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Post by zonagravedad on Sept 23, 2011 17:07:06 GMT
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Post by zonagravedad on Sept 23, 2011 17:17:20 GMT
- Race 1: Keeping the same diameter, a weel accelerates faster than other if it's weight is near the axle ( heavy rim, light tire ). - Race 2: If weight distribution is the same, total wheel weight doesn't matter. - Race 3: Similar conclussion to race 1.
I usually alternate between 12 1/2 an 16" tires ( both using the same 8" rim ) and I've found in practice that small wheels accelerate faster but big ( and heavier ) wheels keep momentum longer and are more stable. Both are fast, but 16" is slightly faster in my opinion ( or, at least, more stable at high speeds ).
When the track is twisty, full of hairpins and a lot of brake areas, I'm faster with 12 1/2 tires, when track is fast, with many fast sweepers and few hairpins, I'm faster with 16" tires.
Of course, you need to have two complete sets of rims to change them reasonabily quick.
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Post by Scottish Cartie Association on Sept 23, 2011 19:13:01 GMT
The observation that "lighter wheels are faster" is a simplification. You're correct that it should be "wheels with a smaller moment of inertia are faster". There is a good discussion of it, plus all the maths involved, here. The interesting thing is that the radius of the wheels in the moment of inertia calculation is cancelled out, leaving only the mass of the wheel to affect the net forces on the cartie. Look at race 2 again. The lighter wheel has the lower moment of inertia, but because it is lighter there is less gravitational force acting on it as well. The two balance out and the net result is that both wheels accelerate at the same rate. However, this is not an accurate model of a cartie, since a cartie also has the mass of the body as well to provide force to accelerate the wheels, and the lighter wheel can be accelerated quicker because it has a lower moment of inertia. When attached to a cartie (all other things being equal) a lighter wheel is faster.
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Post by zonagravedad on Sept 30, 2011 15:19:50 GMT
Hello: Last Euroean Championships were held in a quite aesy ( and boring ) track. See results ( C4 class ) in www.zonagravedad.com/images/Cronicas/2011/Moras_en_Valloire/Resultat_Moras.pdfThe wheel size distribution was:- About 75% of riders used 16" tires in 8" rims. Top 13 riders used them. On-board video ( chrono was 1'21"6 ) www.youtube.com/watch?v=PF4YKDgJjYg- About 25% of riders used 12 1/2 " tires in 8" rims. Best one was 14th classified. On board video ( chrono was 1'21"4 ) www.youtube.com/watch?v=RwOMzm0Sy1Y- Just 1 rider used bicycle 16" spoked wheels and tyres ( UK style but 16" instead of 20" ). He finished 39th. On board video ( chrono was 1'24"1 ) www.youtube.com/watch?v=ILFcjjWfkHQ1st and 3rd videos show the ride style differences between light 16" spoked wheels and heavy 16" with 8" rim. The second one keeps a lot of momentum that makes ride more stable and less "nervous". The results table gives these data:1st row: Total time ( between brackets: position ). 2nd row: Time from start to control point - just before 1st tight bent, all easy open bends, so it only acounts for acceleration and rolling resistance. ( between brackets: position ). 3rd row: Speed at control point.( between brackets: position ). For the best riders using each kind of wheel on its best heat:- Big 16" tyres with 8" rims: Number 461: 1'19"5 ( 1 ) / 34"3 ( 24 ) / 70,7 Km/h ( 9 ) - Small 12 tyres 1/2" with 8" rims. Number 404: 1'21"4 ( 10 ) / 34"1 ( 2 ) / 69,0 Km/h ( 24 ) - Spoked 16" bicycle wheels. Number 428: 1'23"9 ( 29 ) / 34"8 ( 40 ) / 71,7 Km/h ( 1 ) The spoked wheel gave the MAXIMUM SPEED, but it has just the 40th. position in control point. It looks like it has a high speed advantage but it accelerates slower. It's best total time is 29th ( 4,5 seconds slower ) on a track that has jus one hairpin and one 90º bent with sligh braking, everything are easy sweepers. It looks like the max speed gain is much less than the bending speed loss and the worse slow speed acceleration.The 12 1/2 tyre has a very fast intermediate time with a discrete max speed ( it's the fastest accelerating one ). The 16" with 8" rim is average in acceleration, average in max speed but the fastest overall.
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Post by Scottish Cartie Association on Sept 30, 2011 16:25:42 GMT
I could counter your observations by saying that I have seen many carties with small fat wheels, but I have never seen one win anything, but there is no point because the plural of "anecdote" is not "data".
Your analysis doesn't tell you anything about the underlying physics of what is happening, since there are far too many variables between the various carties for you to draw any conclusions. Each cartie has a different mass, drag coefficient, rolling resistance, etc, so to draw the conclusion that their relative performance is related only to the wheel choice is a gross over-simplication.
For a proper experiment you'd need to try different wheel mass on the same cartie to be able to discount other variables, but in fact you don't need to do the experiment because you can work it out using the principle of conservation of energy.
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Post by team-art on Sept 30, 2011 22:10:20 GMT
Hi Calixto, “The second one keeps a lot of momentum that makes ride more stable and less "nervous"Personally I think it’s more likely to be the grip/width of contact patch that makes ride more stable and less "nervous" and not the weight. It might even be the larger air volume giving more damping that helps with more stable and less "nervous" Thanks for making the effort to post all that in English. Great to have your observations/opinions/links. Some very interesting stats there, keep it coming please. Very close times. 20th /61(in Europe) not bad CHeers Rich
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