perfect sprocket size?

[LiveWire]

The speedo accuracy is somewhat irrelevant. My numbers are based on going the same top speed, whatever that is. Gearing for peak HP (12K?) in top gear would be like an 89 tooth sprocket. Gearing lower and letting the engine get closer to peak power will probably allow adding top speed. If you ran a 72 tooth and you were at 9000 RPM in 5th, then you would now be around 9700 in 6th at the same speed. If that speed was 65 and the little extra RPM allows the engine to make more power and pull you higher in speed and RPM, you would be around 80 at 12000.

[fabr]

DAMMIT!!! I posted this thing here and it's fubarred itself somewhere. So here it is again.

22" tire,7400 rpm at the countershaft (1.744 primary reduction @13000 redline rpm)

    Now (3.94 final)                    14/72(5.14 final)             13/72(5.54 final)
1st   40mph                                    31                                      28
2nd   60                                          46                                      42
75                                                   57                                       53
88                                                   67                                      62 
100                                                 77                                      71
113                                                 86                                      80


 Now those assume that you can pull to redline but that will not happen as you saw from the low rpm maxed out at with this too high gearing you have now. Personally I'd take doug up on his offer and use the sprocket he is suggesting. You should check out what the diameter is tho and see if it screws up your ground clearance.

[Voodoochikin04]

i have a 1/4" steel plate on the bottom that runs along the sprocket that would still protect a 72 sprocket, which is 14.blahblah inches diameter.. which only adds 2 inches to my radius...

[fabr]

Others may disagree but with the numbers in front of you you have something to go by instead of a WAG whether or not the WAG was right or wrong. This is the way to do things. Calculate THEN do.

[Voodoochikin04]

thanks fabber!!! gives me a good idea of what i want..   i can pull to redline 1st through 4th gear, it tops out at 9Krpms in 5th gear.. on flat road..   so the 72 it is..  sent doug a pm..

[Doug Heim]

Thanks for the order. Sorry I thought 72 was too big initially. Offer still stands as a swap with a 60 if you decide to change for personal reasons.

Thanks again! 

[fabr]

Want me to run the numbers with a 60/14 and a 60/13 combo tomorrow?

[fabr]

Most of you guys DO know how to do this don't you?

[Carlriddle]

Just ask Master.

[fabr]

That's NOT what i meant!

[Doug Heim]

I know how. Simple math really. I just dont have the time to crunch the numbers. Im busy at work again or Id do it.

[fabr]

Easier than that. Just go to the friendly internet. There's tons of sites with calculators online. I'm too lazy to chew on numbers now days! LOL!!!!!!!

[fabr]

Here's the one I used yesterday. Really easy. There's tons of them out there.

http://www.apexgarage.com/tech/gear_ratios.shtml

 I guess I should mention that the rpm value you use when working with a bike engine or any engine for that matter with a primary reduction is the engine rpm divided by the internal reduction ratio. In voodoos case his redline rpm is 13000 and his primary reduction is 1.744 so you use 7454 as the input rpm. I rounded his off to 7400. There is another variable that can and does make a difference also. Tire diameter is most often used in these calculators. IMO if you want better accuracy you need to measure out the ROLLOUT of the tire and then convert that circumference into diameter for the true tire height.  You may ask how a tire can be a certain height and not have its rollout(circumference) be mathmatically different. It has to do with how a tire contacts the ground. With some tires that run at low pressures(don't we all?) the rollout can be significantly different than the diameter would indicate. Best thing to do is adjust air pressure to running pressure and with the tire on buggy and with buggy at running weight make one revolution of the tire and measure the length of the "footprint" on the ground and convert that true ,real world, circumference into diameter and use that diameter in the calculator.  Yes,I know, low pressure tires grow with higher speeds (think top fuel dragsters)but our speeds will not make much difference.

[fabr]

Oh and in the case of paddles you should do the rollout measurement in the sand. I think that is why a lot of people are surprised that their top end is lower than they expected. Usually they attribute that to tire slip when it well may be due to the tires rollout being significantly less than the diameter would indicate.

[LiveWire]

Calculating from scratch is still an educated guess since there will always be some factors that you did not or could not account for. Once you have a working model, I think it is far better to do calculations to get what you want based on what you have. Another example of this is people calculating spring rates. Your measurements being just a bit off on arm length, pivot point location, shock angle, etc. can all add up to make the end result be off quite a bit. Calculators are a good start, but won't get you perfect. Once you have a machine sitting on springs, you can use the compression of the spring and the amount compressed to determine the spring load at ride height. you can then use that load to calculate how much a different spring would compress so the ride height change.

Engineer pointed out in another post about calculating the strength of a shock tower. The forces that will be applied are a WAG so the calculated result is still questionable. I thought about that one for a bit in terms of what real world test data we have. We know that 1/2" shock bolts are extremely common. I don't think we have heard of any bolts simply sheering off. If the shock tower can handle the force that it takes to sheer a 1/2" bolt, it should never fail. If you set your lever ratio so high as to bend the shock tower, the shock bolt will shear first. More likely, the shock will bend before that though.