Tire Testing Machine
Tires are the most important part of handling. Is the biggest performance improvement you can make. There are many tire choices and just as many opinions on which is the best. In general, most people have settled on 8 deg for rear and 15-30 deg for front. I decided to do some testing myself to try to get some numbers. Real tire test machines is way out of my means. I just wanted to use what is around the house. Is not a formal test, but is just to satisfy my curiosity. Word of warning. Conclusions drawn here are not the final authority. I like to trust them, but that is just my opinon, it actually contradicts what various expert drivers believes.
Tires come in different size and degrees. Normal width are used for front, and wide are for rear. Kyosho makes simi wide that can be used in the front, but testing so far shows there is no advantage to it. Tires come in only one diameter, 20mm ID and 25mm OD for both front and rear. Degree in tires is not about angles, but rather the hardness of the rubber. It is generally accepted that the lower the degree, the more traction and higher degree has less traction. In Kyosho nomenclature tires go from 20 deg to 60 deg. Most aftermarket tires use a different scale that goes from 8 deg to about 35 deg. 8 deg is about equivalent to 20 deg Kyosho tires. For the test, I will be using GPM tires bought from RCXmini.com. It is just because they are the lowest cost tires that allows me to buy multiple samples of different hardness, and I had good luck with them.
Questions I wanted to answer are:
1) Is rear motor faster or is mid motor faster. That answer depends on whether having more weight on the tire improves traction. More weight means more centrifigal forces, so the tire would have to hold on tighter to overcome the added weight. Question becomes whether coefficient of friction goes higher with more weight.
2) Do mini-z tires behave like a car tire with similar slip angle characteristics. Typical full scale car slip angle characteristic curves is shown here. This picture is linke to the http://www.donpalmer.co.uk performance driving web page.
3) What is the difference between different hardness.
By driving around the track and trying different tires and chassis, I have some seat of the pants answers. Mid motor car is faster than a rear motor. Mini-z tires seems to behave like a full size racing car tire in terms of slip angle characteristics. With the GPM tires, I don't find any better traction going from 8 deg to 25 deg tire. In fact, I feel traction actually goes up when you go to a 25 degree tire. 15 degree tires however is more slippery.
A simple tire testing machine was made out of some VEX Radio Shack robot parts I have laying around the house. We had a VEX robot competition at work, and they gave each team a VEX kit. Is worth about $250. We were suppose to give it back after the competition, but that never happened. The kit is chump change to a big company anyway.
Here is the tire
test robot with the remote controller.
Tire is mounted
to the end of the arm at an angle. A protractor is used to set the slip angle.
The arm operates a
kitchen scale that measures side force down to 1 gram. Proper proportioning
is used to calculate force occuring at the tire.
1) This graph answers the question on whether MM or RM is faster. For the McLaren, the weight distribution is 95gram/92gram front and back, or nearly 50%/50% distribution. For the Porsche 934 RM, the weight distribution is 78gram/100gram front/rear or 43%/56% distribution. As seen in the chart below, the wieght on the tire does not affect performance that much. In either case, RM or MM will have about the same side grip in the back.
Same test shows that for the front tire, the performance decrease when there is more weight on the tire. Therefore, a RM will have more front wheel traction than a MM in steady state cornering.
2) and 3) The graph of slip angle versus coef of friction is similar to one generated by full size cars. Most traction is generated at 30 degree slip angle. After that, traction drops. This graph is intersting in that it shows both 8 deg and 25 deg tires have the same traction. Therefore, both should not be any performance difference. That is backed up by my own experiance. However a 25 deg tires wears longer, and because it is stiffer, it tends to stay better on the wheel. That means in general, 25 deg tire should be used. At 35 deg, you start to loose traction.
For the rear tire, 8,15 and 25 degree tires have about the same traction. The 8 and 25 have slightly more traction than the 15 degree tire as in the front case. It is interesting to note that the rear tire eventhough it is wider, has about the same ultimate traction as the narrower front tire. Therefore, a narrow tire can be used on the back for the same performance.
Since the width of the tire have no affect on traction, using the same reasoning then the camber of the wheel should have no affect on the traction either. Test indeed shows that camber of the wheel have no affect on traction. Conclusion, you should pick a camber that gives the most even tire wear.
Note that all these tests are measuring a constant slip on the tire. That means a dynamic friction rather than static friction is measured. With the equipment I have, is difficult to gage peak friction associated with static friction. I assume that higher dynamic friction equals higher static friction. From observing the scale, there is an initial jump in tire side force then it settles on a steady sliding force. That initial jump assuming it is the static friction is about 5-10% higher than the steady state slide side force. Although from reading other web sites about tires, I am not sure if tires are suppose to have static and dynamic coef of friction as in an object sliding. Tires are always flexing and slipping, so the classical static and dynamic friction probably does not really apply here. This is just a start of a tire test. I believe proper testing with good equipment that can measure peak force can yield important handling information about mini-z cars or any other R/C cars.
