COM "rookie" question

[blittle]

I see that a rule of thumb is to have the the 4:1 weight distribution with 4 ounces in the back and 1 in the front. I assume the resulting COM would be 1.4 inches from the rear if my math is right? Does this same rule of thumb apply if I move the rear axle back just enough so that the tire doesn't extend beyond the rear of the car? Thanks for the help

[PWD_addict]

COM does not end up being fixed at 1.4 for a 4:1 car. If you are assuming using standard axle slots and weight at the point directly above the axles, you are probably correct.

Move weight towards the rear of the car and you should be able to achieve about 7/8".

The 4:1 is a good starting point. However, if your Pack/group has a very smooth track, you can get away with much less weight on the front end. We've done well with 4.7:0.3 on a very smooth track.

Our Pack track is very rough so 4:1 seems to be a prerequisite for going fast but I haven't really played around with more aggressive rail-riding instead of front weight. Should have a chance this year, though.

[Stan Pope]

I see that a rule of thumb is to have the the 4:1 weight distribution with 4 ounces in the back and 1 in the front. I assume the resulting COM would be 1.4 inches from the rear if my math is right? Does this same rule of thumb apply if I move the rear axle back just enough so that the tire doesn't extend beyond the rear of the car? Thanks for the help

Make all measurements relative to the axles! That simplifies execution and communications.

For a given weight distribution, the CM location moves forward from the rear axle as the wheelbase increases. So, when citing CM location, it is necessary to also cite wheelbase if meaning is to be conveyed.

[blittle]

I guess that's what throwing me off. I was trying to find the correlation between the front/rear weight ration and the COM location that folks use. For example PWD_addict mentions a 7/8" COM which I assume if from the rear axle, correct? Was trying to convert the 7/8" COM into a front/rear weight ratio.

[Stan Pope]

I guess that's what throwing me off. I was trying to find the correlation between the front/rear weight ration and the COM location that folks use. For example PWD_addict mentions a 7/8" COM which I assume if from the rear axle, correct? Was trying to convert the 7/8" COM into a front/rear weight ratio.

The relationship is mathematical.

See http://www.stanpope.net/cmcomput.htm for a computing aid for CM and weight dist. I don't recall if the formulae are given there or not. If not, you can learn a lot about 'em by dissecting the javascript code!

You can also derive the formulae by computing rotational torques: For instance, the car's weight at the CM (total car weight) multiplied by the distance from the rear axles to the CM should equal the car's weight under the front wheels (with the car resting on its wheels all around) times the wheelbase. Or you can compute from the front axles usually with a bit better accuracy.

[blittle]

Thanks You! The caculator is exactly what I was after. Makes it a little easier to just weigh front/rear for testing different setups!

[PWD_addict]

I guess that's what throwing me off. I was trying to find the correlation between the front/rear weight ration and the COM location that folks use. For example PWD_addict mentions a 7/8" COM which I assume if from the rear axle, correct? Was trying to convert the 7/8" COM into a front/rear weight ratio.

I'm glad you found what you wanted. By 7/8", I meant in front of the rear axle.

[ohiofitter]

I think that most people would agree a good com to have ranges from 3/4 to 1 1/4 inch in front of the rear wheels......I know everyone has there own personal favorite COM but if your in this range I think your car would be good