Rollers: the diameter/resistance connection

[marathon marke]

Does anyone know if 3-inch drums should feel a lot harder to turn over than 4.5-inch drums?

Twenty-five years ago, I had set of the original Al Kreitler rollers, with 4.5-inch diameter drums. I regularly rode them for 2+ hours at a time, even doing "double-centuries" for trade shows to push my sponsor's products. I sold them after using them for a few years.

A few weeks ago, I purchase a set of "reduced diameter", 3-inch rollers from Nashbar. The first thing I've noticed is how much harder it is to keep these things turning than my old Kreitlers. It's not like I'm out of shape. I've logged 6,000 miles since April of this year. My weekly mileage has gone down from 300+ to around 200, since the colder weather is setting in. But right now, I'm even having a problem keeping them turning for over 30 minutes. When I free up the belt from the rollers to see if they roll freely, everything seems okay.

Does anyone have any experience with different drum diameters, and if these two sizes would vary that much in resistance?

 

[tubularglue]

http://www.bikeforums.net/archive/index.php/t-43404.html


i've got 4.5 kreitler's that i need to give back to a friend

and just purchased a set of elite parabolic 3.5" to replace, slight difference.

 

[ambrose]

It would make sense. From a basic physics point of view, obviously depending on what they are made out of and their weight distribution, the 4.5 would have higher inertia (ie they want to keep spinning) (i hope i haven't oversimplified this as i like to do). My personal experience would also agree with this. I think there would also be a difference due to the way the tyre reacts with the rollers. the force would be distributed over a greater area. i believe this would effect how easy they spin up, but am not completley sure.

 

[David Suro]

Physics:

Force x Force arm = Resistance x Resistance Arm

When on the rollers, the force and force arm are constants generated from the rider on the bike. The resistance arm is the radius of the cam of the roller.

So, a cam of 4.5 inch diameter has a radius of 2.25 inches and a cam of a 3 inch roller has a radius of 1.5 inches.

Thus, the 4.5 inch cam rollers have a 50% reducation in rolling resistance when compared to the 3 inch rollers.

Wind resistance in the spokes and bearing resistance in the bicycle are greater at higher speeds, but that still does not change the fact that riding on rollers with larger cams gives a reducation in overall resistance, thus making it easier.

I once rode rollers with 6 inch cams. What fun! The 52 x 13 was a breeze!

 

[orbeas]

training rollers

The other thing to remember is tyre choice, modern tyres are very 'sticky' and make it hard to pedal on rollers, so use a cheap hard type of tyre, or the special tyre made for home trainers.
I have a bike with olf fashioned tubs on and it much easier to ride than my other bike with modern clinchers on which tend to grip the rollers and slow them down.

 

[marathon marke]

Thanks for the replies.
David, after reviewing a link on the Kreitler roller website, it seems your estimations are quite close to what they say as far as how much more resistance a 3" drum will have over a 4.5". I think they said a 40% increase.

That link that tubularglue gave was very interesting too.

And orbeas's suggestion that the newer stickier tires of today will increase rolling resistance makes sense too. In my case though, I'm riding Hutchinson Equinox tires which seem to be a pretty hard, non-sticky compound. I just bought these because they were very cheap and they are actually holding up very well on the road. On the other hand, I wonder if the low 66 tpi of the casing would add more resistance on rollers? Any thoughts?

I have also come to the conclusion after reading the Kreitler website that the 3" rollers won't allow easy spinning, as the 4.5" rollers do. I'm sure this is due to a smaller flywheel effect I get from the smaller diameter rollers. I might try to fabricate my own external flywheel from an old Blackburn windtrainer I have.

Thanks!