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Discussion Starter #1
Does anyone know why the gearing makes so much of a difference when it comes to larger tires? The reason that i put the question out there is because 33 only weigh an average of 8-11 pds more per tire.
 

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The tire size changes the final ratio, AND you are dealing with rotational weight.
 

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The width of the tire directly affects the tread where a lot of the weight is concentrated. It's not just the increase in weight, it's where a lot of the weight is added.
 

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Tire weight has very little to do with it, to the point where it's negligible to anything other than a race car. Tire/rim weights of 10-15lbs more will change the 0-60 time of a car by only 1-3 tenths of a second, as long as the tire is the same diameter. Braking is likewise minimally affected by tire weight but highly affected by tire diameter, but to a somewhat lesser degree because your stock brakes will still be able to lock up the tires at any speed (your brakes are much more powerful than your engine, and braking performance is tire friction limited).

The overall diameter of the tire is what makes the difference. Tires are the final "gear" in the driveline. They're the gear that acts directly on the road, and they use the friction of the rubber rather than teeth in a metal gear. Increasing the diameter of that gear does the same thing as increasing the diameter of any other gear. It reduces the amount of torque being transmitted. Changing the gears in the rear end to compensate and equalize for the tires is the only way to compensate.

Switching to larger diameter tires is the exact same as swapping from say, 3.73 gears for 2.73 gears; tires ARE gears.
 

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If you really want to get technical, larger tires impact performance in two ways on any vehicle with wheels not just race cars and Jeeps. The first is what you are referring to impacts the final drive ratio. The other is the Mass Moment of Inertia (MMI) which affected both by increase of tire radius and the weight or mass of the tire. Braking must counter the MMI so mass of tire absolutely has an impact. Otherwise it would be just as easy to stop a 2 ton 33" tire as it would and ideal weightless tire. With all due respect tires are a lot more than gears.
 

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Discussion Starter #8
Changing the gears in the rear end to compensate and equalize for the tires is the only way to compensate. Switching to larger diameter tires is the exact same as swapping from say, 3.73 gears for 2.73 gears; tires ARE gears.
Very well put, definitely makes more sense.
 

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The " gear change" from bigger tires essentially raises the "final gear" , the tire, in the cog. As well as the additional rolling resistance and weight as minor factors..

Edit... Should read entire thread before answering. The above says it better than I...
 

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If you really want to get technical, larger tires impact performance in two ways on any vehicle with wheels not just race cars and Jeeps. The first is what you are referring to impacts the final drive ratio. The other is the Mass Moment of Inertia (MMI) which affected both by increase of tire radius and the weight or mass of the tire. Braking must counter the MMI so mass of tire absolutely has an impact. Otherwise it would be just as easy to stop a 2 ton 33" tire as it would and ideal weightless tire. With all due respect tires are a lot more than gears.
Yes, moment of inertia makes a difference. The easiest way to see this would be if you had the rear tires off the ground and hit the gas. The heavier tires would take measurably longer to spin up. But once you introduce the resistance of the weight of the vehicle itself, the effect of moment of inertia would be a very small percentage of the change in performance. Once the tire is actually on the ground, the change in leverage due to the fact that the tire really is just a larger gear....plays a much larger role in things.

You can see this same effect in bicycles. I'm an avid road biker, and people get really crazy about weight with bicycles. They use ultra thin tires, and ultra thing tubes, because that lowers moment of inertia. I used to use those super-thin tubes in the tires, and after moving to arizona, I was replacing tubes weekly due to the small thorns all over every road. The "thorn proof" tubes the bike shops sold were more than twice as heavy...but they made no measurable difference to my times. Meanwhile, if you were to switch to different tire diameters on a bicycle...the performance difference would be immediately noticable, and would have to be compensated for by gearing.


So yes, moment of inertia has an effect, but not nearly as much as the raw change in gearing. When it comes to braking, moment of inertia has the same small effect. The leverage (torque) provided by the larger diameter tires isn't anywhere near enough to overcome the brake system's ability to fully lock up the tires, but it does slow it down by a fraction of a second during each cycle of the ABS, which is enough to increase braking distances by 20+ feet from 70mph. The raw weight of the vehicle plays a larger role.

Your example of 2 ton tires compared to weightless tires would of course make a HUGE difference in performance....but tires don't weigh the same as the vehicle. :)
 

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The Unsprung weight of heavy tires and their effect is measureable. There is a formula, (I'll have to look for it again), that determines the total weight effect on a vehicle with heavy wheels and tires.. It makes more difference then you think.. I believe it was somewhere in the neighbourhood of every 20lb extra tire (unsprung/rotational) weight is = to 100lb cargo weight. Again this is just a ballpark and there is a calculation for the ratio of sprung to unsprung weight that is more accurate.
 
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