morey000 wrote:
tribern wrote:
RacerMate Inc (ComupTrainer) uses the term "Press On Force" to describe the function of tightening or loosening the pressure with which the trainer's roller is pressing on the tyre. Increasing Press On Force (tightening) is like having a heavier bike/rider on the road. The extra weight presses the tyre harder onto the ground, increasing the rolling resistance.
If you doubt this, ride your trainer at 3 turns of the knob for 5 minutes at 20mph and without changing gears. Watch your power numbers (or check the average after). Then get off the bike, tighten the knob 2 more turns (increasing the press on force) and ride for 5 minutes at 20mph as before. Compare the power numbers. You will need to hold a higher average wattage to ride the same speed with greater press on force on your trainer/tyre.
Seems to me that both are true. Tightening the roller against the tire will increase the rolling resistance just like being a heavier person. But- it also makes the wheel smaller. Given that I've read enough tire reviews where apparently small differences in Crr equate to 10-15watts- it doesn't seem unreasonable that Press-on-Force would make a similar difference in magnitude. So- these would counter each other out to some effect.
The question is- which effect is larger?- and are they close enough such that a different tire type or air pressure might make a third, equally sized change. I know one way to reduce my virtual power a bit- change from the Conti GP4000S I've got on there now (it's my road bike that goes on and off the trainer), and put on a training tire. Seems to me that there are really too many variables to make this a simple calculation. Yup Nate- we need that roll-down test.
In the end- I guess it doesn't matter all that much. I still put in over 500 in TSS last week, my legs are fatigued, and I'm good.
you're right in the fact that they both make a difference. quoting myself:
nolken wrote:
okay i'll admit, an increased pressure can make it harder to turn the tires. BUT under the weight of an average human or the pressure applied by the roller, it's effects would be so incredibly small you can pretty much not even think about it. a ridiculously small value that isn't even worth considering. the change in radius has a FAR greater effect.
like i said the Crr does make a difference, but it will be much smaller than the effect you get from changing the radius. I will post how to properly set up a trainer to account for these changes, but i just dont have time right now. maybe this weekend. i'll also do a non-scientific test of it this weekend if i have the time.
Reid did say in a previous post:
Reid Weber wrote:
The tests I did with our motor setup all showed that an increase in power was needed to maintain the same speed as tension increased. I measured speed on the bike wheel without adjusting the wheel circumference with each adjustment.
so the Crr and the decreased radius do not cancel each other much.
you know we could just email the guys at KK and ask them.. i'm sure they would have an idea ;)