Silicone Tire Truing
Silicone Tire Truing Process
The below is an 8 step process and involves many hours per rear set of tires. Having trued tires will result in better gripping power of the rear tires and result in better confidence with your cornering speeds.
1. Truing wheels
2. Gluing tires
3. Truing sidewalls
4. Radius tire edges
5. Truing tire tread
6. Polishing w/240 grit
7. Polishing w/320 Grit
8. Polishing w/400 grit
Truing plastic wheels is a wheel/axle relationship. The wheel must be permanently attached to the axle. You must true the plastic wheel while it is on the axle and never remove it. If you true the plastic wheel and then remove it, when you reinstall it , it will not be true in relation to the axle and wobble. I have learned to first remove the plastic wheel and then epoxy it back on to the axle, let the epoxy set over night and then true the plastic wheel. I will clean the axle and as much of the inside of the hub with alcohol so the epoxy bonds with the surfaces. Truing plastic wheels can be challenging as the spur gear will be very close to the wheel/tire making it more difficult to true the wheel and tire.
Metal set screw wheels are much easier and most of the time already come trued. I have found on occasion an out of round metal wheel, but that is easily solved with a little truing. I only take off the bare minimum of metal to get the wheel round.
So now your wheels are trued. I will first mount a tire on to a wheel and look for tread unevenness. Is the tread sagging in the middle? Is the tread bulging in the middle? You want the tread to be as flat as possible as this will help reduce truing time and take less product off of the tire. If you have areas that are low, you can use pin stripping tape to build up the wheel.
If the center rib is too high, you can sand/true the rib down in small increments, sneaking up on the correct diameter. Once you have the tire as flat as possible , remove them and clean the wheel and the inside of the tire with soap and water to remove oils generated from the manufacturing and molding process. You can then glue the wheel and slip the tire on. Slot Car Corner has a new tool that assists slipping the tire onto the wheel and works well. Do not mix up the tires and wheels. The bonding agent of choice is purchased at most local auto parts stores. It is made by "VersaChem", and it is called "Black Magic". It is a Silicone weather-strip adhesive. Apply the adhesive thinly on the wheel and after mounting the tire on to the wheel, spin the tire on the wheel to help spread out the adhesive evenly. Let this set up for at least 45-60 minutes. I will let it set up for 24 hours. When installing the tire onto the wheel, there will some glue squeeze out. Remove any evidence of glue from the sidewalls of the tire.
The next step is to remove any rounded sidewalls from the tire. Make sure the tire is spinning so the bottom of the tire is spinning away from you. Both Super Tire and Quick Slicks have rounded sidewalls and these need to be removed. The reasoning is the tread needs to have wheel support directly under the tread. So you can use a multi bladed razor or 150 grit sand paper. All the sand paper used in the truing process is made by 3M , it is Wet/Dry 413Q. I went to a body shop supply store to get my sand paper.
When using the razor, you want to hold it mostly vertical against the sidewall with a slight cant ( angle ) inwards. Hold the razor almost vertical and while the tire is spinning ( 8 volts from the power supply ), rub it up against the sidewall with a slight cant towards the middle of the tire. You want the sidewall to be angled about 1-2 degrees inward. . This will reduce the tread contact patch just a bit. Do not press the sidewall to hard with the razor as you can destroy a tire very easily, but it can make quick work eliminating the rounded sidewall.
When using the 150 grit sand paper, you will follow the same process but you must keep the sand paper hydrated with a water/dish soap mixture. I mount the sand paper to a popsicle stick to make it easier to handle and the popsicle stick is a flat surface. I got my popsicle sticks from Hobby Lobby. Do this to the outside and inside of the tire.
Radius tire edges
So now you have trued the wheels, glued the tires to the wheels and removed any rounded sidewalls and slightly angled the sidewalls inwards. Now you will radius the edges, the area where the sidewall meets the tread. You can Do this with a multi bladed razor or using 150 grit sand paper. Hold the razor vertical up against the sidewall towards the top/outside edge of the tire and while rubbing it against the tire slightly, curve the razor 90 degrees so it ends up horizontal, almost parallel with the tire tread. The rule of thumb is to create a 1/8th inch radius.
And although I have done this, I am now starting to create a slightly smaller radius. This also will reduce some of the tread contact patch, but it is the radius on the tire that gets the tire to bite going around the corner, which is the best way to lower lap times. Repeat the razor truing until you get the radius you desire. Do this to both the outside and inside edge of the tires, although the inside edge can be a slightly smaller radius then the outside edge. Again, be careful not to press the razor too hard on the tire or it will gash and destroy the tire.
Using 150 grit sand paper ( mounted on a popsicle stick ) is the same process, but you must keep the paper hydrated. It just takes longer. I think you actually have a bit more control of the radius process using the sand paper.
Truing the tire tread
You've trued the wheels, glued the tires to the wheels, canted the sidewalls and radiused the tire edges. So finally it is time to true the tire tread. This is the only step that actually true's the tire tread. For plastic wheels you will use your Tire Razor or Area 3 tire truer. For set screw wheels, I use a Hudy truer as it is much quicker. Mount the 150 grit sand paper to the sanding block and at 8 volts from your power supply, lower the tire while it is rotating on to the sanding block while you are moving sanding block left to right and back and forth. You must keep the contact patch between the tire and sand paper hydrated.
The resistance of the tire on the sand paper should be set to 50 amps on your power supply. You do not want to overheat the silicone as the surface will get gummy and then you have just destroyed the tire. The amp draw will change so be vigilant and keep adjusting the tire pressure to the sand paper to maintain the 50 amp draw. Don't be stingy with the sand paper, let it do the work. 12" x 12" sheets of sand paper are about $1.10 per .
During this step, stop the tire once in a while, dry it off and check the tread surface. The part of the surface that is dull and grainy is where the sand paper is contacting/sanding the tire. If you have any shiny parts on the surface, you still have more sanding to do. Keep going until the entire tread surface is dull and grainy. It's grainy because your using 150 grit sand paper.
While the tire is spinning, I can check the roundness of the tire by rubbing my finger nail against the tread. If it feels smooth, I'm close to being done. If I feel vibrations, I still have more truing to do. Over time, you will develop a feel for this. Make sure to keep everything hydrated. Once this is done, flip the tire/axle assembly over so it will spin in the opposite direction and true just a few minutes more. This step alone will take up to about 45-60 minutes. But get it right, this is the business end of the 8 steps.
Polishing with 240 Grit
Once you have the tire trued, you want the surface to be as flat as possible. So now you will start the process of polishing the tread to get it flat and smooth. This step is more structured. Tires still in your truing machine, attach the 240 grit to the sanding block and again , move the sanding block back and forth. You are looking to get rid of the graininess of the 150 grit paper. Again, maintain the 50 amp draw. Add drops of water to keep hydrated. This step will get the tread to look more smooth. When you add drops of water to keep hydrated, it will eventually dry up in about 5 minutes. Add drops of water 3 times ( about 15 minutes ) to the contact area of the tire and sand paper and when you are done, flip the tire over and polish in the opposite direction for the same period of time and drops of water. Maybe about 40-45 minutes total for this one tire. And now you will sand the sidewalls and the radius with 240 grit sand paper on the popsicle stick to get those surfaces smoother also. Keep hydrated.
Polishing with 320 Grit
This step is the same as the 240 grit. And you're looking for the tread surface, radius and sidewall to look even smoother than with the 240 grit.
Polishing with 400 Grit
Again, this process is the same as with the 240 and 320 grit sand paper. Here you are looking to have the tire look like it just came out of the mold, very, very smooth.
Some people have gone as far as 600 grit sand paper, but that is smoother than the track surface and after about 30 laps, the tread surface will end up being about the same as the 400 grit sand paper. You can try 600 if you like. On average, you can expect to remove about .035 -.040 of tire diameter total.
Now you are done. But the tires still need to be broken in on the track. This will take about 30 or so laps. You will notice a great improvement in cornering speed, thus building more confidence in getting around corners.
I want to point out that this tire truing process is not my idea. I give credit to and thank Rob Hall out of Clover Leaf Racing just outside of Detroit and his tire truing partner, Smokio ( don't know his real name ) from Slot Car Corner. Rob spent many hours on the phone and texting with me to guide me through this process. Rob was very generous with his time. He has trued, to date, about 1,000 sets of rear tires. It takes Rob about 7-9 hours per set. Smokio can do it in about 5-6 hours, and it takes me about 6-7 hours. My tires still do not look as good as Robs, but with time and practice, I will improve with every set, and so will you.