Challenger Posted December 24, 2008 Share Posted December 24, 2008 Is the moment greater for a 17 inch rim or a 16 inch rim with equal overall tire height, width? Quote Link to comment Share on other sites More sharing options...
garvice Posted December 24, 2008 Share Posted December 24, 2008 It would depend on where the weight is. If the 17 inch wheel puts the weight closer to the edge then yes it will be a greater moment. I can't say for sure, but I would bet that putting the rim further to the edge of the wheel is putting the weight closer to the edge of the wheel (I think that the metal is heavier than rubber. I.E. rim is going to be heavier than tire, so larger wheel smaller tire is heavier wheel and lighter section of the wheel is a thinner section), so I would say that the moment is greater with a 17 inch wheel. It would be easy to do the math though. Just find the weight of the two rims and the weights of the two tires. Quote Link to comment Share on other sites More sharing options...
garvice Posted December 24, 2008 Share Posted December 24, 2008 Actually (Been thinking more, It's Christmas eve so I am a bit slow). The heaviest part of most rims would have to be the outer edge (Greatest amount of metal). So moving this edge closer to the outside of the Wheel would make a greater moment. The Tire has most of its weight at the outside edge as well. So using a lower profile tire shouldn't change the moment a great deal. Quote Link to comment Share on other sites More sharing options...
240hoke Posted December 24, 2008 Share Posted December 24, 2008 That is a ridiculously vague question, however Tires are usually heavier then your wheels...unless you have steel wheels or some blinging chrome 22's. I took a quick look on Kuhmo's website at tire weights and a 17" tire of the same overall diameter is going to weight several pounds less then a 16" tire since you are losing sidewall. Then I took a look at a wheels specifically Kosei K1's, 16" weighs 15lb, while the 17" weighs 16.8. The total combo is actually going to be lighter in a 17 then a 16". Im going to guess the MOI may be very close to the same if not lower for the larger wheels....but I think is a moot point anyway. Quote Link to comment Share on other sites More sharing options...
johnc Posted December 24, 2008 Share Posted December 24, 2008 I did this research a few years ago and it mirrors what 240hoke posted above. But, it is very tire and rim specific so you must compare the same brand and model of tire and the same brand and model of wheel. A 1" OD wheel difference is not that big a deal as far as MOI is concerned. Going from a 15" to a 17" is more significant. Quote Link to comment Share on other sites More sharing options...
Challenger Posted December 24, 2008 Author Share Posted December 24, 2008 I guess it was kind of vague, It was late. Maybe just imagine two identical wheels with the same design. Just one had been stretched the extra inch. (1/2 inch longer spokes) Same with the tire, both tires are made of the same compound, just one has a larger ID. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 24, 2008 Share Posted December 24, 2008 Assuming the same wheel manufacturer, and even if the two wheels were the same weight, the moment arm of the larger wheel will be longer because of the rim's construction at the outermost end. In other words, it's not the spokes that are heavy. Its the "rim of the rim" and the drop center that weighs a lot. Tires are heavy so using a lower profile tire may save more weight than using a smaller rim, but if you compare equal profile tires (again, same for same) then I don't see how the larger rim could possibly have an advantage. The larger rim has the drop center further from the hub, and the tire has its mass further from the hub as well. Contrast that to the lower cg that one gets from running a smaller tire and resultant lower amount of weight transfer, and I think the advantages of a smaller rim/tire combo are complete. Less weight, smaller moment arm, lower cg, less weight transfer. The cost of all of these benefits is the smaller (shorter) tread patch, and the limitation as to the size of brakes that will fit inside the smaller wheel. You see the same discussions with flywheels, people talk about a heavier flywheel acting the same as a lighter one because the weight is removed from the "inertia ring". John also points out that the rim and tire choice makes a huge difference as well, so that makes this a theoretical and not so much a practical discussion. Quote Link to comment Share on other sites More sharing options...
tube80z Posted December 25, 2008 Share Posted December 25, 2008 If you're really curious you can measure this by placing a known weight on the outside of the tire and measure the period as if it were a pendulum. Think 20 pound block of metal hooked to the tire with a nylon strap. Remember F(measured in hertz) is =1/T (period of the pendulum). And T=2*Pi*SQRT(I/m*g*L) m is your mass g is the gravitational constant L is the length from center And I is what you want to measure (moment). Now you have a simple experiment to keep you busy over the holidays. Merry Christmas, Cary Quote Link to comment Share on other sites More sharing options...
Challenger Posted December 25, 2008 Author Share Posted December 25, 2008 Yes you could attach equal weights to the two and then see which weight hits the ground first. Problem is Id need to get a 17 RBR and tires for my 16's. Would be an expensive experiment... Or if someone knows a function for the mass distribution of a 17 in and 16 RBR... Or even easier is just roll the two. Which one speeds up the quickest will have the smaller moment. Correct? Quote Link to comment Share on other sites More sharing options...
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