Tyre Truing  

Why

The Importance of being Round

"The wheels are only small, and they look round, what's all this about?"

Little rubber tyres are mass-produced in molds, as are the hubs/rims. These may not be completely round - especially since the wheels are so small - they would have to be made with 32 times more precision to equal real world wheels and tyres.

"OK - they might not be TOTALLY round, so what?"

If the wheel is slightly off-centre, or if it has some lumpy bits that are at a slightly bigger radius than others, as the wheel turns, it will raise and lower the car. Things get worse as the revs get higher, because the high spots will tend to bounce the car, making it less stable. Also, during each 'bounce', the wheel will not be touching the track. In bad cases, your tyre might only touch the track 10% of the time! This is going to affect grip, which will affect acceleration and cornering ability.

"Oh OK, that seems quite important"

Told you so

Flat Tyres

"So the object of all this is to make the wheels round?"

Well - that's a big part of it, but we also want to make them, flat

"let the air out? I didn't even know they had any in to start with"

a new tyre
- in this case an Ortman replacement tyre

No no - I mean the profile of the tyre, as seen from the front. The tyres do not come perfect by any means, and the way they sit on the hubs/rims mean , if you look at the tyre from the front, they may have a bulge in the middle, or they my have too ridges at either side.

"Oh yeah, what of it?"

Well, when the car sits on the track, the only the bulge or ridges will be contacting the track. So that might be 50% of the area of tyre touching, instead of all of it. Again this means correspondingly less grip than if it was flat.

"Oh right. Well what should I do about it?"

Read the next bit on how to true tyres.

How

The wheels on the slot car go round and round

"is it tricky?"

No.

"Is it expensive?"

No - look if you listened, you'd see!

Next I will give a basic procedure that will see you right, and all you will need is your car, a bit of track and power, and some sand paper.

"do I need to do all four wheels?"

For most cars, we are only talking about the rear wheels, which drive the car. I could explain why, but can we just get on with it?

Steps

1: Ideally - take the body off the car. This will stop bits of rubber ending up in the car, in the gears and in the motor. Also - it's a good idea to oil and grease bits before you start. See another article for that! This step can be skipped if you prefer to be shoddy.

2: Place the car on the track facing the wrong direction. NB - some people prefer to have the car facing the right direction so as only to run the motor and gears in the correct way, but the truing is easier if you place it on the wrong way. Also, choose a flat straight piece of track.

3: Slide the sandpaper under the rear wheels. Most types of sandpaper work, get some general purpose stuff from a DIY store if you don't have any lying about. I have used grits between 100 and 500. If you use finer grits, it will take longer, but if you are careful it will probably give marginally better results.

4: With one hand, hold the rear of the car, so that you will be able to lower it to touch the sand paper. With your second hand, hold the sandpaper. The car will try to pull the sandpaper underneath itself if you have the car in reverse, or it will try to push the paper away if you have it forwards. The car will also try to drive away.

5: With your third hand, pull the throttle maybe half way so the tyre spins quite fast. If you don't have a third hand you will have to develop dexterous feet or be inventive.

6: Lower the wheels onto the paper, mainly allowing the weight of the car to apply the pressure. Slide the car from side to side a bit. After a few seconds, raise the car again

lower the car gently

7: We have raised the car for 3 reasons -

    7a allow the motor to cool. We don't want to end up with round wheels but a burnt out motor.

    7b allow the rubber to cool - if the rubber gets too hot, it will melt and change consistency, becoming less grippy than unmelted rubber

    7c see how our truing is going

8:

notice only the centre is sanded. This would be the only contact patch if you stopped truing now!

(sort of 7c but in more detail) Assess the state of the tyre now. You will be able to see how flat the profile is on the tyres - by looking to see which parts of the tyre have been sanded. At first, it will be only the centre of the bulge, or the 2 ridges. As you continue, these areas will widen until you can see that the whole of the tyre has been sanded.

If you look at the sandpaper - depending on the type of tyre and colour of the paper - you may be able to see the trails of rubber taken off. Again, this will start as the high spots, and eventually be a patch the width of the tyre.

9: If the tyre isn't finished, go back to step 6 and sand some more. When sanding, also see whether the car seems to be trying to hop about, or if it is feeling smooth, this will be some guide to how round it is. But usually, by the time the wheel is flat, it is also round. It is quite difficult to assess the roundness without expensive stuff like a rolling road.

10: clean off all the bits of rubber from the chassis

10b - finished - almost! See below

The easiest way is to use your girlfriend's/boyfriend's nail file/emery board. Hold the wheels off the track, apply some throttle and bring the file in at 45 degrees. Sand off the edges, changing the angle of the board to give a nice rounded edge. Again, 3 arms are required, sorry.

the pit crew are hard at work

Some more steps in case you are step crazy

In the same way that the tyre can be imperfect, so can the wheel (Hub / Rim) which it sits on.   Also bits of sprue (molding lumps) may still be there, making lumps in the tyre. To get the best results, the hub should be trued as well.   If you are going to true the hub, do this before truing the tyre, as after truing the hub the tyre will sit differently.

After all the truing, many racers like to glue the tyres to the hubs. This is because, when the wheel is spinning, the rubber tyre expands, and without the glue, can separate from the hub. Don't worry, it won't come off, but it won't be giving the optimum grip either.

centre mold line and sprue have been sanded off

To do this, peel back the tyre and apply some glue around the wheel, and then plonk the tyre back. Superglue is often used for this. Repeat on the other edge of the tyre. Allow to dry before running the wheel, you don't want to send superglue flying into your eyes.

Fly away from here!
Just a quick note: some tyres, especially those on Fly Classics - are very difficult to true. The rubber does not seem to sand and melts very easily. Don't get disparaged, get new tyres from another make like Slot-It or Ortman or someone.

Results

"So how much quicker will my cars actually be?"

It kind of depends on how bad the car was to start with. I was going to time 10 laps before truing the new tyres, then time 10 laps after, and show you the amazing temporal difference. Unfortunately I forgot until it was all too late.

But it will vary from car to car, depending on how round the tyres were to start with. Try it, you won't be dissappointed!

Thanks to Astro for this article

We're looking forward to more articles    
in our SLAP My SlotCar Series.