[Nonfractal]

I found that tearing chunks out of a tyre occurs when too much pressure is applied.

[slotloco]

I found that tearing chunks out of a tyre occurs when too much pressure is applied.

Have to echo NF 
And nail varnish holds okay?  And doesn't cause any reaction in the tyre compound that would degrade them, or cause them to swell?
I've never trusted it anywhere near soft tyres, as most nail varnish has some pretty volatile solvents in it.
I am very particular about types of glue to use.
- but also, looking at that tyre - Mon dieu !!! what have you done to it. 
Hard to be sure without a partially side on shot, but it looks dreadfully deformed as if it was even fitted to the wrong size rim, or not glued in correct position, as the shoulder appears out of place.

I would say, we true ultra-grips on a weekly basis, across 15 plus guys, I haven't heard of anyone stuffing a tyre for ages.
A few guys are running Thunderslots now, and retaining the stock tyres, so they must be gluing and truing those okay.
( I haven't bought any Thunders myself to know the tyres,  preferring to retain my Slot.its, and prove they are just as quick in 65-80 sports.)

I PRESUME you are truing one tyre at a time, so you get a feel of the loading?  These drop arm lathes are rubbish when used to true two tyres at a time.
None of the brands, even the RSM when using the ball race 2nd arm, ever lower exactly parallel, and you never get equal loading on both tyres.
The claw mounts don't help with accurate/parallel seating of the two bushes anyway, it's just a limitation of the concept.

Usually I true on the slightly downward leaning side, as this naturally puts a slight camber onto the outside of the tyre, and provided better hook up[ and bite when you load the tyre up into the corner. Then you switch the axle around. You can mark the handle you turn on the lathe, to try and get both tyres the same diameter, but really, you also need to check them with a good set of digital calipers. 

As for grits, most people use finer than me, 600 is apparently quite common, but most guys in our club have 2 or 3 grits,
and they go progressively finer.
- You can put one grit on one end of the sanding plate, and another on the other end. 
We also mostly have a couple of plates, so we can swap out plates with different grits on them.


- A guy in our club makes up plates for all these brands of lathes, and also spare split pulleys,
and an advanced wide pulley which is better when doing alloy wheels where you can push the axle right through

[Model Murdering]

Interesting picture ...

... in that is shows a ribbon of damage, BUT only across the right third as shown above.

Close examination of the left side shows an odd irregular looking loop that looks like it's ready to let loose if you were to persist.

At a glance, Captain Obvious notes that the material has a cohesion problem with itself. The nasty reaction to the grinding action looks an awful lot like trying to grind, sand, or file on a poorly mixed product that isnt uniformly catalyzed. Most of us have had a dreaded part A and part B incident ... LOL. They might just be unsuitable for slot car consumption, as many H0 enthusiasts discovered about certain tires. There's tires, and there's "tireds" (of trying to make it work)

Unless proven otherwise, Im not always keen on a petroleum distillate coolant for things that arent metal. One might try a common liquid dish detergent, maybe slightly thinned; or not. Plenty slippery, prevents fouling; and maybe wount devour the sticky back of your grit. 240 might be too big a bite. Go up a hundred or two, and try a nonvolatile coolant.

All this spitballiing with my usual disclaimer of: I have been wrong before, but nothing ventured is nothing gained.

Good luck!

Bill

[slotloco]

Interesting picture ...

... in that is shows a ribbon of damage, BUT only across the right third as shown above.

Close examination of the left side shows an odd irregular looking loop that looks like it's ready to let loose if you were to persist.

At a glance, Captain Obvious notes that the material has a cohesion problem with itself. The nasty reaction to the grinding action looks an awful lot like trying to grind, sand, or file on a poorly mixed product that isnt uniformly catalyzed. Most of us have had a dreaded part A and part B incident ... LOL. They might just be unsuitable for slot car consumption, as many H0 enthusiasts discovered about certain tires. There's tires, and there's "tireds" (of trying to make it work)

Unless proven otherwise, Im not always keen on a petroleum distillate coolant for things that arent metal. One might try a common liquid dish detergent, maybe slightly thinned; or not. Plenty slippery, prevents fouling; and maybe wount devour the sticky back of your grit. 240 might be too big a bite. Go up a hundred or two, and try a nonvolatile coolant.

All this spitballiing with my usual disclaimer of: I have been wrong before, but nothing ventured is nothing gained.

Good luck!

Bill

Are we talking about the same picture MM? 
I don't see anything that correlates to what you describe.
But I do see a tyre that looks misfitted to the hub on the inside, because the inside wall looks bulbous, the outside wall appears to be sheer/straight.
It looks to me like it has been cooked, overheated and melted, and that various bits have then rolled into little strings like miniature dough-sticks.

- I HAVE managed to create a few of these in years past myself, mostly from F22 or NSR Extremes + an infusion of too much welly of the kind that Non-Fractal described. :)

[woodcote]

These drop arm lathes are rubbish when used to true two tyres at a time.
None of the brands, even the RSM when using the ball race 2nd arm, ever lower exactly parallel, and you never get equal loading on both tyres.
The claw mounts don't help with accurate/parallel seating of the two bushes anyway, it's just a limitation of the concept.

Usually I true on the slightly downward leaning side, as this naturally puts a slight camber onto the outside of the tyre, and provided better hook up[ and bite when you load the tyre up into the corner. Then you switch the axle around. You can mark the handle you turn on the lathe, to try and get both tyres the same diameter, but really, you also need to check them with a good set of digital calipers.

I got to try the German version (RSM?) and was really disappointed that it didn’t true both tyres flat - therefore loading one tyre more than the other (potentially damaging that tyre). I’d thought that was the point of the tool - truing tyres on plastic wheels on the axle. I chose not to spend £150 + postage and stuck with my DIY truer - despite its deficiencies, it did a better job for what I needed...

[OXO cube]

hmmmm 
thanks guys 
all points noted 
will re-address using your tips 
and let you know how things progress ( or not hehehe )
maybe I'm just too heavy handed ... we will see
thanks again 

kev

[slotloco]

Yes, the same issue remains that there is play in the swinging arm, and the vertical height screw is also not centred. 
I have (and use) an RSM-3 myself, but only for doing plastic wheels one at a time. Most guys in our club have one of RSM, Professor Motor, and the odd predecessor - the Tyre True in this vein. I don't think anyone in our mottley crew has a Tire Razor, as they were only imported briefly by a guy who didn't stay in the hobby long as a supplier.
Actually a couple of guys made their own lathes too.... not sure if they are still using them. 

But going carefully one wheel at a time, guys in our club get their tyres as good as mine, and I have all the tools and (most of the) time in the book.

We also have a gun engineer in club, who also is good with CAD and has an Endor 5 pro. I think he is currently tossing up between 3D printing a Hudy lookalike that is available for download, or creating his own lathe from scratch.
I wonder if anyone here has printed up and is using any of these.

[Ricko60]

I really like the 3D tire truers - real KOOL! I use a Hudy to true my tires!

[Kevan]

Up until this week I've trued tyres at 5V but decided to try at the lowest voltage possible without the motor stalling whilst grinding, 2.8V, keep the amp draw below 1.9A and...wait...I have to say the results are better than expected.

[Brumos RSR]

Up until this week I've trued tyres at 5V but decided to try at the lowest voltage possible without the motor stalling whilst grinding, 2.8V, keep the amp draw below 1.9A and...wait...I have to say the results are better than expected.

I was going to suggest 3 Volts, or like Kevan says slow.  I use 600 grit, 800 grit, 1500, then 2000 for a polishing.  Also don’t do this with the tire dry. Use something to keep the tires cool, water, dish soap, etc your choice.