I'm looking to buy a Cervelo Dual in the next couple of weeks, but am torn between the 51 and 54 size. I'm 5'8" with a 30.5" inseam. According to bike-fitting websites I've found, I should be looking at a 51.9cm frame size. The Cervelo seat tube adjusts b/t 74 and 79 degrees, which means that the 51 size would fit me at a 75 degree angle (53.5 cm top tube length at that angle). However, I've read that the steeper the seat tube angle the better so keeping it at 79 degrees is preferred. The 54 size at 79 degrees yields a 53.0 cm top tube length.

Does anyone have any experience fitting a Cervelo that might be helpful? Should I go for a smaller frame size and adjust the seat tube accordingly? Or should I go for the larger frame size? Thanks!

Is that a "true" or pants inseam?

Off the bat, I'd say 54.

I'd look here: https://www.slowtwitch.com/dealersurvey/fistcoaches.html to see if there are any shops near where you live with experience in fitting tri bikes.

I have a cervelo as well.

I'm also 5'8 with a 30.5" inseam (it's true!).

My road bikes are all 54's

My Cervelo is a 51, and fits like a glove.

Go to a qualified bike fitter and test ride the 51 properly setup, you'll be surprised.

Enjoy.

I'm with everyone else. Find a shop that carries them. Make sure they have one in stock before you drive too far, and go get fit. Most shops will order a size in if they don't have it for you to try.

I'm 5'11". When I was test riding tri bikes, I was riding on a 54 p2sl. I ended up buying a QR, but the 54 Cervelo would have worked for me.

yeah, if you're spending enough money to buy a cervelo, the shop should give you a proper fitting.

If you are between sizes, I'd suggest the smaller of the two sizes for TT frames.

But as everyone else is saying a sizing by a good fitter is a worthwhile initial investment prior to purchasing.

I ride 56-57cm road bikes (183cm tall - 86cm inseam) and have recently bought a 52cm Alloy P3 which I am currently building up. I was sceptical about going that small, as the general rule of thumb is to get a top tube ~2cm down from your road bike. But I went and borrowed a friends 52 cm P3 (who is identical in fit) and went out on a 60km training ride on the bike. Note though I am still pretty flexible in the back and hamstrings so can ride more aggressively...

The smaller frame and forward position is also fine for me for Tri's and TT's. As I don't race under UCI rules, so am not restricted by UCI rulings in regard to seat position behind the BB. Which would be near impossible to meet with the extra small frame and forward seat position.

I have a P2SL (very similar to the Dual) and it is a 54cm frame. I'm 5'-9" with a 30" pants inseam (short legs, long torso). The bike fits me extremely well and I have the seat in the forward position.

There is so much to fitting a tri bike... go to a good shop in your area. If you live in Michigan or northern Ohio then I would highly recommend BikeSport of Michigan in Dearborn. Tom Demerly is very experienced and you will not go away without a proper fit.
- tom

I had to double check to make sure I didn't make this post. 54 roadie and a 51 P2 SL

Yup...........I have the P2SL as well!

Funny.

I do love the bike, and its FAST!

Hmmmmm. I didn't get the fast version. Bummer.

I'm in a bit of a predicament. I really want to order a Cervelo P2SL but there are absolutely no local shops even close to me that sell Cervelo and the one that is closet isn't being all that cooperative with assisting me.
I've used the competivecyclist fit calculator and compared results to Cervelo's geometry tables and I'm still not positive which size I should be purchasing.
I'm 6' with a 35" inseam. I fully realize the best thing for me to be doing is getting properly fitted but it just isn't going to be possible.
If anyone could recommend an on-line shop that I should deal with and a suggestion for Cervelo size, I'd appreciate it.
Thanks!