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Gearing

3K views 10 replies 7 participants last post by  Bigtone 
#1 ·
I should know this but I often get confused with gearing so help please:
Assuming the pinion and tyres stay the same size does a gear with more teeth give a higher top speed but less acceleration or lower top speed and more acceleration?

Example - 12 tooth pinion & 29 tooth gear vs 12 tooth pinion & 31 tooth gear - which has higher top speed?
 
#2 · (Edited by Moderator)
lower the overall ratio more top speed, less acceleration and braking

so
12/29 = 2.41
12/31 = 2.58

So the motor needs to turn that many times to make one revolution of the wheel

if you want and more accurate measure and work out physically what the difference is you need to add in overall wheel/tyre diameter in to equation as that will make a difference

Matt
 
#4 ·
Hi Brian.
Matt is quite correct, don't forget your rear wheel/tyre diameter.
The overall ratio, sometimes called in slot car circles, 'roll out'. This is the actual distance the rear wheels cover per 1 revolution of the motor.
Something else to keep in mind is motor torque.
If you have a small can, very high reving motor, then the greater the numerical ratio (eg 4-1,) then the faster acceleration.
If your running a super Hi-torque 300gr+ 'Large can' (boxer type), this can often accelerate quicker with a 'taller' gear ratio (eg smaller numerical ratio =2.5-1).
These motor should obey the laws of gearing just the same...but in the real world...they don't!
Regards Bill.
 
#5 ·
That's interesting Bill. I have not experienced the high torque motor effect you mention and have always thought that the lower the ratio, the more torque at the rear axle and therefore the more acceleration as per basic physics. Do you think the culprit is excessive wheelspin?

Cheers,

Keith
 
#6 ·
Leaving aside obvious restrictions on pinion/gear combo such as mesh etc. does it make any difference how you achieve the required ratio? Can you just change the number of teeth on one of them or is it a balancing act between the two?
 
#8 ·
Hi Keith.
First thing to say is I am talking about a Club racing environment, with good power continuity...mostly wood tracks.
You are right, with poor tyres any initial acceleration with a Hi-Torgue large can motor gets eaten up in wheel spin, so taller gearing helps transfer the power to the track. With really grippy tyres then 'short' gearing results in wheel stands, as soon as you hit the power. Don't get me wrong, a high ratio-low gearing, will give the most instant response and better brakes, but with the very Hi-torque motors the best ratios on larger club tracks are always in the region of 2.5-1 or even taller.
The massive torque of these motors shrugs off the very tall gearing with ease, and the huge 'roll out' means they leap forward on power covering huge track distance for quite short throtle bursts.
Brian,
As Keith said the ratio however you achieve it is the same, but gear mesh will differ, often the more teeth invollved the better the mesh.
Regards Bill.
 
#10 ·
Some gears are more efficient than others, it's not just a question of number of teeth.
Efficient = less power wasted in poor gear mesh.

Some makes offer pinions all the same size with different numbers of teeth. The highest and (sometimes) lowest number of teeth often don't mesh as well.
Generally slot car pinions have too few teeth to allow an ideal tooth form to be achieved. So gears with more smaller teeth often mesh better.
Of course some types of gear are made better than others, that certainly makes a difference.
 
#11 ·
In BSCRA racing the most popular pitch gears are 64, 72 and 80 to enable the use of small gears. The top drivers use EDM pinions to ensure a smooth mesh and they are about £17 a piece, even turning the motor shaft down to 1.5mm so a 6 tooth pinion can be used. The best gears seem to be all Czech made.

Tony
 
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