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Old 12-30-2002, 11:22 PM   #1
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Default front/rear weight bias?

Im working on a project and am wondering if/what is the best front rear weight bias for a car.

My initial thoughts was perfect 50/50, but are there advantages to using say 40% front 60% rear?

Any tips would be great.
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Old 12-30-2002, 11:48 PM   #2
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Basically the more weight you have towards the front of the car the more steering it will have, atleast that's my experience with changing the weight bias.
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Old 01-01-2003, 05:10 AM   #3
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I'm quite sure, that a rear wheel driven car need something else than a 4WD. And a front wheel driven are different from the two others....
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Old 01-01-2003, 12:52 PM   #4
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I guess i should add that it is 4wd.

Im thinking making it 50/50 with extra battery slots forward and backward so i can change it easily.
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Old 01-01-2003, 08:34 PM   #5
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I find that an electric fourwheel drive touring RC car needs to be about 2-4 ounces heavier in the back than the front with a total weight of about 53.7 ounces. Some weight will transfer forward in a corner and the weight split will be closer to 50-50 during cornering if you preload the back a bit.
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Old 01-01-2003, 10:33 PM   #6
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Thanks John.

Keep the advice coming.

thanks
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Old 01-01-2003, 11:02 PM   #7
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I don't know if this holds true to 4wd r/c cars, but in a full size rear wheel drive stock car, more front weight will make the car push, more rear makes it loose. This is because more weight carries more momentum. A heavier front will resist turning, and a heavier rear will resist straightening out, making the car loose in the middle and exit of the turn.
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Old 01-02-2003, 01:23 AM   #8
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Quote:
Originally posted by JeffC
I don't know if this holds true to 4wd r/c cars, but in a full size rear wheel drive stock car, more front weight will make the car push, more rear makes it loose. This is because more weight carries more momentum. A heavier front will resist turning, and a heavier rear will resist straightening out, making the car loose in the middle and exit of the turn.
One thing that many people seemed to have ignored so far is the fact that RC cars have much lower ground pressure than their full sized counterpart.

For a tire footprint that is 1/100 that of a full sized car, there is only 1/1000 of the weight on that tire. That translates into 1/10 the per unit area pressure on that tire compared to a full sized car. All else assumed similar (tire compound etc), let us not forget that for tires, the graph of traction vs pressure (weight) over it is convex ( inverted U shaped).
Now full sized car tires with their higher per unit ground pressure operate closer to the "falling" part of the curve where more weight over the axle actually causes it to grip less. I suspect that the much lower per unit ground pressure of RC car tires actually means they operate on the "rising" part of the curve, where having more weight over a particular axle actually means more traction over that axle
Remember not so long ago where super narrow 21mm tires were all the rage??
I think ist all about the scale factor here, teh physics are the same just that the scale factor has to be taken into account.

Cheers
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Old 01-02-2003, 08:36 AM   #9
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If you start with a car that has 50-50 weight split (or 60%front and 40% back like a fullsize sedan) and move weight forward you will have less steering traction or will create a push. The front tires will be less efficient. If you preload the back of an RC car a bit say by 4 ounces and then you move weight forward then you will create more steering traction or less push you are getting closer to the ideal 50-50 split.

Last edited by John Stranahan; 01-02-2003 at 08:45 AM.
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Old 01-02-2003, 09:10 AM   #10
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Default All good in theory

Guy's;

One point that has been missed is that most of the R/C Cars Electric and Nitro leave little or no room to move things around to change the weight bias though some electrics allow for different battery positions.
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Old 01-02-2003, 09:41 AM   #11
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On the TC3 I can arrange weight all the way from 5.5 oz heavier in the back to 1 ounce heavier. This is plenty of adjustment. The optimum is somewhere in the middle. My car works very well at 2.4 ounces heavier in the back. My Losi truck had plenty of battery adjustment as well. It is two wheel drive with a much higher center of gravity so the truck had 21 ounces more in the back. I used to run the battery 1/2 inch back in the tray. The factories make an attempt to place the weight in a good place to start with and allow some adjustment with ballast or battery and electronics placement.
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Old 01-02-2003, 03:54 PM   #12
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I would like to remind you that this is a project car and i can place the battery, motor and electronics anywhere i please, so there is infinite room for adjustment.

I disagree that more weight in the front makes a car push. Looking at Barry Baker's TC3 which he tuned for max steering, he has the battery placed in the forward slot (i also drive my TC3 with the configuration, but i wanted proof from a truely reputable source).

Lets keep this debate going.
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Old 01-02-2003, 08:39 PM   #13
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hey guys i'm no pro, but i get very into theory as well. i think that steering is often generalized by more or less. of course there are three types of situations. they are off power, on power, and constant velocity situations. usually you trade one for the other. i think jeff c is really on to something with the argument that he made. momentum is a large factor in determining steering characteristics. a light front end will respond quicker to input and will turn in quicker and posibly to a higher magnitude. a heavy front end tends to feel dumpy early in the turn, but really exits turns well due to the extra weight. the idea that once in motion, it stays in motion. in a constant velocity situation, a near 50/50 balance tends to dominate as neither end wants to swing out due to centrifugal forces.

i think it's very difficult to find the right balance. roll centers and spring stiffness can play a large part in the "apparent" weight balance durring cornering. i will say that i've not known an rc touring car to have >50/50 bias toward the front. usually between 45 and 48 for the front. my rclab comes close at 700/705 grams without body, and i'm getting an off power oversteer maybe for this reason. my idea would be to shoot for 50/50 when everything is foreward, and as much difference as possible when everything is rearward. options are a great thing to have when you're the lone ranger with a one of a kind touring chassis.

well that's my too sense. good luck! belt or shaft? hehe.
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Old 01-02-2003, 08:57 PM   #14
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Default i'm getting an off power oversteer

seaball;

You may be transfering too much weight to the front wheels off power. More Droop in the rear, raise the front RH slightly, stiffer springs in the front, front oil. I had this problem with one of my Nitro cars and Thad Garner (HPI) told me to increase rear droop and that fixed my problem. Only took a couple of mm to fix the problem.
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Old 01-02-2003, 09:02 PM   #15
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Wink mab_man20;

Quote:
I would like to remind you that this is a project car
Can't you see that no one cares about your project car. We are all TOO busy arguing among ourselves about this Weight Bias thing that someone posted.

Funny though how a simple question CAN really turn into a BIG arguement online......

Last edited by popsracer; 01-02-2003 at 09:05 PM.
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