Adding to Kayce's advice on tyres, if you can't get yellows to give grip for eight minutes then try pink rears. Rears should be pink or yellow for mere mortal drivers, the Olly's of this world can drive magenta rears but most of us can't!
I'm not sure what Kayce means by slip in the diff. If it means lossening the diff until it slips when you punch the throttle, then that is not good advice. A 12th diff must never slip at all under any conditions. It must, as Kayce says, be smooth as silk in action.
You can turn a 12th car into an understeer in, oversteer out machine by having a diff that is stiff or notchy in action. Stiff, as in overtightening a TC front diff to get more traction out of a corner, or notchy as in when you turn it it feels like a ratchet. A 12th diff must be glass smooth in action. If you contra-rotate both rear wheels, it should have no resistance at all; with strong motor magnets you should be able to get the diff to work without turning the motor.
At the same time, there must be no slip at all. Grasp the right rear wheel in one hand, with you thumb and index finger holding the spur gear. Now try and turn the left wheel with your left hand. If you can get the diff to slip (the left wheel turns) it is too loose. If the left wheel won't turn (give it a good try, but don't wrench it) it is set right. Now try contra-rotating the wheels. If you can feel any resistance, then rebuild it again.
Try the diff of any 12th National A finalist and this is what you will find. Smooth is in, slip is out! If anyone would like a demo of how to build a diff like that, then please ask. I can get down to Ardent most weekends and would be happy to bring my kit down and help anyone learn how it is done - once seen, never forgotten! PM me if that helps - I'll be down there this Saturday - and I'll give you contact details.
Good to see such an interest in 12th, so please don't hesitate to come on here and ask questions. Our next National is at Hereford (see
www.brca.org, 12th Section, for details) and after that there is one at Tamworth in December. Come into the pits are ask any of us for help, we're pleased to sort your cars out, give you all our set-ups and loan you bits to try if you need them. HTH
