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rolling resistance


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say to compare, what kind of percentage difference in power transmitted to the rolling surface would be noticeable between:

7.50 R 16 / similar eg 235/85 R 16 on 16" rims [5.5" width]

285/55 R 18 on 18" rims [possibly 7.5-8" width]

[all should be in the region of 31" total laden effective diameter]

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well my understanding may not be perfect either, although I'd think that gearing is related mainly only to the effective diameter of the wheel + tyre [although a bit of the power business must be relevant to gearing too, the less power transmitted to the road, the less possible higher gears are], while the higher the profile of the tyre, unless it is massively overinflated, there'll be a certain amount of rolling resistance basically from the need to keep moving the 'flatter bit' underneath the tyre, around, plus the 'bow wave' effect of doing so.

I hope I'm describing it relatively clearly, that's my understanding of it at least.

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more than anything else, the tread pattern is going to affect your rolling resistance.

assuming they are the same tread bias ( like road / AT ) then i reckon you'd feel about a 5% difference as a guestimate in terms of getting upto speed. If your chaning from MT to road bies tread, then i reckon the road tyres would see a noticable improvement

However, you may also find that the wider tyres tramline a little bit more on certain roads ( follow the bumps and cambers ) . I find that to the be the case when i've gone to wider tyres.

Another thing will be you'll possibly find the low speed steering heavier too.

You'd need to specify which treads types. To me, they'll make the most difference

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I really should have 70% road tyres for the places I drive, although for snow etcetera I've so far tended to 50/50 tyres, seem a nice balance to me, so I'll stick with similar tread, while hopefully improving road manners with the width/lower profile; the width a mixed blessing off-road.

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The height of the tyre will affect the amount of speed transmitted, the pattern/construction/sidewall height/mass of vehicle/road surface etc will affect the amount of power transmitted. The first one is easy to calculate, draw a line and roll the vehicle etc. The second's much more vague, and changes dynamically as you drive too (ie in cornering).

I've been dealing with this as I develop the GPS "Rolling Road". I basically take a best guess for aero and rolling resistance coefficients as part of the calibration. At the moment that guess sits around 0.1 for BFG TracEdges (but I suspect that contains an awful lot of mechanical losses and transmission windage as well).

Bosch's Handbuch Automobil puts pneumatic tyres on a tarmac road at 0.025, on a 'field' at 0.1-0.35. Interestingly, steel wheel on a rail comes in at 0.002.

Be careful with people who rate tyres as 50/50 on/offroad or similar - compared to what?!?



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good stuff, I certainly understand better now at least. I suppose the power difference is kind of evanescent until the V8 engine is properly [overhauled it looks as though it is now, low oil pressure apparently being a sign that mains need replacing] then put in the 110, although a lower profile would at least improve cornering on winding mountain roads.


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