Between bike sizes.. need help!

missyschorpion

Hey guys!  Wanted to pick your expert triathlon brains and see if you could all help me out with a bike size/fit issue.  I currently ride a Women's XS 47cm Trek speed concept with 650 wheels.  I recently rented a Trek speed concept in a Women's size S 50cm with 700 wheels for the Florida 70.3, as the bike shop did not carry XS rentals.  Overall I felt really good on the size S.  I am not sure if it was mental, but I felt like it was easier to maintain faster speeds with less effort on the bigger frame.  I don't ride with a power meter and didn't wear a HR monitor for the race so I am purely basing that on RPE.  

I am 5'4" and have a relatively petite frame.  I think overall, I am between sizes and could make either the 47cm or 50cm frame work- I just wanted to see if anyone has thoughts on which is best.  With the 50cm frame, the advantage is the 700 wheels- which are generally more easily accessible, as well as the physics of a bigger wheel covering more distance per revolution. However, with the 47cm frame and 650 wheels the advantage is that the bike is potentially a bit lighter and more aero.

I am considering upgrading my current bike, so any feedback you guys can provide would be so appreciated!  Thanks  

MattAaronson

Melissa, honestly you should go see a professional bike fitter and have them set you up. If you're in the market for a new bike it is very typical that half or more of the ~$300 they will charge you would be credited toward the purchase of a bike from them. You need therefore to find a store with a very good fitter that also carries the brands of bike you may be interested in buying.

Al Truscott

Having ridden a 650c TT bike for the past 15 years, I'd advise going with 700 if at all possible. Simply because of the limited wheel, tire, and tube options (eg, no latex tubes) for 650. I' d switch myself, except I save $1000 a year on bike shipment costs d/t my bike's S&S couplers allowing transport in a 26" suitcase.

missyschorpion

@Matt: Thanks for the advice! I actually was just fitted a few months ago on the XS, I currently ride. I am heading into the bike shop on Friday to speak with the bike fitter who performed my fit to see what he thinks.

@Al: I definitely agree with you about the limited selection on the smaller wheels- which has been a problem in the past. Any thoughts on performance with the bigger vs. smaller wheels?

Peter Noyes

I went to a fitter and found that I do much better on a larger Tri Bike. I am a 5'9" male and ride a 54cm road bike, but I ride a 56cm Tri Bike. I have a long torso and long arms and short legs. I don't think it is unusual to feel more comfortable on a larger bike.

[Deleted User]

Melissa, we are the same height and similar body type.  I have a Trek SC 9.5 size small.  It came with a 170 crank length and I had a bike fit with Todd @ ttbikefit.  I was comfortable with the fit but then decided to go with a compact crank. I also went with a shorter crank, 165.  After the changing cranks I got a retul bike fit.  With the shorter crank length I was able to raise my seat, which opened up my hips and made me more aero. I could have gone with a XS frame but decided to go with a S due to the wheel size. When I had to replace my bike last year I didn't even consider going with an XS.

Al Truscott

Not trying to sell you on 650 ... but given proper gearing, meaning the same gear inches on each size bike (wheels roll the same distance for a given pedal stroke) the 650 might be considered to have a slight advantage, with less wheel weight and thus angular momentm to overcome.

As part of my recent overhaul, I, too, now have 165 cranks, makes for easier fit to comfortable yest very aero position.

rbogle

todd @ ttbikefit can do a pre-buy fitting via video in which he'll give you recommendations on brand frames (due to geometry differences) and sizing. It may be worth your while to collect more info that way. He's probably one of the best fitters around, and arguably the best at remote/online fittings. 

missyschorpion

@Peter @Teri @Al @Rian: Thank you all for the help! I will reach out to Todd @ ttbikefit and see what he thinks. It seems the general consensus is that a shorter crank is also something to consider. I appreciate all of your feedback. I will let you all know what I end up choosing.