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Old 12-21-2006, 11:11 AM   #1
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Default Help with gearing

I'm having a problem with gearing. Whenever I go to a new track I am really bad on deciding gearing. I'm always way under geared or over geared. I have a Xray T2 and have all the gearing charts but when gearing my car I can never decide which way to go and where to start. Someone once told me when I first started to gear the car so it tops out on the back strait right before you let off for the turn. Im getting tried of having to walk around and find people with the same car and bug them for gearing help.
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Old 12-21-2006, 11:12 AM   #2
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Quote:
Originally Posted by willcunningham
I'm having a problem with gearing. Whenever I go to a new track I am really bad on deciding gearing. I'm always way under geared or over geared. I have a Xray T2 and have all the gearing charts but when gearing my car I can never decide which way to go and where to start. Someone once told me when I first started to gear the car so it tops out on the back strait right before you let off for the turn. Im getting tried of having to walk around and find people with the same car and bug them for gearing help.
what motor?
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Old 12-21-2006, 11:39 AM   #3
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With 19t I usually run EA Komodo with the 767 brush In stock I run EA Co27 with 767 brush
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Old 12-21-2006, 01:03 PM   #4
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With 19t I usually run EA Komodo with the 767 brush In stock I run EA Co27 with 767 brush
komodos i hear people gearing them in the 6 range and 6.5 for the CO
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Old 12-21-2006, 01:26 PM   #5
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alright thanks alot ill start there next race
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Old 12-21-2006, 01:27 PM   #6
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you wouldn't happen to know what gearing would be good for MDI would you?
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Old 12-21-2006, 05:16 PM   #7
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In stock I always go by motor temperature after a 5 minute heat. If it's over 140degF, I go down. If it's below 125, I go up.
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Old 12-21-2006, 05:52 PM   #8
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ok thanks
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Old 12-21-2006, 08:29 PM   #9
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you wouldn't happen to know what gearing would be good for MDI would you?
havent been to mdi yet next month will be my first time. Tim Potter should be able to help you
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Old 12-22-2006, 08:39 AM   #10
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Quote:
Originally Posted by willcunningham
With 19t I usually run EA Komodo with the 767 brush In stock I run EA Co27 with 767 brush
Probably this belongs in the stock motor thread where I am sure it has been discussed before.

If you think and talk in terms of rollout, that will help. FDR is OK if you always run roughly the same diameter tires, but rollout takes tire diameter out of the equation, making it easier to ask 1 question on gearing and everyone talks the same numbers. Some will be inches, e.g. 1.00, others millimeters like 25.4. In case you don't have the formula handy, rollout = tire diameter x 3.14 / diff ratio / spur teeth * pinion teeth. I measure tire diameter with a mm ruler, and if I'm careful it gets me close enough. If you use mm for diameter, you get mm rollout (distance wheels go per motor revolution, usually like 20 or 30 something), or if inches you get inches rollout (usually between 1 and 1.2 or so). 25.4 mm per inch if you convert.

You will find rollout goes up or down depending on how much low vs. high speed areas on the track plus how flowing it is, e.g. how much you have to come off throttle.

Let's take the CO27 for example. You can probably approach the 19t similarly, but I have no experience there.

For large open tracks, e.g. over 100 feet and 10+ feet lanes, the CO27 will run 1.20 to 1.28 range depending on temperatures (lower if really hot). In midsummer, mid 90's on asphalt, about 1.20 to 1.22 was the limit, hitting 165-180F range, sometimes 190's but that's hard on the motors. I can put more like 1.24 on cooler days. Personally I like to keep motors to 165F max to protect the magnets. But I have to go higher sometimes, just keep in mind heat can be hard on magnets so they may need re-zapped sooner. For smaller to medium carpet tracks, you might be more like 1.0 to 1.04.

For a typical size spur, the difference between, say, 1.0 and 1.2 might be just 7 or 8 teeth on the pinion. If you keep track of things, and know say you need for example a 44 tooth pinion on the big track, then it's 36 or 37 teeth on the small track. Medium sized is halfway in between. If you keep track of the best rollout at each track (it's ok, just ask the top guys what rollout they are running) then after a couple of times, you should probably be within 2 teeth of optimum just by looking at the track.

Besides, it's always a fair question to ask the pros at a new track, what's a good starting point for rollout on a ___ motor ? When I have a gearing question I always try to ask the local pros. We have several including some who travel a lot.

Motor temp is useful for finding the limit and is a good comparision to performance on the track. You always want to gear for best lap times. If the track has a lot of tight areas and one long stretch, you'll top out early there to have enough grunt in the infield. If it's all pretty flowing with no slow areas, the "top out near the end of the stretch" might work ok. But it's no substitute for an accurate comparison of lap times in practice or even in the heats. I find it works even with me and I'm not the best driver at the track.
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