I'm not a Cervelo dealer, but what I know about them is that the 51 cm size seems right to me based upon your size. You might want to email Gerard or Tom for an opinion.

The traditional way of measuring for frame size is to multiply your inseam in cm by .67. Thirty inches is about 76.2 cm x .67 = 51.054 cm.

Based upon this formula, the 51 cm frame seems spot on. Of course different manufacturers may measure their bikes slightly differently. Also make sure you are measuring your inseam correctly. Stand in sock feet, in your bike shorts, back against a wall. Push a book up against the crotch with about as much pressure as you feel when riding. Have an assistant measure from the ground to the top of the book.

I don't see how the information given makes sense...

I'm 5'9" w/ a 31.5" (80cm) inseam.

Using CG's formula I would be on a 53.6cm frame. Fine. Sounds reasonable, BUT Andy is .5" shorter than I and his inseam is 1.5" less, which my amazing math skills tells (hopefully?) that his torso is 1" LONGER than mine. So... Why oh why would Andy ride a smaller frame than me? Especially one that is 1" (2.5cm) shorter. It seems we have a 2" discrepancy.

* I'm not saying I should actually ride a 53.6cm. I'm just using the rule of thumb to make a point. Which is... Given your torso length (and knowing very little else) 51cm sounds smallish to me.

Frankly, I can't see spending that kind of cash on a bike unless you are professionally fitted for, or have a chance to ride first. Seriously, you would be faster buying a $500 plane ticket, flying to Louisiana, getting a $200 (guessing) fitting from John Cobb and then buying a $2000 P2K or similar. Same amount of cash, but the right fit.

Bottomline: There is more to fit than height and inseam. Hold onto your $3K until you talk to someone that asks the right questions and/or you can get fitted and/or you can ride a couple of the bikes you are thinking of buying.

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Justin in Austin, get it? :)

Cool races:
- Redman
- Desoto American Triple T

The x.67 formula was actually taken from Greg LeMond who uses this to determine frame size and was being used as a basic guideline. Assuming that his inseam of 76.2 cm was measured correctly, this would indicate that a 51 cm frame is correct, at least according to this method of determining frame size.

It's more than just frame size but from here you can compensate for torso or leg length with stem length and seat height adjustment. Any given frame size can fit many variations of riders with similiar heights but different body proportions. Cervelos are designed with quite a range of adjustment.

The next size in a Cervelo is 54 cm, which according to LeMond's formula seems a bit big for this rider. As I stated, I'm not a dealer and suggested that he should email Gerard for an opinion. That's a bit cheaper than spending $700. to visit John Cobb in Louisiana.

I don't like the tone of my previous post after reading it again. My apologies. I wasn't trying to say you were wrong CG, and I realize that it wasn't your formula. I'm just trying to raise some eyebrows. Doesn't it seem strange that a person with a shorter torso would be fitted to a larger bike? It just seems to me that tri-bike fitting is more about top tube length fitting than seat height fitting. I read on another BB of a female 5'10" riding a 48cm bike because her legs were long / torso short. Completely violates GL's formula, no?

Take another example. Two people with the same inseam, but one person is 1" taller than the other (easy if we are talking M/F, my wife is 2.5" shorter than I with less .5" shorter inseam). Presumably we would fit these people on the same bike and then as you pointed out (and here is where it gets fun IMHO) you can just adjust stem length. Common stems range over 30mm, so assuming all other things being equal (flexibility, potbelly, ability to see through top of head, etc.) we could fit these two people on the same bike only changing the stems. My question, then is, wouldn't the handling of these bikes be very different. Here we have two different height people and they are going to get very different handling bikes. Aren't they? Is one more right than the other? Given the range of stem lengths (30mm at least) does that mean everyone has a 3cm range of frame sizes to choose from?

Is someone on a 51cm bike with a 120mm stem, fitting differently than that same someone with a 54cm frame and a 90mm stem.

*I'm not considering variations in headtube length as that affects fit. Should I?

More questions than answers really...

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Justin in Austin, get it? :)

Cool races:
- Redman
- Desoto American Triple T

Justin, Thanks for the clarification. I guess to clarify my post... I do plan on getting fit by someone who knows what they are doing, but I plan on buying the bike first. Just want to get the right frame size and from reading Tom Demerly's posts in the past, I gathered that test rides don't mean squat and proper frame sizing based on measurements. Maybe I'm way off base...

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Andy

'You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Thinsg Are As They Are.'

I'm 5'8", 30 in inseam. I was fit by a local dealer on a 52cm P3, steep seat angle set-up. I had originally thought I'd be on a 51cm, but the fitter was very certain of his decision and I had to defer to his knowledge. I was happy to stay on 700c wheels. I'd be curious to be fit by a FIST graduate and see what their thoughts are and if their frame size selection would be the same.

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"I spent a lot of money on booze, birds and fast cars, the rest I just squandered" -George Best

Justin, I definately see your point. My wife is five inches(12 cm) shorter than I am but her inseam is only about .5 cm shorter than mine. She has long legs/short torso even by female standards whereas I have short legs/long torso. She rides a one size smaller P2K than do I because of this.

These sizing formulas are based only as a guideline for the "normal" proportioned rider and doesn't take into consideration unusual anatomical variations. That's why I suggested to the original poster that he should email Gerard.

I'd make sure to get my inseam, torso, height and possibly arm measurements taken before buying a frame.

I bought a Cervelo One from www.insideoutsports.com, and they used the following information to pick out the right size. I made sure to ask them if this was an okay bike for me, or if there was a better fitting bike. Really helpful people, I would definitely recommend them for online sales if you don't have a shop in your area. Good luck,

http://insideoutsports.com/info/bikesize.asp

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"What the mind can conceive and believe, the mind and body can achieve; and those who stay will be champions."