XRAY - Model racing cars

Hopefully this makes sense !

I have just put on a larger spur gear and to get the same gear ratio I was using I need a larger pinion ???

Does this mean you can still have the same ratio, but depending on spur gear size can effect acceleration and top speed.

yes you do now need a larger pinion, you may be able to achieve the same ration, or a very similar ratio.

the larger spur and pinion will effect the acceleration of the car if you have the same ratio, the car may feel less punchy. this is because of the increased rotational mass now in the drivetrain.
but this shouldnt effect your top speed, and the effect of the larger spur and pinion gear shouldnt realy make that much difference to your acceleration.

hope this helps,

chris

i have noticed a difference if you use the same ratio and larger spur and pinion, but yes the actual change in the ratio does have the larger effect

The numbers you need to match is rollout. As long as the roll out stays the same, then you will have the same accel and top speed. I did some testing of my own and found a very slight change in my average lap times on the test track. The car does handle differently with the larger spur setup because the motor rides lower. I tested with 87/27 and 93/29 with rollouts of 1.146 and 1.152 respectively. In competition, I am only about 1/2 second off the pace to run with the nitro guys.

What motor and surface do you run on?

When I run stock indoors I usually run and OVERALL ratio of about 8:1 and outdoors with stock about 6.5:1. Both are for 64mm diameter rubber tires and I do vary it a tiny bit from tight tracks to open sweepy tracks.

To calculate this, use (Spur / Pinion) x Internal ratio.

I always try to keep the motor as low as possible (after all, raising a 200g mass even just a fraction of a mm will effect the C of G). Overall ratio and wheel size is far more important that just changing a pinion.

James

Not to burst anyone's bubble, but what is everyone talking about?

If you keep the same gear ratio, regardless of pinion/spur size, your top speed will remain the same (simple physics and mathematics). By increasing the spur AND pinion together, you ARE increasing the rotating mass, but it's by a very small amount. I'm not convinced you will notice that change on the track, because the mass is not where you will feel the changes, unless you go to the extreme. At the same gear ratio, whether using small gears or big gears, the motor will always spin at the same RPM in relation to your final drive RPM. CHANGING your gear ratio is what you want to experiment with...Why? read below:

Electric motors never produce peak power at the beginning or end of their RPM range. Take one of my P2K's, which produces it's most power (63 watts) at around 15k RPM. BTW, Don't EVER trust labels put on by motor companies. So, let's look at how this relates to on-track experience when using a 93t spur, 24t pinion, T1FK04 with low-ratio pulley set (1.77)...

Gear Ratio = 93/24 = 3.875 (the pinion turns 3.875 times for every revolution of the spur)

15,000RPM/Gear Ratio = 15,000/3.875 = 3,870.97

3,870.97/Pulley Ratio = 3,870.97/1.77 = 2,186.99

So far, at 15k Motor RPM, we are spinning the axles and wheels at 2,186.99 RPM. Using my Sorex 28R tires, I find the circumference of my tires are approx. 7.875". So, how fast am I going on the track?

2,186.99 x 7.875 = 17,222.55"/min or 1,435.21'/min, which is 15.71mph. If you are saying, at this point, so what! I don't care how fast in mph my car is going! You are correct, it doesn't matter in mph how fast your car is going, but what kind of track are you running on?

If you are constantly driving the car at 3/4 to full throttle around the track, then you want to gear the car to place your motor's peak output in the 3/4 to WOT range, which you can easily find by knowing how fast your are going at WOT. The reason is because you will accelerate faster out of the corners because you are in the motor's power band, thus increasing your average speed...and on-road is all about raising that average speed. However, keep in mind what Big Jimmy G said, the lower the CG the better the car will feel. Also, don't everheat the motor...I usually run mine in the 150-170 range at the end of 5 minutes on an outdoor track. But I also know someone that's pretty quick running in the 200 range. These temps are stock motors.

(DISCLAIMER) These are just my opinions and I don't expect anyone to agree. Your driving style and car condition may affect your specific situation!