XRAY - Model racing cars

Hi everyone,

I would like to run this by everyone before I proceed to test. Maybe someone has already tried and can tell me results.

Theory - Lower Centripetal force = faster corner speed. Centripetal Force is CF= m(v)2/r.

I have a circular asphalt track that has a circumference of 210 feet. My car currently weighs 1,426 grams. With my current tires and setup I can run a 5.4 second lap (about 26.5 mph). Based on the centripetal force equation, if i reduce my car weight I can increase my speed around the circle and decrease my lap times.

I can decrease my weight by using a lower rated mah lipo battery. (110 g vs 265 g and a net reduction of 155 grams).

Has anyone used a 2000 mah lipo battery? How much run time do you get? (I need around 5 min).

R. Schumacher

Practically will this apply? Will I see faster lap times by reducing weight? or is there an "optimum" weight for these cars?

I would have thought there would be a point where there wouldn't be enough weight pressing down on the tyres to create grip.

I thought about doing a test like this before, but never found the time...

Here is my thoughts:

If you run a circle as fast as you can, there are two forces that are balanced: centripetal force and friction force of the tires.

As you mentioned centripetal force ist proportional to mass.

If tire friction was dry friction, you would have the maximum possible friction force also proportional to mass:
Ff=µ*Fn with µ=coefficient of friction and Fn=weight=m*g and g=gravitational acceleration (9.81 m/s²).

If you are at maximum cornerspeed you can set them equal:

Ff=CF
µ*m*g=m*v²/r

as you see mass is on both sides of the eqation and you can calculate cornerspeed:

v²=µ*g*r or v=squareroot(µ*g*r)

However µ is not constant for tyres, and depends from weight, speed, camber, slip angle, temperature and other parameters, so there will be an influence of weight on cornerspeed.

Another aspect is aerodynamic forces: if you have some aerodynamic downforce, this will increas cornerspeed because it raises the friction force without adding weight, so your centripetal force stays constant. This effect is bigger, if the car has less mass.




Keep us informed...

Best regards
David

assuming camber, slip angle and velocity effects are minimal... the weight (force) on the outside tire to generate grip comes from the centrifugal force from the mass of the car acting around the roll center. The centrifugal force or acceleration comes from the constant velocity in a circle.

tires lose efficiency as the weight on them increases, so with a lighter car you can drive faster to generate the same weight/grip relationship as the heavier car. ie. All things being equal the same tire will give up earlier on the heavy car because the tires reach the limit earlier.

This is convenient, otherwise my go-karting excuse for being beaten by 10 year old kids who can drive is out the window

Great discussion. Thanks for the formula David. It makes sense that there is an optimal weight, but there are many variables that play into the traction or friction of the tires. I am going to attempt to calculate the optimum weight based on some normal baseline variables like track type (Asphalt), track temp, tire compond. Aero downforce will be a "good guy" force which will increase my traction without increasing mass. So if I calculate the optiomal weight, any aero downforce will equate to increased speed.... Theoretically.

Thanks for book reference gameover. I am an engineer as well. Your comment on the go carts made me laugh.

R. Schumacher

I currently can consistently record a 5.4 second lap. I ordered the lighter battery today. This will reduce my car weight by 155 grams. (From 1426 to 1271 grams). It should have a measurable impact. Based on my calculations I should be able to acheive a 5.1 second lap consistently. I will post the results.

R. Schumacher

I received the lighter battery. It reduced my car weight by 185 grams. Total car weight is 1249 grams with the lighter battery. I can gett 77 laps on it which is plenty since our races are 40 laps.

I was able to reduce my track times by 2 tenths of a second. The track temperature is considerably cooler (55 degrees F) than my previous runs, so I expect the improvement to get better since the friction will improve with a higher track temperature.

More to come but it appears to have a definite positive impact.

R. Schumacher

The Track temperatures have finally increased to acheive good grip. I was able to record 5.0 second laps with the lighter battery. The car is on rails. The lighter weight has reduced my lap times by .2 to .3 sec. I have found that this only has a significant impact if the grip is good. I am extremely happy with the results.