650c Tri/TT

diy

A friend of mine who's quite petite is taking up triathlon. Training for and doing triathlons is her only reason for cycling. She is very strong, flexible and fit, but only 5'3. Her budget means that we were trawling the used bike adverts.

Given that she comes up as XS in most models, I've found her a 650c wheel based carbon tri/road bike. Is there any downside to have 650c wheels over 700c - Inner tube, wheel and tyre choices seem a bit limited, but given her size could this actually be right for her?

Opinions?

Giraffoto

Rolling resistance will be fractionally higher with the slightly smaller wheels, but at 5'3" I'd guess her reduced frontal area will offset that with aerodynamic gains.

imposter2.0

Lots of youth riders are comfortably on 700c bikes at 5'3" or less - I would avoid 650c if at all possible, as tyre and wheel choice is severely limited.

Also, I don't see how wheel size has any effect on rolling resistance, but I am open to correction...

diy

Problem was, very little to choose from with a budget of £350, but quite a few Planet X Stealths used. My thinking is two seasons for a £100 loss.

simon_masterson

Works for Emma Pooley. The tyre and wheel choices are indeed a bit limited (I have a lo pro with a 650c front), but they are available, and it's well worth considering if you're smaller in stature. SJSCycles still seem to have Conti Podiums (tub) in 650x19mm, which are pretty good. Try places that cater to wheelchair sports, as they use them too.

Giraffoto

Imposter wrote:

Also, I don't see how wheel size has any effect on rolling resistance, but I am open to correction...

Disclaimer 1 - this is a comparison "All other things being equal". Identical tyres, rims etc., just a different radius.
Disclaimer 2 - you're not being corrected here, you've asked a reasonable question

Rolling resistance is the resistance of the wheel to being rolled along the road. It is largely caused by the wheel having to get over any unevenness in the road. If we imagine that you're rolling along and encounter a 5mm high ridge in the tarmac, you experience a resistance as your tyre compresses and the wheel lifts slightly to go over it. The resistance is directed towards your hub, and if we imagine a line going through the point where your tyre touches the bump and through your hub, that's the direction of the resistance. The direction of the resistance is almost vertical, because your wheel is quite big compared to the bump you've just gone over, so it won't slow you down much because it doesn't have much of a horizontal component.
With a smaller wheel or a bigger bump, the direction of resistance is further from the vertical, so it has more of a horizontal component and so more of a tendency to slow you down. So, all other things being equal, smaller wheels = more rolling resistance. But it won't be much, and reduced frontal area will be far more significant at time trial speeds

diy

Yep - as I understood it, bumps have more impact on smaller wheels because they (the bumps) are proportionally larger. They are more prone to skidding too for the same reason and also the CoG is lower which also decreases the ability flatten the tyre under breaking.

But on the other hand they tend to be a fraction lighter, a bit stiffer and faster to accelerate on.

Thanks for the feedback - I was in two minds to recommend the 650c, and its hard when the lass with the cash is clueless about this, so is just saying "I trust your advice". the fool

fenix

I'd go with 700c if I were you - just more choices.

If £350 is her budget for a bike then maybe Tri isnt the sport. Entry fees are big and there's an awful lot of kit you need.