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MG Midget and Sprite Technical - tyre pressures at speed

Hi all any racers out there i can borrow a bit of your experience , i was sprinting at lyyden at wend and on the top bend (top of hairy hill ) i was getting a bit of under steer not mega loads just enough for me to have to scrub off speed so it would turn in to nice controlable over steer , my speec is frontline front end ( avo shocks turned up to 4 on a scale of 14? i think ) 400 lb springs 11/16 s antirollbar, toyo t1r s 205 50 15 tyres running at 28 psi (cold ) i dont think the suspension is set up to hard iv seen it lent in photos so im thinking tyre pressures any one any ideas cheers if you dont ask you dont know Dal :-)
darren1800 frogeye

The trouble with sprints is you don't run long enough to get any heat into them. For Yoko 32 or 48Rs you need to run soft, maybe as low as 20 - 24. Caterhams will run them as low as 18. I don't have personal experience of Proxes but would have thought you could start with the book pressures and drop the front to 24?
David Smith(davidDOTsmithATstonesDOTcom

Darren,

I had issues with understeer at Wiscombe hillclimb and the main hairpins and added a rear ARB to counter it. While adding/increasing a front ARB can help stabilise the car compared to standard it does lead to more terminal understeer. The ARB I added was from the rear of a Vauxhall Chevette and re-bent to fit, about 1/2" OD IIRC and fitted to angle iron mounts bolted to the lever arm damper positions, modified lever arm drop links connection it to the axle.
David Billington

Agree with David.

One of the reasons that car makers recommend higher pressures for high speed cruising is to make the tyre less deformable and therefore prevent it overheating through rapid deformation.

For sprints, you need the opposite - deform the tyre as it rolls with low pressure to get heat to build up.

Conventional suspension theory says reduce understeer bu softening front: roll-bar, spring, damper rates.

Your figures all seem high for a Frog with a light engine.

A
Anthony

I run 175/60 AO48's at 20 psi cold (sometimes 18 on the rears)on the racer and Lydden was my best race last year - worked brilliantly there - Peter May flat rear springs, solid aluminium spring pads, uprated dampers and front springs and 11/16 arb on solid mounts.
John Collinson

i like the idea of a vauxhall chevette arb on the rear.. could you not fit a midget front arb on the rear? has anyone gave it a thought??
sm white

SM,

The chevette ARB has a dip in the middle to clear the propshaft and with slight re-bending fit well with the chevette mounts on angle iron bolted to the damper mounting holes. The lever arm drop links had Us welded to the ends to fit the rubber inserts in the forged eyelets at the ends of the ARB. It was one solution that was easy at a time when I had limited facilities. These days might look at a front ARB, I think others may have done this already, or just make one from scratch from suitable material.

David Billington

With my 4-link rear Modsports car we just went round the scrappy until we found one that fitted across the width of the car, can't remember what but IIRC we used an Alfa panhard rod.

The ARB had a sleeve over each end which slid slong both it and the lower tie bar so it was adjustable. This meant we didn't have to have *exactly* the right one. David S, it may still be in your stores as you had what was left of my modsports car (RIP).

HOWEVER - how legal it is in speed events I don't know. Need to re-check the regs.
Max T

Er, nope I just have the vestigial rear bodywork and 2 door skins. I remember the Panhard rod, it was a funny U-section in pressed tin, nothing like as cool-looking as a Peter May one !
David Smith(davidDOTsmithATstonesDOTcom

This thread was discussed between 31/03/2008 and 04/04/2008

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