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Old 09-24-2003, 05:48 PM   #16
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I guess what I was getting at was comparing car to car, so things like pinnion size and wheels/tires would not be part of the equation.

It seems like my initial view of it (that I have had people at the track try and tell me is wrong, which I never bought) was correct, or at least held in common belief.

I watch people trying to shave weight off of their chassis all the time, but it seems to me that having a light drivetrain is going to have more effect on your car's performance than an ultra lightweight car in general.

From the motor's perspective, you only "see" the rotating parts, which all sit in a "frictionless" (for our purposes) environment, save for the tires on the racing surface (the rest being bearings). The actual weight of the car is a constant gravitational force downward, which is only felt by the drivetrain at each of the four contact points (tires). So the car's overall weight is still obviously a factor, but seemingly much less so than the rotating bits themselves.

Or another way I choose to look at it:

lighter drivetrain - primarily changing acceleration
lighter car - primarily changing side-to-side handling characteristics, for better or for worse
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Old 09-24-2003, 06:03 PM   #17
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Now that you explain ti, it makes a whole lot of sense.
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Old 09-24-2003, 06:15 PM   #18
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great thread.

it is important to note that these rotating parts are both, rotating, and translating in space. so actually the motor will benefit from overall weight reduction and rotational weight reduction. more from the latter, as it is contributes to both, as per my aformentioned topic sentence.

another thing to consider is the distance from the axis of rotation that the center of mass for half of the cross section is. what the hell am i saying? basically, given the same weight, a smaller dia piece will spool up faster, or have less rotational resistance to accelleration. simply because the average mass is nearer the cender. it has to do with moments of inertia (1st and 2nd), which increase exponentially with distance the center of mass is from the axis.

this comes to play more in differential design, where rotational mass/momentum is compromised by the slip force from drive torque. ideally we would locate the balls as close to the center as possible for quick spool up, but the drive torque at that location would be so high that that it would require high diff compression, quickly wearing, and maybe warping, the various diff parts.

we assume manufacturing engineers have run the calculations and have arrived at an optimal design for our application. or do we?

if the geometry has been optimized, the next step is to optimize the material. ceramic diff balls are a great example of this, as they are lighter, harder, and the sphericity is usually much better. the diffs with 10 or 12 1/8" balls are great candidates for dropping the big bucks for these. note that elemental tungsten is much heavier than the chrome steel that diffs come with. opt for ceramic before all the tungsten carbide (mini shotputs )

well, i've expounded for today. sorry for the long read, but i know i enjoy reading this kind of stuff. by the way, where's john stranahan been?
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Old 09-24-2003, 07:03 PM   #19
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seaball- YOu KILL-no SLAY me everytime with your posts!! I am so glad you race at our track-so fun to talk to!!! awesome stuff.

Now-is Sphericity really a word-come-on hahaha!!!

if you guys dont know Seaball-if he cannot buy the modification he wants to make his car better-by god that wont stop him. he'll cut drill/dremel his own pieces. Pulleys, CVD's bulkheads, belts, layshafts...He has no theoretical limit to his minds ability to create!!
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Old 09-24-2003, 07:09 PM   #20
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DOH-talk about going off track...

Since this is actually a comparison of drivetrains strictly by weight and rotational mass-I would have to say thats not feasible for the application we are talking about. That is because the SD, EVO3 and FTTC3 drivetrains-though very similar in design and weight I bet-react totally different to the same battery and motor, ESC, and radio input. And I dont think it has anything to do with weight.

I think the design of the chassis (flex), materials of the gears and rigidity of the bulkheads and/or diff cases plays a HUGe role.
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Old 09-24-2003, 09:02 PM   #21
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OK for keeping car to car the same lets go with this scenario
LIGHT TC3
composite bones,carbon shaft, plastic diffs.

HEAVY TC3
steel bones, alu shaft,steel diffs.

The lightweight tc3 should accel much quicker than the heavy drivetrain tc3.
The heavy drivetrain TC3 should accel more slowly than the lightweight TC3 because of the weight that has to be spun to get moving. In turn this will make it harder to stop by the parts inertia to keep it rolling.
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Old 09-24-2003, 10:46 PM   #22
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Under no circumstances is an R/C car benefited by a heavier rotating mass. In fact, there is only one reason for tuning a 1:1 race car with flywheel weight- because the stored energy in a rotating flywheel is what the car uses to accelerate it from a standing start and on gear changes. Since an R/C car has no use for stored energy that would only hinder decleration, there is no reason to have it. That includes the pinion gear. Bottom line is: the more rotational unsprung weight you must turn, the slower the car will accelerate.
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Old 09-24-2003, 10:54 PM   #23
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Seaball, you are awesome.....I consider myself fairly intelligent and I catch myself adding some of your words to my dictionary.....I can honestly say that your use of the word sphericity is a first for me

Keep up the R & D

Eric
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Old 09-24-2003, 11:38 PM   #24
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I don't think anyone would say that a car would benefit from a heavier rotating mass. The question to be answered, though, is can any reasonable comparison between two similar drivetrains -- using the TC3 and SD as examples from my earlier post -- be made based on weight?

As rayhuang points out, similar looking drivetrains may not be so similar after all. However, aside from acceleration and perhaps top speed, what else does a drivetrain effect? Many people talk about "more steering" and "oversteer" from shaft-drive cars versus belt drive, but I find that to be a side effect of quicker acceleration, and not a separate handling characteristic. The only potential way to affect handling would be if the front or rear wheels were being over/under-driven, but I don't see that happening in most cars.

I am curious as to how you think the rigidity of the bulkheads/differential housings would affect the rotational speed. Chassis flex perhaps, but it would have to be an extreme case. Most chassis flex I observe is side-to-side, but the bulkheads themselves are not flexing -- otherwise we would see very serious problems arise!

Gear material is a good point to raise, although I think that would apply more in a friction and wear sense than a power transfer sense. Also, it seems that most cars these days are using the same material to make their diff gears.

Anyway, good discussion so far!
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Old 09-24-2003, 11:41 PM   #25
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Quote:
Originally posted by seaball
we assume manufacturing engineers have run the calculations and have arrived at an optimal design for our application. or do we?
Another interesting point here. How much thought goes into the actual physics when designing these cars? Are actually weights and forces being calculated, or tested outside of a car/track environment? Any computer simulation or anything of that nature?

Or is common R/C design more of a hit and miss proposition?

I assume it varies from company to company and location to location, based on availability of true engineers and equipment. Just guesses, though.
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Old 09-25-2003, 01:55 AM   #26
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You have to also take into account wich type of car and racing you are doing. If you are into offroad a lighter drive train will just help your car spin out faster(if its 2wd). However in touring car stock the lighter the better that is for sure. They you bring in 4-cell 1/12 stock and even the pinion weight can be a factor. Now mod touring car outdoor on rubber tires a heavyier drive train will help smooth out transitions and make the car eaiser to drive.

Just some food for thought
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Old 09-25-2003, 02:43 AM   #27
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Two things totally overlooked in this thread are the direction of rotation and the gyro-effect that turns the direction of force 90 degrees.
The tires are without a doubt the parts of the car with the greatest inertia. If you have ever tried to have a car in your hand and accelerate and tehn brake you will notice how the car first tries to lift the front and then the back. Now, consider a belt driven car. Have you ever wondered why the motor is always placed so that the pinion is pointing to the right. This is simply to counter at least some of the inertia of the tires. A shaft driven car on the other hand don't has this counter effect by the rotating armature, but has because of it some additional sideway forces which make the car handle in a slightly different way in left- and right-hand corners. The armature and the cardan rotating in to opposite directions help a bit, but don't even nearly cancel each other out.
Another thing I mentioned was the gyro effect. I take it that everyone knows what that is. It can be demonstrated in a RC-car by again picking the 4wd car up to your hand. Then accelerate the tires to a reasonable speed and suddenly turn the wheels to left and right. Doing this the car tries first to raise the left side and then the right side. So, by having wheels, spurs, etc... with a low inertia will actually distribute the weight more evenly to the inside and outside tires in a corner. Again, in belt cars the armature aids this because it rotates in a different direction.


I talked a lot about tires what is specifically what you didn't ask about. However the effect of the drive-train are a lot similar, but have a lesser effect. Seaball hit the nail in the head by bringing it up about the r^2 relation between the rotating weight and the energy stored. This means that all the hollow axles etc are quite useless in an inertia point of view compared to belts and spurs. A belt can be consider like a rotating mass having the entire weight of the belt on the radius of the pulley. If the two pulleys are of a different size, the larger one will dominate. If you would like make an accurate comparison between two drive trains it would be easy, but not so important as the directions of the rotating components.

BR/ JesseT
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Old 09-25-2003, 06:36 AM   #28
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This is kinda off topic, but I always here about unsprung wheight.So what is it?
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Old 09-25-2003, 07:18 AM   #29
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It is the weigth of the parts that have to move with the tires and the track and not with the suspended and sprung chassis. So basicly it's the wheels, tires, hubs, bearings, drive shaft, and partly the wishbones and dampers.
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Old 09-25-2003, 01:11 PM   #30
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An easy experiment:

I got some tires, which are 5 grams heavier each, than my usual tires. The interesting about this is, that it highly illustrates the effect:

Drive some laps with one type of tires, then some laps with the others.

Of course, it'll influence on handling, but when it comes to acceleration and deceleration, it's very easy to feel the difference. Especially since wheels can be changed so fast, that it's easy to feel the change.

Try measure the weight of your own tires, maybe you can do some testing here yourself.

What surprised me, was the less braking with the heavier tires, I often entered the corners too fast.... Not surprising, when re-thinking about it, but it wasn't what first came to mind, when I was planning the test.

JesseT got some good points about motor placement, that is, belt versus shaft.
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