Rolling Resistance

[omgwtfbyobbq]

Does anyone have any info on what bearing friction is compared to rolling friction? I've seen that increasing tire diameter lowers rolling resistance marginally, but I haven't found anything stating why it does that. Since rolling resistance is supposedly caused by both the ground and tire deforming, it follows that a harder compound and more pressure reduce the tire deformation and rolling resistance, but increases in diameter shouldn't do anything between the tire and the road. It will however, result in the bearings spinning less compared to the tire, so if the resistance of the bearings/races is greater than the resistance of the tire/ground, an increase in diameter should minimize this up to a point, and lower the total force the tire has to overcome. Thoughts, doughnuts, skunks?

Anyone (who doesn't have sealed bearings, I H8 'em ) try synthetic greases?

[GeoMetry]

Another thing to concider is the rotational weight. In general when you increase the diameter you end up increasing the weight of the wheel/tire. I don't claim to understand it but this is something that can have a measurable negative effect.

[Silveredwings]

I would think the larger diameter would have atleast 2 (qualitative) effects:
1. the larger diameter tire might mean less angular deflection where the rubber meets the road (sorry ).
2. a larger tire should much more easily roll over the irregularities in the road surface.

[onegammyleg]

All sounds to me like a good reason to drive slower.

[Silveredwings]

Originally Posted by onegammyleg

All sounds to me like a good reason to drive slower.

I didn't think rolling resistance was speed-dependent.

[JanGeo]

just a little dependant - linear - not exponential like air drag

[omgwtfbyobbq]

Hi Silveredwings, aren't No.1 and No.2 the same thing (1=>2), or for that matter, how much does angular defelection depend on tread/pressure? What about the additional weight of a larger wheel/tire?

I had a comment that states that a reduction bearing rotation helps, a little.

Bearing drag is fairly low compared with tire resistance, but increasing the ratio of wheel diameter to bearing diameter will help. A little.

And also that

If the width, load and pressure are the same, bigger diameter should have less tread and sidewall distortion, so it should have lower losses due to hysteresis, however it will have a higher polar moment.

So, fewer losses due to tire flex, but more mass. Given how much compound/tread desgn seems to play a part it seems like size isn't much of a concern at all... Which seems pretty obvious after checking out greenseal's 2002(?) report.

[JanGeo]

If that was true then Bicycle tires would be small. The bottom has to be flat to match the road so the larger the diameter the less it has to flex to get flat on the road.

[omgwtfbyobbq]

But bicycle tires are small.

[JanGeo]

26 inch diameter about the size of a car tire is not what I consider small - 10 -12 inches is small.

[omgwtfbyobbq]

Oh you mean wheel diameter. If you hold a 26" bicycle wheel/tire up to a 14" car/tire you'll find they aren't very far off in terms of size (for instance my road bikes tires/wheels are only a few inches taller than the 13" tires/wheels on my car, and a bit smaller than the 15" tires/wheels on my truck), since car tires have way more sidewall compared to bike tires. The change in angle wrt the outer diameter isn't very different. In fact, other confounds, like lack of a suspension in many bikes, leads to small bike tires having a lot of give to them, so it's not exactly a tomato tomatoe deal compared to cars, where the suspension dissapates energy from rough riding.

[Silveredwings]

Originally Posted by omgwtfbyobbq

Hi Silveredwings, aren't No.1 and No.2 the same thing (1=>2), or for that matter, how much does angular defelection depend on tread/pressure? What about the additional weight of a larger wheel/tire?

I had a comment that states that a reduction bearing rotation helps, a little.

And also that

So, fewer losses due to tire flex, but more mass. Given how much compound/tread desgn seems to play a part it seems like size isn't much of a concern at all... Which seems pretty obvious after checking out greenseal's 2002(?) report.

regarding 1 vs. 2: You make a good point. I do think they are different. Treadflex on smooth ground may prove to be more a function of the tire's design, whereas a tire's ability to roll over irregularities seems to be a direct function of diameter. Look at exaggerations: which would be more difficult to roll over a curb, a 3" wheel, or a 60" wheel? The road just seems to be a series of irregularities.

[omgwtfbyobbq]

Ah I gotcha, the designated inflation pressure can be different for different tires which impacts treadflex, but diameter/mass is directly related to the polar moment and the ability to roll over much rougher obstacles. I see... Thanks for the info!