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Old 09-24-2003, 12:24 PM   #1
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Default Drivetrain Weight/Mass - Does it matter?

I have heard all sorts of different opinions from people about which drivetrains work the best, and most importantly, whether the rotating mass of the drivetrain matters as much as others say it does.

Which brings the question:

If you take two similar drivetrains -- say, the TC3 and SD -- and examine every part of the rotating mass, meaning the spur, main shaft, pilot shafts, differentials, CVDs, and axles, would the lighter drivetrain have any sort of advantage?

Wheels/tires would not be part of the equation since they can be equalized from one car to the next.

Another concept, then, would be bringing belts and shafts into the equation. Can any useful comparison be made between the weight of a belt car's drivetrain versus the weight of a shaft cars drivetrain?

Just curious to see what people say. If you have physics to back up your statements, all the better!
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Old 09-24-2003, 12:29 PM   #2
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rotating mass always makes a difference. Its more weight the motor has rotate. Same goes with the weight of the car itself.

So if you took two shaft cars, and one had a much lower rotating mass drivetrain, it's advantage would of course be "less rotating mass" resulting in better acceleration, increased runtime, less generated heat.
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Old 09-24-2003, 01:31 PM   #3
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what he said... If you're gonna shave weight off the car, I'd start with the parts that rotate first.
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Old 09-24-2003, 01:36 PM   #4
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but if you had a car with more rotating mass, wouldn't it roll longer after acceleration? more mass simply gives the car more inertia, wouldn't it? assuming both the car with more rotating mass and the car with less rotating mass has the same aerodynamics and the rest of the stuff is the same

food for thought.

i asked this question before, but about the mass of the spur & pinions only
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Old 09-24-2003, 01:38 PM   #5
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That is also true, but you don't really gain any advantage to having more inertia. As a matter of fact, having more inertia might hinder your performance diving through the corners...however you DO gain advantage being able to accelerate faster.... more punch is what it is all about.
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Old 09-24-2003, 01:38 PM   #6
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sometimes the extra weight is good though if you look at it as a flywheel which serves to store energy and helps to smooth the power delivery out.......could be a blessing to a beginner or on a loose track surface.
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Old 09-24-2003, 01:41 PM   #7
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in regards to trimming weight, rotating weight versus static weight will always be more beneficial. take real cars for example. manufacturers always specify a rating for horsepower. what they fail to emphasize is that this rating is taken at the crank, not to the wheel. for example, my wrx is said to have 226 hp stock, but when put on a dyno, it's only putting around 180+ hp. what happened to the other 46 ponies? drivetrain happened. after going through all the driveshafts and gearboxes and blah, blah... this is what you end up with.

i'm not saying that an efficient drivetrain is the magic answer to speed, but i think rotational weight's importance is sometimes taken for granted.
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Old 09-24-2003, 01:42 PM   #8
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Quote:
Originally posted by holycow
That is also true, but you don't really gain any advantage to having more inertia. As a matter of fact, having more inertia might hinder your performance diving through the corners...however you DO gain advantage being able to accelerate faster.... more punch is what it is all about.
i believe a lighter rotating mass accelrates faster than a heavier rotating mass.

think about it
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Old 09-24-2003, 01:44 PM   #9
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hey future real its nothing to do with this thread but i wanted to get your attention.
could you lock the thread 'iam tring'
as it is insulting and we can do without people like that here.
thanx
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Old 09-24-2003, 01:44 PM   #10
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Quote:
Originally posted by Lowrance
sometimes the extra weight is good though if you look at it as a flywheel which serves to store energy and helps to smooth the power delivery out.......could be a blessing to a beginner or on a loose track surface.
i concur. i had a lightened flywheel on my old integra gsr, and it was rather touchy on the bottom end. it took awhile to get used to it. actually, i don't think i ever got comfortable with it. maybe i just can't drive?


... i guess that's why i'm into rc
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Old 09-24-2003, 01:54 PM   #11
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flywheel, not exactly the best example, but would do.

in plain english, i think the best way to describe it is that

with a lighter rotating mass, accelrating is more "direct", faster, jerkier?

but with a heavier rotating mass, the gears/stuff acts as a buffer, and makes the accelration smoother.

oh and i dont own a car, how sad. but i'm happy with my r/c car nevertheless
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Old 09-24-2003, 03:03 PM   #12
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I've always tried to have the least rotating weight possible. It makes the car much more responsive to throttle inputs. It accelerates quicker, decelerates or "shuts down" quicker. It improves the effeciency of the drivetrain increasing runtime. I've just never found a situation where the drivetrain could be "too light!" Yes, it is different to drive. You need to be smoother with the throttle, both accelerating and decelerating. Personally, I think if the drivetrain is free enough (good bearings cleaned well, nothing bound) the car will roll just fine off power. It won't roll as fast, but you can always just let off the trigger slower or later......

I also race a full size stock car. We do everything we can to keep especially rotating and unsprung weight to a minimum.

I've just always felt that a lighter drivetrain can potentially give the best performance, after you adapt your driving to it.

there's my two cents
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Old 09-24-2003, 03:33 PM   #13
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Having a heavy pinion, however, will act as a flywheel, and be beneficial. The effect is multiplied by the gear ratio, so by adding a little momentum to the pinion, its like adding a lot of momentum somwehere else. For this reason, I suppose the lightest parts on your car shoudl be the CVDs.
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Old 09-24-2003, 03:53 PM   #14
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Quote:
Originally posted by RC Paperboy
Having a heavy pinion, however, will act as a flywheel, and be beneficial. The effect is multiplied by the gear ratio, so by adding a little momentum to the pinion, its like adding a lot of momentum somwehere else. For this reason, I suppose the lightest parts on your car shoudl be the CVDs.
but wouldnt it take more power/force to rotate a heavy pinion? think about it.

the thing is we do not want inertia
heavier rotating mass = more intertia

inertia makes it harder to get moving, and harder to stop as well.
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Old 09-24-2003, 05:36 PM   #15
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i try and ahve the lightest rotating weight possible, thats what the motor turns to get the power to the ground.
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