Motoring Discussion > BMW X3 E83 SUV - New rear tyres, less grip?! Miscellaneous
Thread Author: Auntie Lockbrakes Replies: 16
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New Thu 16 Dec 10 03:55 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Auntie Lockbrakes
I guess this question can apply to any RWD vehicle: I've recently put new tyres on the rear whilst the fronts are approx 70% worn. Now, when navigating the hilly corners around here the traction control is kicking-in much more frequently mid-bend...

Am I right in presuming the car is essentially understeering and scrubbing the fronts because of the increased traction through the rear wheels? Or...? I never was very good at physics...

Would I be better off putting the new tyres on the front and swapping the worns to the rear? Car is essentially RWD, the 4WD system only sends power to the front on demand. TIA.
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New Thu 16 Dec 10 07:29 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Manatee
>> Am I right in presuming the car is essentially understeering and scrubbing the fronts because
>> of the increased traction through the rear wheels?

Insufficient data!

>> Would I be better off putting the new tyres on the front and swapping the
>> worns to the rear?

Almost certainly not. Tyreco advice is 'new on the rear' regardless of drive end.

Are you just going too fast?
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New Thu 16 Dec 10 07:31 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Fenlander
The new tyres on the rear are in the right place on your BMW.

I wonder what tread depth you call 70% worn? If they're at 3mm or less I'd replace the front ones as well. Once tyres fall under 4mm they will reveal themselves if driven briskly under adverse conditions so I take extra care and then change around 3mm.
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New Thu 16 Dec 10 07:31 
 BMW X3 E83 SUV - New rear tyres, less grip?! - NeilS
It could be the understeering with the rears pushing the nose wide but that would be quite unusual with a rwd layout.

With fronts 70% worn that's quite a difference between the rear 2 and front 2 and wouldn't be recommended on some awd or 4wd vehicles as it can stress out the drive components due to different rolling circumferences of the tyres. I don't know how the BMW system works, does your manual recommend anything on tyre changes?
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New Thu 16 Dec 10 08:46 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Manatee
Good call on the tyre size/type. The new tyres should always be the same brand and type, and preferably they should all be replaced together. Wear aside, there can be big differences between different tyres at the same nominal size. The Michelins on the CRV are significantly bigger than the Bridgestones that were on it previously.
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New Thu 16 Dec 10 08:50 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Statistical Outlier
One of the things the traction control looks for is a difference in rotational speed between wheels, indicating slip. Could be that there is a sufficient difference in circumference between the new rears and old fronts to be triggering this on a bend where the difference between front outside and rear inside would be most accentuated?

I believe the advice generally is to rotate tyres front to back every few thousand miles to avoid this sort of thing on any 4x4 system. Avoids this sort of irritation and some potentially very expensive wear related problems.
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New Thu 16 Dec 10 13:49 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Skoda
What kind of tyres are the new ones? If they're rubbish and the old ones are decent then...

Tread depth doesn't matter, it's not what gives you grip on a plain dry road.
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New Thu 16 Dec 10 14:03 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Manatee

>> Tread depth doesn't matter,

It might in this case if it affects the rolling radius too much.
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New Thu 16 Dec 10 20:55 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Glaikit Wee Scunner Snr. {P}
IIRC you are meant to keep the same tread depth front and rear by regularly rotating the tyres.
As I understand it this is to avoid confusing and overheating the transmission.
You may have turned the X3 into a part time AWD by not getting new tyres on the front also.
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New Thu 16 Dec 10 21:14 
 BMW X3 E83 SUV - New rear tyres, less grip?! - MD
What about pressures?
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New Thu 16 Dec 10 21:22 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Manatee
What's your theory Martin?
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New Thu 16 Dec 10 21:39 
 BMW X3 E83 SUV - New rear tyres, less grip?! - -
How many miles since these new tyres were fitted (still running in?), also what are they and indeed whats on the front....good point from Mart what pressures?

Obviously a different beast but one of our family had some cheap'n'nasty Chinese ditchfinders fitted to the rear of the now departed 320td Compact, slightest bit of damp and the thing would throw the back end out far too quickly.
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New Thu 16 Dec 10 21:43 
 BMW X3 E83 SUV - New rear tyres, less grip?! - MD
Given that they are supposed to be the same circumference won't pressure have a bearing on this?
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New Thu 16 Dec 10 22:38 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Auntie Lockbrakes
Thanks for the input so far guys. All 4 tyres are Goodyear, same size/spec all round (excuse the pun!). All 4 have the same pressure - 38psi in old money I think. The new pair are less than 500 miles old and roads have been bone dry.

Aside, the car does have an irritating low pressure monitor, which lights up when it thinks a tyre is low, having calculated a difference in rolling circumference. This has not happened, so no reason to suspect a significant difference in circumference.

I'm not driving very far/often over Xmas, so we'll see when the STC kicks in again...
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New Thu 16 Dec 10 23:19 
 BMW X3 E83 SUV - New rear tyres, less grip?! - rtj70
If the X3 is all wheel drive, shouldn't all four tyres have worn at roughly the same rate? I know power is probably normally sent 60:40 between rear and front.

In an all wheel drive car with automatic transmission, not swapping all four tyres at the same time can damage the transmission. It does on some Volvos at least. A puncture can turn out expensive.
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New Fri 17 Dec 10 01:40 
 BMW X3 E83 SUV - New rear tyres, less grip?! - Skoda
Hmmm most versions of xDrive (although not all) don't use LSDs or any other form of mechanical lockup on either axle -- you wont get the run around on the spare wheel for a week and find you need a new tyre and a full diff rebuild with new EDIT: friction plates.

Some versions of xDrive, the early X5's, don't have any lockup mechanism for the centre differential either. You could run all 4 wheels on different circumferences and not wear anything out, or wind up the transmission. They were fixed 38% torque to the front.

The x3 i don't know, i've fired up TIS here and it doesn't give much to go on, refers the technician to follow the step by step instructions on screen of the DIS -- Great!

It kind of hints that windup is possible (there's all sorts of warnings that aren't present on the documents for the early X5) so i guess it must be the modern xDrive with the centre wet clutch -- it can divert 2-98% of torque to either axle.

Guessing the X3 is using that version of the xDrive system.
Last edited by: Skoda on Fri 17 Dec 10 at 01:40
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New Fri 17 Dec 10 21:00 
 BMW X3 E83 SUV - New rear tyres, less grip?! - -

>> I think. The new pair are less than 500 miles old and roads have been
>> bone dry.

I doubt they are fully bedded in yet, it takes up to 2000 miles to fully bed the tyres on the pick up (Vred Winters excepted).

38psi does seem a bit on the large side though, i would have thought 32 to 35 would be a normal pressure but that's a guess.

MB's are notoriously misleading when you read the makers tyre spec label inside the filler cap, the primary pressure given is say 36, but when you delve further into the silhouettes they reveal this is for speeds in excess of 100mph and full passenger loads, when you work things out you have to minus that figure by 4psi for 99% of your driving.

I seem to recall BM's tyre pressures being similarly quoted, might be worth checking that figure.
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