keesue

I seem to recall that stepping down a frame size with a slightly shorter top tube is beneficial in a racing bike; specifically, if one rides a 55cm frame with a 55.5/56.5cm top tube in road bikes, would one benefit in stepping down to a 54cm frame with a 54.8 top tube in a racing bike? To keep the same cockpit length, the stem would have to change from 110mm to 120mm and flipping the stem up would keep the same drop configuration. If I recall the article correctly, the smaller frame would allow one to hammer better.

I guess I'm asking is a smaller frame beneficial in a racing bike as opposed to a more relaxed road bike? Do any of you have a racing bike that is a size smaller than your road bike?

urbanknight

Beneficial for what? Crits? Centuries? Road races?

mugatu

everything is a race


I knew a fitter who insisted that every bike, even the grocery getter be fitted down to the mm, and that optimal fit = optimal power output, and there is should never be a reason to change the fit for different events
if you had to, then it wasn't right in the first place

keesue

Road racing.

keesue

Actually, every one of my bikes is fitted the same way with the emphasis on comfort over the long haul as opposed to more aggressive riding. I seem to recall that racing benefits from a 'slightly' smaller frame. What has prompted the question is I have an opportunity to get a bike - which is a racing bike I have really wanted - to round out my stable. It is a 54 with a 54.8 TT and my bikes are all 55's (with 55.5/56.5 top tubes). I can only ride the bike to test it so I can't get a longer time in the saddle to experience it, hence my question.

veloboy971

The only reason I can think of that one would prefer a smaller frame would be the ability to get the stem to saddle height larger than they would be able to with the "correct size" frame, allowing for a more aero position.

patentcad

Here's my Cervelo. There are plenty of spacers on the stem and lots of seat post showing. That means I could also ride a 58 (this is a 56cm). The 56 frame sizes always seem to fit me. Despite the aggressive appearance of this set up, I can (and have) ridden it comfortably for 5-6 hours at a time. I had some low back discomfort early on, but now it's fine. I'm 5'11", shorter torso, longer legs (33-34" inseam). I really think you should be able to set up a racing bike you can ride all day long in comfort. Doesn't have to be quite as aggressive as my bike, small differences in things like handlebar height make huge differences in comfort.

keesue

Ah, that was it. I couldn't remember. Thanks.

keesue

Thanks, much, this helps a lot. If you could ride a 58 and you ride a 56, it appears you have a more aggressive position on that 56 which is what I thought. Looking at your ride and picturing a 58, it appears the ratio would be the same as a 54 to a 55/56 with that more aggressive stance. I would work with bar height by rise and fit by length (10mm difference which may be offset by a slightly higher rise). I have slightly shorter legs than torso which is what put me on a 55 with the 56.5 TT but the LBS suggested that if I were buying a racing bike, a shorter top tube on one size smaller (54 v 55) would be better. Your picture really helps.

keesue

And that is a killer bike BTW. I have my eyes on a Specialized Tarmac Pro SL. How long is your stem?

pdedes

choosing the smaller size frame is the stiffer option. great for racing and what i've been doing for 20 years. lots of exposed seatpost and long stems. pcad's bike is a great example.

MONGO!

The whole point of running a smaller frame is to get more saddle to bar drop.

cshell

Off topic, but what's that grey thing hanging under your computer?

DaveSSS 

Don't overlook changes to the STA. A 54cm TT is only 1cm shorter than the 55cm if the STA is the same. If the STA on the smaller frame is 1 degree steeper, the fit would be nearly the same.

Reach measurements also lie. A 54cm Cervelo is not 1cm longer in reach than a 51cm. It's about 16mm, with the measurement taken at the same stack height.

If the fit of the smaller frame is not changed, the only difference might be a very slightly shorter wheelbase. You also have to look at the HTA and fork offset. Those can be changed and make the wheelbase of the smaller frame nearly the same as the next larger size.

urbanknight

No necessarily. If the STA is steeper, you would likely want to set the saddle further back to achieve the same fore/aft position, which would effectively increase the reach back to that full 1cm. A small point, but a valid one nonetheless.

Agreed on the wheelbase, though, and I think that's one reason racers like smaller frames. Shorter wheelbases have sharper handling.

botto 

do a lot of charity rides?

urbanknight

Those are races. They must be. At least once a month, someone posts on how they won one.

DaveSSS 

You're agreeing with me on both counts. I said the fit on both would be about the same and you noted exactly why. To get the saddle in the SAME position relative to the BB, the saddle has to be moved back on the frame with the steeper STA. When comparing the fit of a frame it's always assumed that the saddle is in the same position relative to the BB, otherwise you're talking about a different fit.

If you want to get real picky, the difference in reach is (cosA -cosB) times the c-c frame size. In this size, the steeper STA increases the reach by 8-9mm per degree.

fa63

I have very similar measurements to Mr. Pcad's (I am inch taller at 6'00", 33.5" cycling inseam) and the bike I currently ride has a 549mm top tube with a 110mm stem (73.5 STA/72.5 HTA). I guess you could call it a 55cm frame (the manufacturer calls it a Medium). While convention would say that the bike is too small for me (bike shops always try to put me on 58cm bikes), it is the best fitting bike I have owned. The setup looks fairly aggressive in pictures, but it is comfortable (at least for me) in that it allows me to stretch out and just pedal.



Today's compact frames and threadless stems have changed the fitting game.

urbanknight

Ahh yes, we are in agreement then. It just doesn't pan out for me because if I were to go down one size on almost any brand, or even the same size on some brands, I would not be able to put the saddle in the right position... at least not without at least 35mm of setback on the post.

botto 

how old are you?

patentcad

Plastic adapter for mounting things on stems, it has since been removed.

mugatu

no, mostly coffee runs, but I've dialed them back a bit, that's how I could afford my new Cervelo

TheKillerPenguin

Keep in mind that a smaller frame doesn't necessarily work for everyone, as not everyone is flexible enough to take advantage of a large saddle to bar drop. In my case, I'm 6'2'' and ride a 58cm, but it works for me because I have a short torso.

Jay Andriot

The main purpose of sizing down is to get a larger drop in the handlebars. Exposing an extra length of seatpost is not going to make for a stiffer bike. Going with a smaller frame and flipping the stem will defeat the purpose of going with a smaller frame, don't bother with that. If you look at the images above you'll notice that they both have a good deal of drop from the seat to the bars even with Pcad's extra spacers. If you are not looking for a 4 to 5" drop then stay with a larger frame.