Rolling resistance question

[matticus]

I've never looked at the numbers (doesn't affect my life!) but at a low enough speed, your weight (+bike) will be a much bigger number than your rolling resistance, won't it? (especially considering the edge condition of trying to keep velocity above zero)

 

[MichaelW2]

All true, and obviously the speed at which there is a change over between whether AR or RR dominates relies on many factors, but I think I'm right in saying that in most cases that speed will be pretty close to zero.

AR will almost always dominate.

I have no idea what any of this has to do with cruising speed.

According to an unreferenced graph ( with no description of data, bikes, riders, sample size etc) at

https://bicycles.stackexchange.com/...ed-does-aerodynamics-of-a-bike-come-into-play

RR = AR at about 12mph.
Below that speed, RR dominates drag forces, above it, AR dominates.
12mph is also a very common every day cruising speed

 

[Ajax Bay]

Climbing a steep ^^ hill (?6mph), compared to force due to gravity, both rolling resistance and air drag are small.

A linear relationship. Proportional. Double the speed, double the resistance?
Now resistance and power are not the same thing...

I suggest the rolling resistance force is not dependent on speed.
"Calculating rolling resistance
This equation is used to calculate the force of rolling resistance from each tyre:
FRR = CRR x weight

• FRR is the rolling resistance force.
• CRR is the coefficient of rolling resistance (a number used to rate each tyre at a given pressure),
• weight is the force due to gravity being exerted on each tyre (mg).

This equation shows that speed does not affect the force due to rolling resistance. The type of tyre and the mass of the bike plus rider are the only two variables."

Maybe you meant rolling resistance in power terms (in which case 'linear' with speed = tick).

 

[faster]

According to an unreferenced graph ( with no description of data, bikes, riders, sample size etc) at

https://bicycles.stackexchange.com/...ed-does-aerodynamics-of-a-bike-come-into-play

RR = AR at about 12mph.
Below that speed, RR dominates drag forces, above it, AR dominates.
12mph is also a very common every day cruising speed

That's an interesting graph! The speed is much higher than I expected. I'm not entirely sure it all rings true though, as I was under the impression that in absolute terms rolling resistance was only a handful of watts (see that bicycle rolling resistance website), whereas to do 12mph would require quite a few watts - maybe 100? I just can't imagine rolling resistance being that high. Maybe that is the graph for a really grim set of puncture proof tyres like Marathon +?

I'd heard, and thought it sounded quite reasonable, that at even at 10mph 90% of your effort was being consumed by air resistance (I realise there are other losses, transmission losses etc., but I think these are negligible compared to RR and AR). I have absolutely no data/source to back this up though!

On the other hand, I do feel like the reduction in effort when using a decent set of tyres is easily noticeable, so maybe it is right.

I still have no idea what this has to do with cruising speed.

 

[Ming the Merciless]

Go to this website and find the two tyres you have

https://www.bicyclerollingresistance.com/

The one with lower rolling resistance will be better for your hill climbing. Grip is of no practical issue unless it's wet and really steep

Depending on which tyres it's quite easy to have, say, 10 watts difference in resistance between pairs of tyres which is quite a lot

Not when climbing a steep hill it won’t. At that speed the difference will be much lower. Remember rolling resistance wattage losses scale linearly with speed. So 10 watts at 18 mph will only be 3 watts at 6 mph.

 

[Ming the Merciless]

@brucers As has been made clear either tyre is fine, it really doesn’t matter. Good luck with your steep hill. Do you know how steep and long the hill is?

 

[brucers]

@brucers As has been made clear either tyre is fine, it really doesn’t matter. Good luck with your steep hill. Do you know how steep and long the hill is?

It seems that way. Atm I have the higher resistance on and will go with them for now. The hill is probably about a third of a mile but the gradiant changes 4 times but I have no idea of the percentage ratio.

 

[Grant Fondo]

I am working at conquering a steep hill near where I live. I have 2 sets of tyres. All other things being equal, which am I better off using for the climb - do ones with a higher rolling resistance gain me any advantage on the hill due to their greater grip or is it no different to being on the flat? No other aspect is of concern, just which are best for the hill climb.

Always remember tackling the Golf Course hill on Lantau HK on MTB with 2.2 knobblies, switched to Schwalbe Kojaks and flew up it

 

[brucers]

Always remember tackling the Golf Course hill on Lantau HK on MTB with 2.2 knobblies, switched to Schwalbe Kojaks and flew up it

Whatever tyres I have, I will not get to flying speed. Haha.

 

[Grant Fondo]

Whatever tyres I have, I will not get to flying speed. Haha.

Going down was the hard bit, ie.not flying off the end of sharp corners

 

[MichaelW2]

That's an interesting graph! The speed is much higher than I expected. I'm not entirely sure it all rings true though, as I was under the impression that in absolute terms rolling resistance was only a handful of watts (see that bicycle rolling resistance website), whereas to do 12mph would require quite a few watts - maybe 100? I just can't imagine rolling resistance being that high. Maybe that is the graph for a really grim set of puncture proof tyres like Marathon +?

I'd heard, and thought it sounded quite reasonable, that at even at 10mph 90% of your effort was being consumed by air resistance (I realise there are other losses, transmission losses etc., but I think these are negligible compared to RR and AR). I have absolutely no data/source to back this up though!

On the other hand, I do feel like the reduction in effort when using a decent set of tyres is easily noticeable, so maybe it is right.

I still have no idea what this has to do with cruising speed.

According to:
http://www.gribble.org/cycling/power_v_speed.html
At 12mph, RR is about 20 watts and AR about 30 watts. You need to output about 50watts. The variables can be keyed into the calculator.

 

[Ajax Bay]

Steady speed along the flat at 12kph, RR in watts is 8W, Drag is 17W.
Climbing a 1:10 slope (0.1) the additional power needed (maintaining 12kph which is not unreasonable up a 1:10 hill) is 255W.
So as I said, RR and Drag are 'small'.
Assumptions: Drive train eff = 96%
Mass of bike + rider = 80kg
CRR = 0.003
Rho (air density) = 1.2kgm-3
CdA = 0.39
v = 3.3ms-1 (no wind)

 

[roley poley]

many factors ...a tyre with a smaller diameter will give you a lower gear from slick to pokey out knobbles ?