Mathematicians please

[Phil S]

If a 160lb rider on 700x25c tyres at 110psi needs to put out 219w to achieve 24mph on rollers, will a 150-152lb rider on 700x23c tyres require more or less watts at the same speed, if so how much? Sorry I was a linguist at school and a disaster at maths.

 

[Dietmar]

If a 160lb rider on 700x25c tyres at 110psi needs to put out 219w to achieve 24mph on rollers, will a 150-152lb rider on 700x23c tyres require more or less watts at the same speed, if so how much? Sorry I was a linguist at school and a disaster at maths.

This is not a question of mathematics but of physics, and the physics of what you are asking is quite complex. As a mathematician, I would simply tell you that I have no idea, since you do not give me any equations that I could apply to this problem. As a physicist or an engineer, I would tell you that I do not have enough information to determine what the combined effect of the simultaneous change in the two parameters you mention (tire size and load) might be qualitatively, let alone quantitatively.

 

[alienator]

If a 160lb rider on ......blah, blah, blah........you can't get a numerical answer.

Ooops. I forgot the riders were of two different sizes.......
The following can contribute to differences in power output (these are just some things not all):

• The efficiency of each rider.
• The difference in drivetrain efficiency between bikes
• state of maintenance of each bike
• differences in bearing drag between wheelset
• Inner tube type
• Differences in rolling resistance for each bike
• Changes in efficiency of the rollers as a result of the loads from two different riders.

If the bikes are different, then that adds more variables to consider. Then you need to consider how large of a difference must be to be considered significant: would it be a difference in tenths, hundreths, ten-thousandths,.....

 

[acoggan]

If a 160lb rider on 700x25c tyres at 110psi needs to put out 219w to achieve 24mph on rollers, will a 150-152lb rider on 700x23c tyres require more or less watts at the same speed, if so how much? Sorry I was a linguist at school and a disaster at maths.

I would expect that the power requirement would be very similar.

 

[frenchyge]

If a 160lb rider on 700x25c tyres at 110psi needs to put out 219w to achieve 24mph on rollers, will a 150-152lb rider on 700x23c tyres require more or less watts at the same speed, if so how much?

Same tire (other than size), or different brands. If different, which brand/model for each?

Sorry I was a linguist at school and a disaster at maths.

Is it safe to assume that the 219w was measured with a powertap, while the other rider doesn't have a powermeter? If so, could you just put the 23c tire on the powertap wheel and measure the smaller rider's output directly?

 

[Phil S]

Thanks, Andy's answer is the one I'm looking for, i.e. the relatively simple one. It's to determine the size of the rollers I need for my purchase. Kreitler's website gives a wattage and speed charge with typical power outputs for a rider slightly heavier than me and with fatter tyres.
http://www.kreitler.com/wattage.php

...but I may end up getting Cycleops rollers with resistance settings instead. Decisions, decisions.

 

[root]

The power requirements would be very similar. This is on rollers and the standard things like air drag of both the rider and the tires do not come into play so much as they would on the road. But even on the road, weight of the rider does not come into play on flats so much as aero dynamics of the rider and the bike, rolling resistance of the tires etc.

I would guess the rolling resistance of the tire could play the biggest effect here, and if that is not much different then the power would be very very similar.

 

[mattyb]

Been down this research road before. The above is correct.
You are not propelling the bike forward, you are turn the roller, so if you use the exact same bike and measure power from two different weighted riders for the same speed, the power output will be the same. Why? You are still turning the same roller. It requires the same power to get it to spin whether you weigh 50kgs or 100kgs.

From everything that I could find, the tyre should not make a significant difference.
The only significant difference will be the effect on the rider. 24mph will be a different percentage of FTP.

 

[artemidorus]

You are still turning the same roller. It requires the same power to get it to spin whether you weigh 50kgs or 100kgs.

I don't agree, a heavier rider will deform a given tyre more on the same roller, and a wider tyre will deform over the roller in a different way. I don't think that the power differences would be big in the scenario described.

 

[alienator]

I don't agree, a heavier rider will deform a given tyre more on the same roller, and a wider tyre will deform over the roller in a different way. I don't think that the power differences would be big in the scenario described.

You are right, as I said earlier. However, Crr can produce significant power differences. Bigger rider-->increased Crr. Bigger tire-->decreased Crr. Rollers are a great place to use the biggest tires you can: you get the least rolling resistance; you get the best tire wear; and the increased aero cross section of the bigger tire is not a factor. Given how significant tire wear is on rollers, using the biggest tires possible seems like a gimme.

 

[artemidorus]

You are right, as I said earlier.

Sorry, sometimes these "debates" seem to become opinion polls, and I wanted register my vote for what seems the right answer.