way more than the geometry numbers off of the internet. This is one area where a little professional help is a good thing.
???alex the engineer
Aug 17, 2001 11:41 AM
Gee, I'm 6'2" tall, have a 34" inseam, and I'm perfectly happy with my 25" (63cm) c-c frame. I have 175mm cranks, too! A frame that size would work for a dirt bike, but it's too small for you in a road bike.
you're confused....C-40
Aug 18, 2001 12:36 PM
You haven't read the Cannondale geometry charts correctly. C'dales are measured from the center of the the bottom bracket to the top of the top tube. The actual seat tube length is irrelavent, since it's extended above the top tube.

Treks are measured weird, stating the seat tube length to be the frame size.

Basically a 56cm Trek is most similar to a 54cm Cannondale. You have to consider the difference in seat tube angle when comparing the top tube lengths. The 56cm Trek has a top tube length that is about 1cm longer than the Cannondale, after the correction for STA is considered.
I think he is correctGregJ
Aug 18, 2001 1:36 PM
C-40, I do not think you are correct. The Cdale geometry chart is a little confusing. The picture shows a measure D that is center to top of top tube. That is not the number they use to label to the bikes. That number is shown in a column under what they call size, which I assume is the c-c measure. What Cannondale calls a 56 at the top of the chart is 58.7 c-top of top tube. The original poster is correct. I think a Cdale 53 will almost certainly be too small and 56 should be about right. Definately check this out at a shop and ride these bikes.
The Cannondale drawing is wrong and more...C-40
Aug 19, 2001 6:52 AM
Cannondale goofed on their website drawing. I never looked at it before responding, because I've got a catalog to refer to. The D dimension should be to the top of the seat tube, not the top of the top tube. The size is also c-t, not c-c. This is obvious if you calculate the standover height for any model. I've also owned several and I'm quite familar with their measurments.

Another thing that I forgot to address is that both sizes that Keebs64 considered are too small, if he has a true 34 inch inseam, as measured in bare feet. The 56cm has a standover height of 31.9 inches, which is certainly the smallest frame that he would want to consider. A 58cm frame has a standover height of 32.6 inches, which still gives plenty of clearance. Keebs64 didn't give his height, so it's hard to know if a 56 or 58 would fit best. The odd sizes are not available standard.

The Trek frames sizes are 2cm smaller than Cannnondale. Someone with a 34 inch inseam should be looking at a 58 or 60cm in a Trek.
Thanks...Keebs64
Aug 20, 2001 11:15 AM
I'll be heading down to my Bike store tonight to get sized. I see what you mean by the way the different measurements are taken. 58cm Cannondale & 60cm Trek are the closest to achieve a true 56cm C-C. BTW, my height is 5'-11" (34" inseam, bare feet to pubic bone)