[merkit the grof]

I have read two comments about silicons, which are "relative truth".

Please allow me to add my relative truth to the relative truth expressed in the quoted post.
As a relative (gettit???) newcomer I didn't race in the 60s and therefore, didn't develop the same allergic resentment to silicon tyres that others seem to have.
However, I am reasonably curious by nature and was keen to find out for myself if there was any truth in the old ""silicon tyres destroy grip" stories. I must point out that I have no skin in the game, I cast my own urethane tyres and could just as easily make them out of silicon.

First urethane tyres and their dust: I race at a club called SSRC (wooden track with a floorpaint finish, a bit abrasive), we have different race nights, one for modern cars using Slot It and/or NSR tyres; and one for vintage cars using mostly Ortmans and other urethane tyres. There is a definite drop in grip on the night following the urethane classes, but it is mild and the track clears up easily after 50 or 60 practice laps.
Back to silicon tyres: Many years ago I prepared a car with silicon tyres (Slot It group C) and lent it to someone (without telling) just to see what would happen. When directly following him, I did notice a fair drop in grip, nothing too dramatic but enough to lose a couple of places in the results if you followed him in all 4 heats. At that point I put the intense dislike of silicon shown by others down to mostly just another "old wives tale" from back in the days.

Fast forward to last year; I entered a meeting run on "the big track in the little village" Aka, the Green Monster in Champagne Mouton (wood track with a gloss finish). The club there is very found of silicon tyres, one of the members makes his own and others use a mix of those and vintage silicon on their cars.
During practice on the Saturday, my 1/24th car (fitted with Ortman urethane tyres) was doing 9.2s laps. By the Sunday, after the locals had been practicing, I was down to 10.2s, but still fast enough to do reasonably well.

This year went differently. The 1/32nd races took place on the Saturday, and the track was good with plenty of grip.
On the Sunday things changed during the "Thingy" race where everyone (bar a couple) was using silicon tyres. 
In the thingy race, I had no grip whatsoever (using my own urethane tyres) and finished dead last. On we went to the 1/24th scratch class and I was struggling from the off, with a best lap of 11.2 (by then 2s slower than last year) in the last segment, my best lap was 13.8s, total disaster.
When I looked at the tyres on both cars, they had like a waxy finish. Anxious to find out more, I went to the track the following week and ran some practice laps, it took around 250 laps and about 30 tyre cleaning rounds before the grip returned and I was able to lap down to 9.4s
The inescapable conclusion is that silicon tyres can dramatically affect grip for others and mixing is best avoided. The biggest issue is that the problem is quite unpredictable and some urethane/rubber tyres will retain a fair amount of grip, but others will not, and it is impossible to tell until you are actually racing.
Relative truth?


Joel

[Kevan]

He does say quite clearly

So the use of silicon and soft rubber together there isn't a great idea.

[Anthony B]

Nice summary Joel!! And I agree as I have experienced this as well.

I am no tech genius and so my question is why/how does silicone do this? If it 'cleans' the track, I assume it must pick up some kind of coating, and yet sillies do not seem to lose grip.?

I guess the answer is - if 1 driver uses sillies - everyone must - or none use it at all (as per Roncole and Bordo), and as Urethanes are so freely available there is no excuse not to use only them if thats what the rules state.

[merkit the grof]

I am not at all sure it does clean the track, the CM track is gloss, it doesn't visibly rubber up, there is nothing for the silicons to pick up.

Silicon itself might be inert, but there is no telling what additives have been thrown into the mix during the manufacture of the tyres. On the day, Don Siegel said that it was the modern silicons that were the culprits, but, if that was the case, the old timers who came across silicons in the 60s wouldn't have such an aversion to them.
I don't think 1 driver would make a massive difference, what happened that day is that all bar 2 drivers used silicons in the thingy race.


Joel

[Gordon Steadman]

I'm one of those 60s racers with an aversion to silicone tyres. I may have mentioned this before (ha!) but the firsttime I came across them was at a Luton 24 hour race in 67.

Half way through, one team fitted silicones and I had to follow them on the lane they used. It was like driving on ice. Admittedly we were using the standard spongies and goop but we immediately protested and they were told to swap back to proper tyres!

The effect on urethane is not quite as bad but still bad enough for me to win a first leg going away and then come absolutely nowhere when following a silicone user on the same lane.

I refuse to race where they are used. Don't suppose they will miss me but I do miss racing. I think the Green Monster team have agreed to ban them for the big meets but Bordeaux still allow them at the December meet so that's out for me.

[Anthony B]

Gordon - why deny the pleasure of racing - simply get 1 or 2 sets to use only there?

[Gordon Steadman]

It's the principle of the thing....init