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Old 10-05-2001, 08:15 AM   #1
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Default newbie here have some quick ???'s

ok i have a 9turn triple motor on my hpi sport2 and some guy said that i had to cut my comm. my question is what is a comm and why do i have to cut it. also i was wondering what is the best spur and pinion combo. i was informed that with my sport2 a 93/21 combo was best. is this true. also will i notice a difference in speed if i change my pinion and spur. lastly, and i appreciate all the help guys, what is some tire recommendations for running in cali around 6pm. the sun is still out but its not as hot as during the day. also i will be dragging with a friend on normal blacktop. not the best surface. with my cars stock tires(slicks) when i floor it i just start to go in circles. i am not even controlling the car and it just does it. either that or i just pitch to the side really bad. so what tires and foam u guys suggest. i just want the best for dragging on this poor surface. peace. thanx again
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Old 10-05-2001, 08:40 AM   #2
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Well, let's start with the tires:

Foam tires generally get the best traction, but they don't last long. I would get some belted rubber tires. For just messing around I like the HPI Advan tires. They have good traction, and last pretty long.

The piece on the left of the pic, the round cylinder shapped copper piece is the comm. You need a lathe to do it. If you don't have one, ask at your local hobby shop or at the local track. I'm sure someone there has one.

As for the spur/pinion combination, that sounds a little high to me. Once HPI's web site gets back up (www.hpiracing.com) they will have some setup info and gear ratio charts there. You will notice a difference if you change your pinion and/or spur.

If you take the number of teeth on the spur gear and divide them by the number of teeth on the pinion gear you get your gear ratio. Take your combination: 93 divided by 21 = 4.42. If you raise that ratio, you will have more top end speed, but less low end. If you lower that ratio you will have more low end but your top speed will be lower. To lower the ratio, use a smaller pinion or larger spur. To raise the ratio, use a larger pinion or smaller spur. But don't change too much. If you over gear a motor (make it try to run a gear ratio that is too high) you will overheat the motor, and possibly damage your speed control.
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Old 10-05-2001, 12:41 PM   #3
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Default About gearing;

If you're running on a track, here is a good guide: If your car reach max speed at about 3/4 of the longest straight, then you got the right gear. If it reach max speed earlier, you'll probably need a higher gear ratio and vice versa.

Another good guide is the temperature; Drive the car for some minutes. If you can hold your finger on the motor for about 5-10 seconds, without burning, then you are doing fine. If it's much warmer than that, you'll need a lower gear ratio. Otherwise, you might turn the motor into smoke.
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Old 10-05-2001, 02:17 PM   #4
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Here go here it will give you a walk through of cutting a comm. Byran has done a great job on the site, he also has forum. www.balakracing.com is his site and the link to the comm cutting is http://www.balakracing.com/comm.htm Hope it helps. P.s. your tyres are spinning because your running a very powerful motor, a nine turn is phyco heck Masami ran an 8 double to win the worlds. Your looking at having to cut the comm very frequently(your LHS should have this service) plus your run time unless you have great cells won't be too good. I suggest you buy a seperate armature with more turns it will be easier to control the power and not as much work. Also you will need to be smooth to get the power to the ground because like you said the wheels just spin and it also puts strain on the drivetrain witch is not needed.

Last edited by fatdoggy; 10-05-2001 at 02:27 PM.
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