I'm trying to figure this out

I’m no rocket scientist. This one has me a little befuddled.

I ride my road bike on rollers quite a lot in the off-season. I have Kreitler Challenger rollers and I don’t use any resistance aside from the gears. Yesterday I did a 2 hour session and averaged about 34 mph. That’s a pretty good aerobic workout for an old man.

Afterward, UPS delivered my new forkstand and I mounted my tri-bike to it and set it up on the rollers. My tri-bike has 650 wheels. On my road bike I cruise at 36 mph in the hardest gear (on the rollers, of course). The best I could manage on the tri bike with the 650 wheels was 30 mph.

This actually makes intuitive sense to me. A 53x11 at a given cadence is gonna turn the wheel so many times, no matter what size wheel … and the smaller wheel will travel a shorter distance. What doesn’t make sense to me is, how come when I go out on the road I don’t experience anything like that … in fact, it seems the difference goes in the opposite direction? Obviously, on the rollers, all considerations of aerodynamics are not in play, but the differences seem so large. Is it just a matter of the gears I’m able to ride in out on the open road?

Can anyone explain the physics going on here?

I’ll give this a shot. 2 hours on rollers is a great workout for any aged man or woman, keep up the good work.

It is my understanding that the single biggest factor causing resistance to a bicycle rider is drag, and that the single largest perpetrator of causing drag is you…the rider. On your road bike, sitting up on the hoods you are a sail!!! If you have a good, low aero position on your tri bike you have eliminated potentially up to 50% of the drag you present to the wind while riding your road bike. That is huge, and probably the biggest factor in the speed vs. effort differences you are experiencing. The slightly lower drag profile of the 650 tt bike is measurable but small, and it probably weighs a bit less than a run of the mill 700c road bike, there are exceptions of course.

I raced and trained this past season on a 700 c 53.5 degree of seat angle road bike. For races and aero training I put a fast forward seat post, aero bars, and took out all the spacers for a below average but still aero position. My aero equipment was all heavy, adding at least a few pounds to my setup, but I was capable of holding 15% faster speeds over the same TT distances while aero.

I hope this explains a little of what you were looking for. The aerodynamic aspect you mentioned that is eliminated on rollers, is the biggest force we fight on the road.

In other words, as you imply, to ride the same speed in the same gear on a 650 wheel you have to pedal faster because the gear is effectively easier. You either weren’t doing that(and it’s quite a bit faster) or you have imported extra resistance into the system.

Not sure about physics but I’d be interested to know what your limiter is when riding the tri bike on the rollers - I suspect its cadence. Out on a limb but if it’s the case that the reason you can’t go 36mph on your tri bike is that 36 and 30 mph both represent your maximum cadence after which point you get inefficient and clunky then you’re not comparing your max speed. Truth may be that if you could put 700s on your tri bike you would go 36 too and thus the aero difference on the road becomes a lot more modest.