Trainer and ambient temperature.

Eebijeebi

I have a Jet Fluid pro or 2 - not sure.
Ive been looking at my indoor training figures for last year against this and average power is up but average 'speed' is down for the matching period, i.e. it would suggest that it's physically a bit harder to spin the drum this year.
I was thinking that it may be temperature related as I'm now in a cold garage instead of a warm lounge, but I looked at the few summer sessions I did and they too seemed to be a bit 'slower' in relation to watts too.
I know it doesn't make any difference (except in my head) but I don't like mysteries.
Does anyone know if these things deteriorate and change resistance over time perhaps? I'm not looking for an excuse to buy a new one but it would be nice to have consistency in the figures.

BrandonA

I go at zero miles an hour on my turbo as I don't actually move anywhere.

How are you measuring speed? Is it wheel resolutions? If it is then are you using a different gear? Or is your tyre pressure different?

sungod

the fluid viscosity will vary with temperature, this results in lower 'speed' vs. true power during warm up

you mention power, is this direct from a power meter, or extrapolated from wheel revolutions on the turbo? if it's the latter, it'll underestimate power until it heats up

BSRU

I found with my Fluid 2, that the colder it is the more power is required to generate a given back wheel speed.
But eventually once the fluid has warmed up, after 20 to 30 minutes less power is required for a given back wheel speed.

Eebijeebi

Power is from power meter.
"Speed' is from Garmin speed/cadence sensor, so yes, wheel rotations.

Things I get -
The temperature/viscosity of the fluid on warm up.

Things I don't get -
Why does tyre pressure make a difference (as long as the tyre isn't slipping?)
Why would gearing make a difference?

sungod

only power counts, the 'speed' on the turbo is irrelevant, ignore it

re different pressure/gearing, any physically different system will behave differently, this will include differences in frictional losses in the drivetrain and hysteresis losses in the tyre

markhewitt1978

Speed on the turbo is completely meaningless. Power is the only variable to look at.

Eebijeebi

I did say 'I know it doesn't make any difference' in the original post. I'm just trying to understand what causes it to be different now and take more watts to spin the drum at the same speed.
Call it a curious mind, but it seems there's no obvious answer. Perhaps the drum bearings aren't as free as they were. What ever the cause, it always took more watts on the turbo than it did on a flat road to get the same 'speed', now it's just more so.

markhewitt1978

Eebijeebi wrote:

I did say 'I know it doesn't make any difference' in the original post. I'm just trying to understand what causes it to be different now and take more watts to spin the drum at the same speed.
Call it a curious mind, but it seems there's no obvious answer. Perhaps the drum bearings aren't as free as they were. What ever the cause, it always took more watts on the turbo than it did on a flat road to get the same 'speed', now it's just more so.

As you say it's probably do to with the bearings now having a years worth of wear on them plus the grease will have evaporated a little etc so probably no surprise it's running a little less freely than it did last year.

buckmulligan

Are sure the tyre pressure and roller tension (i.e. how much the roller is tightened up to the tyre) are the same as previously? These make a big difference in the amount of watts lost to rolling resistance and would thus affect your rear wheel speed.

Doubling the roller tension will essentially double the virtual weight on the rear wheel; power to overcome rolling resistance = Crr * Mass * velocity * g, so you can do the maths yourself to see if that accounts for the discrepancies you're seeing but at ~200W you're looking in the order of 2-4 kph for reasonable variations in tyre pressure and mass.

Eebijeebi

Ok, so it's not just whether or not the wheel can slip it's about how hard the roller is pressing against it too?
Presumably the tighter on the wheel the harder to turn.
So, no I can't say that the roller is clamped up to the same tightness, indeed I don't see how I could do that with mine. It's likely to vary every time the bike goes on/off.
So, ambient temperature (cold garage, not warm kitchen), tyre pressure and tightness of roller on wheel can all have an effect.
That certainly explains variations and satisfies my curiosity, thanks to all.