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Learn Me: quickchange rearends


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Unfortunately, there is not a lot of info out there on this subject.

The issue here is, there are so many variables that it would be difficult to narrow all the info down to an easy to understand lesson.


Here are some variables that affect your gear selection:


Track size--even quarter mile tracks can vary in gear depending on length of straights or corners or banking. You would not use the same gear at a relatively flat track, such as the old Longhorn track, as you would at, say Slinger in Wisconsin, where they get around the place over two seconds faster.


Engine Combination-- You may be limited by rpm based on your particular engine combination, or class of car run.

For example, a SLM with a concept may run a gear from 5:45 to around 5:65, and expect to turn about mid-8000 rpm.

That same SLM with a crate, such as the ASA series, has a rpm limit of 6300, controlled by a chip. To get maximum use of the available rpm, you will be running a gear closer to 4:22 or 4:33.

Also with the engine, your cam selection will dictate the gear to some degree. Every cam will have a "sweet spot" or rpm range that it really pulls at. Your engine builder can help you with this. You may find that the car lugs a little off the corner, but really takes off just before you are letting off for corner entry.

You may find that changing the gear ratio might pick the rpm up off the corner a little, making it quicker through the initial part of the straight, and the the car will run off the cam about the time you are letting off for corner anyway. As an example, at SAS with a SLM, you may go from a 5:45 to a 5:50 or 60 something, to let the engine wind a little. Remember, you can pass easier if you can out-pull them off the corner.


Bear in mind, though, that engine component weight will affect all this also.

Example--you have a modified at SAS with a TAMS legal 2 barrell, which would be a basic 350 combination. Your ballpark gear may be around 5:10 to 5:29.

But if you break your engine, and borrow a friend's stroker 383, you may need to pull a 4:70 or 4:86 gear.

That big crankshaft will not turn like the smaller one, so you would make use of the engine torque.


Another consideration to all this is the guy behind the wheel. If you are a driver that lets off early, and stabs that gas pedal quicker, you may find yourself with a slightly different gear than the guy who runs 3 lengths too deep in the corner and has to wait to get back in the gas.


The basic jist of all this stuff is the best way to determine the bear is to practice with different gears and record your lap times. Basically trial and error. There are basic starting points for gearing at all our tracks, and most racers willingly get you in the ball park. Remember,also, weather. You will run quicker at night, when everything is cooler, so you may want go up a little on the gear.


Hope all this helps a little,


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Not sure if this is what you mean, but this calculator will ball park you for a ratio.



The problem here is you need to know the top speed you expect to achieve, and that is difficult to determine for oval cars.

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The most practical approach is to make an intelligent guess based on whatever information you have but stay conservative. From there, make a practice run and based on results, adjust as needed. As an example try Thunderhill, 3/8th mile. You can ask fellow competetitors and get several answers based on what engine combination they run. If they will also tell you what RPM range that gets them then you have a real good start.


If they say "I run a 5.30 and turn 7000", and the cars are generally the same size and weight, then relate that to what RPM you want to turn as a percentage: If you want 6500, then 6500/7000 X 5.3 = 4.92. You have a 4.80 and a 5.10, run the 4.8 first and see how it runs...


If you can't get that much info you have to make a real WAG and just start somewhere. If you run with a a lower gear in the tranny, then at least you can shift up to lower the RPM on the first practice and still get useful information. A guess for Thunder hill is somewhere between 55 - 75 MPH top end based on class. So you pick something really off like 3.50. You make the first practice and the car pull 4000.. No problem, you want 6500 take 6500/4000 X 3.50 = 5.69. The engine is in a better RPM range so figure it will pull 300 more based on that so the real gear to try first is what you had plus what you expect to gain 4000 +300 = 4300. now 6500/4300 X 3.5 = 5.29. If you have it, fine if not drop in your 5.10 and see how it runs, adjust again as required.


There are programs like Circle Track Analyzer that allow you to put in track specifications (length, banking, turn radius) and will give you a good starting estimate as well but you need to have detailed track info for it to work with.


Good Luck

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