Toe-in Specs with Semi-Slick Tyres

[kayjay48]

How much toe-in are people using with semi-slick tyres?
My car is running 205/60x13" Nankang AR1 tyres. Front springs are 600lb/in from Motobuild and front camber is 1.5 degrees negative.
The car handles well round corners but is rather twitchy in a straight line.
It currently has 1mm toe-in per side, but I'm wondering whether a little more toe in might help.

[scotty]

Have you lowered the car? I would also check the bump steer.

[kayjay48]

Yes, it is lowered, around 50mm front and rear.
There would be very little deflection of the high rate front springs under normal cruise conditions on a smooth road, so I don't think the problem is due to bump steer.

[scotty]

Should the bump steer be out, checking the toe in without a driver in the seat will be different to with one in, just a little point but as you wish.

[kayjay48]

Thanks for the suggestion. I put around 60kg in each of the front footwells of the car, and another 40kg on the bonnet, and the toe-in didn't change.
It doesn't look like bump steer is the cause of my problem but good to know for peace of mind.
I have checked with Motobuild, who supplied the springs, and they said the lowering shouldn't change the steering alignment/characteristics.

[scotty]

Nice motor, where are you based? Just a small thing, the tow eye has to be in a contrast colour

[kayjay48]

Thanks. I'm in Auckland, New Zealand. NZ regs don't require the tow hook to be a contrast colour. Just that it's either red, orange or yellow (and the tow position is marked).
What are the suspension specs on your car?
I'm running AVO adjustable rear shocks with reset lowered stock rear springs and an MGB front sway bar.
The car is having it's first outing on a local race track this weekend for some testing prior to a winter race season starting in June.

[Nik]

I've been looking but i can't find it, back in the 80-90s I printed out a series of lines on a strip, I then taped it around the central adjusting bar, so I could make adjustments knowing each line was 0.1deg. It should be easy enough to work out once you have the pitch of the thread and the difference in arm lengths. If you do a practice day you could adjust try, adjust try and see what works best, just lift the bonnet break the locknuts adjust, tighten the locknuts and off you go, it was the easiest car I've ever adjusted the toe on.
More toe in will get rid of the twitch on straight but at the cost of cornering, when it steers nice on the straights you may find it's heavier through the bends it will be down which you prefer and as with everything the best compromise. I believe a lot of race drivers drive cars that are not nice on straights but come to life on the bends you may find on tracks with long straights you want a different setup to short twisty tracks?

[scotty]

Car is lowered to min ride height for UK, very uprated front springs and custom built rears. Shocks have to be original bodies but we have two arms on the fronts. Front and back have adjustable valves and we can adjust bump and rebound. The front antiroll bar follows the original 1959 unit which is mounted across the front bulkhead just ahead of the master cylinders with link rods coming up from the lower spring pans through holes in the inner wings. This is very stiff and we have very little roll across the front. We also run full droop limiting.