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Fine tuning (tyre) grip

PostPosted: Wed 21 Apr, 2010 12:49 pm
by wixwacing
I had an opportunity to take the Jag out on a local board track last week. The track in question was a lot more technical than the previous one it had been on. Cornering ability ranked high on the agenda and on the first track, when pressed hard, the Jag showed a tendency to 'cock a leg' under pressure. Between tests I decided to apply a recently learnt preparation to the tyres to improve its corner performance.

The problem was that in a race situation with the model pushed hard, there was a tendency for it to tip up and eventually deslot, sometimes roll over style. I identified the problem as too much grip in the tyres when cornering. The tyre edges were radiused and the model run again. Slight improvement, but still a 'knife edge' situation under pressure. The model has an inherent high centre of gravity and the first solution might be to add some low down ballast to keep the model upright. The downside is that extra weight will affect acceleration and extend the braking zone. Both undesirables as for as body tuning is concerned.

The next solution is to reduce the effective grip of the tyre in a corner to allow the model to 'wash off' a little bit of corner speed and allow it to navigate bends more like a non magnet racer should. So, how do we go about it? The solution has been at hand for a long time but it was a case of spotting it which has taken me time.

When racing some of those highly raceable Ninco NC1 touring cars, it became apparent to me that you could get some models, like the Audi A4, to tip in corners when running extra sticky tyres. Once the standard ribbed tyres were refitted good cornering behaviour was restored, but at the expense of some good straightline acceleration. What was needed was something between the two, Mmmmmmmm? Once again, whilst trying to tame a problem model, the Autoart Lamborghini Murcielago, with the massively wide MJK tyres on the rear of that model, cornering was also a bit of a lottery. In desperation I decided to reduce the grip of the tyres by cutting some slots in the tread area. The first attempt was the complete width of the tyre and it soon became evident that this was too much, allowing the rear of the model to flail viciously on corner exit. Obviously there were too many grooves in the tread.


With the next pair of tyres I approached the problem from the opposite direction! I cut one groove in the outer edge of the tyres and tested. A minor but significant improvement was experienced. I then cut an extra groove besides the first and retested. The solution was at hand and along with radiusing the outer edge of the tyre, I had achieved a tyre which gave good straight line grip. So could I apply this to the Jag?

Back to the Jag tyres and not a lot to play with here, the tyres are barely 7.0 m.m. wide so there wasn't going to be too many groove. I settled for four grooves to start with and the tyre was mounted on a spare hub fixed to a dummy axle and installed in my drill press. The tyre /axle was spun up at about 1200 rpm and the point of a modeller's file was carefully applied to the tread area of the spinning tyre. Being careful not to overdo it I lightly pressed the file into the tyre which it cut quite easily. This created a rough groove slightly deeper than it was wide. Happy with that I cut another three grooves into the tread surface, repeating this process on all four tyres. The tyres were refitted to the model and it was packed up for the next race outing for a test.


Once on the track it was blatantly obvious the tyre grooving had been a success. The grooves had allowed the model to brake away controllably in the corner and still retain almost all its straight line speed. Problem solved. Being unconvinced by the many exponents regarding the 'tripod' theory of chassis setup, I have the Jag running on all four wheels. This contributes greatly to its corner stability too. But when tyres touch the track and unless the model has steering and independently rotating front wheels, there is an inherent risk that too much tyre contact will cause the model to 'snow plough' in corners. It is for this reason I have ribbed the front tyres too. This reduces the front end drag of the model and increases the model's front end ability to 'side slip' in corners. Minimising any unusual behaviour patterns caused by front end drag.

So, I am very pleased with the results and I would dare say that the Jag handling is greatly improved, not that it was that bad to start with! So if you have any model which is causing you cornering grief, before you start adding weight, address the items which are more important firstly.