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Roller resistance inversly proportional to tire size??(7 posts)

Roller resistance inversly proportional to tire size??Cima Coppi
Oct 12, 2001 5:48 AM
Good morning everyone,

I have accidentally discovered a bit of a physics enigma regarding my rollers and different sizes and brands of tires I use. I seems that when I ride my Michelin Axial Sports (700x20), they significantly increase the resistance of the rollers. This is in comparison to my Conti Ultra 3000's (700x25), which make my roller rides fast with little to no resistance. Also, there is no external resistance adjustment on my rollers, so this was an apples to apples comparison. To prove the rolling diameter is different between the two tires, the diameter measurements I recorded are 2064mm for the Michelins, and 2096mm for the Conti's (done for cyclometer calibration). This equates to a 1.5% difference in rolling diameter between the two tires. For this comparison, only the tires where changed out on the bikes existing wheels. Nothing else was changed.

While riding on the rollers the other night, I had on the Conti's, and was riding 53/15 at 38kph at 145BPM on the HRM for 40 minutes. This is in contrast to normally riding with the Michelin's (I bought them specifically for riding the rollers because they were cheap) in which I can ride 39/17 at 25kph and roughly 145BPM. With the Michelin's on the bike, when I move to 53/17, I normally ride at 31kph and my heart rate gets up into the high 160's and low 170's for the same 40 minutes.

Is it possible that reducing overall wheel+tire diameter could have the same effect as reducing the drum diameter of the roller (i.e. Kreitler Dyno-Myte's)? I know that the wheel diameter difference between the 700x20's and the 25's is much less than the difference between the drum options on Kreitler rollers. Or, is could this be due to tread compound differences (I believe the Michelin's have softer compounds than the Conti's)? Or a combination of the two?

As always, I appreciate your insight and opinions. TIA

re: Michelin Sportscyclopathic
Oct 12, 2001 6:10 AM
suck big time no matter what size they are
they are only 33TPI and have very sticky rubber
I know for the fact that Michelin Sports cost me at least 1.5-2mph on century when compared to Conti GP3000.
I had a feeling this was the case, but...Cima Coppi
Oct 12, 2001 6:27 AM
Out on the road, they do not feel like they hold back like they do on the rollers. Can the rollers exagerate the resistance of the tires?

Oct 12, 2001 7:02 AM
there's a believe that most of resistance come from sidewall deflection, not from contact patch. Obviously deflection will be much higher on rollers, and 33TPI isn't something easy to bend.

Also contact patch would be different and probably causes slip on the edges /have to admit Sports are really sticky comparing to low/mid level Conties/, more resistance

other thing I am using much wider tires (28mm) this should extravagate the difference

interestingly Conti engineer I spoke to told me that 25mm Conties have slightly lower rolling resistance then 23mm due to the fact that contact patch is more round
correct observationjacques
Oct 12, 2001 6:54 AM
Everything else being the same, the smaller the wheel the larger the rolling resistance. It's elementary physics. It's also one argument against small front wheel time trial bikes.

The reason you don't notice it on the road is because the combined forward-moving rider-bicycle mass plus the rotational mass of the wheels easily overcome this added resistance.

On rollers, however, all you have is rotational wheel mass. That's so little, especially if you're using light racing wheels, that you notice added rolling resistance immediately.
Jacques is rightcyclopathic
Oct 12, 2001 7:07 AM
at 20mph 80% of your energy goes to fight grade/air resistance, only 10-20% against rolling. Opposite on rollers :)
I certainly have no complaints...Cima Coppi
Oct 12, 2001 7:12 AM
As I have discovered a cheap way to gain resistance!! The Michelin tires cost me all of $7/tire during a Performance sale.

Thanks all.