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Old Codger

Understeer & Tyre Size

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My G31 Xdrive is running 20 x 8.5 ET25 Front with 255/35 tyres 20 x 10 ET41 Rear with 285/30 Tyres.

 

I’ll be tracking the car soon and as she suffers from chronic understeer - which takes the edge off high-speed cornering antics – it’s a situation I’d like to improve upon. 

 

Tyre size requires careful consideration because of the AWD aspect, big differences don’t do the transfer box any favours.

 

I don’t think 19” rims will fit as the disc diameter is 395mm with humungous callipers that are already tight to the rim, so I guess I’m stuck with 20s or above. 

 

Anyone got a suggestion as to what tyre, or wheel & tyre combo to put on the front for trackdays?

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I would have thought suspension would have a bigger impact on understeer than tyres, 30/35 profile tyres aren't going to flex too much

 

A decent set of Bilstein B16 dampers or similar with a possible spring change might be the way to go

 

I'd contact Lorcan on the f30.bimmerpost forum, he's from Rossiters that seem to have a good rep with suspension upgrades

 

 

 

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I don’t think your tyres would be the the issue, apart from maybe the compound, but not the size. 
Maybe a good set of non RFTs might offer better grip in those circumstances, as they will flex more, rather than skip and judder across the road. (Assuming you are running OE RFTs)
Aside from already mentioned suspension hardware changes, I would also look at some simple tweaks, steering geometry, 4WA etc, with the track in mind, but also bearing in mind that these cars aren’t made for high speed cornering, nor for tracking. 
Of course, not forgetting that UK roads are very poor, with incorrect camber on bends, lumps and bumps meaning much less tyre contact with the road than you would find on a good track, so high speed cornering on our roads is always something I avoid. 

Apologies if you mean you are already experiencing the understeer on a track, rather than just on the roads. 

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Doing track days on my motor bike I would always have to lower the tire pressures by 1/4 to a 1/3 for the track. This would ensure with the extra heat you would maintain a decent tyre footprint to give good grip.

 

Would this same rule apply to track days in cars?

 

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Dare i say its simply the nature of the beast? AWD and a big heavy car and probably never intended as a track weapon? Its a continental express.

 

My e28 could be made to under steer on track as standard until i changed the top mounts for more camber and changed the ARB and suspension.

 

It was better but not a race car. What i found though was the more changes i made for track days the worse it was on the road and it still rolled too much on track.

 

Maybe just leave be and adopt slow in fast out?

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5 minutes ago, duncan-uk said:

Well there is that approach....:lol:

PS that doesn't work in the wet going into the chicane at Donnington on a bike !!!!

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