Front Tyre Sizes

I found that increasing the front width to 245 (from 235) made the steering more prone to following the camber of the road. I increased the width to provide more rim protection.

Speedo sensor is in the diff, so the front wheels don't effect them.

Wider tires tend to do as Timm say and follow the grooves in the road, especially if the alignment is off.

Zitat von Noggie;106524

Speedo sensor is in the diff, so the front wheels don't effect them.

Wider tires tend to do as Timm say and follow the grooves in the road, especially if the alignment is off.

Get spherical bearings for the upper control arms and kiss this tramlining issue good bye.
I'm running 255/40/17 front and 275/35/17 rear and had a horrible tramlining problem. But using 235s in front is not road legal in combination with the 275s in the rear. After switching to the bearings I had a massively reduced tram lining effect (plus some other advantages).

Zitat von Noggie;106524

Speedo sensor is in the diff, so the front wheels don't effect them.

Isn't the front wheel speed compared to the rear wheel speed to detect wheel spin?

I think if your front and rear tyre diameters differ by more than 3% you will have problems. I did this once and my ASC T used to cut in under moderately hard acceleration and cut the power to the rear wheels.

Absutely correct Difference in size of better circumferrence would confuse the ABS.

I suggest that you consult "Anleitungen", "Zusatzbetriebsanleitungen", where 18" sizes and pressures are listed.

Best regards

Peter

You can check for % speed difference of any tyre sizes here:
http://www.ukzero.com/calculator1.htm

I run the standard sizes for 18" wheels as in the E31 handbook:
245/40 Fronts and 285/35 Rears.
These have no speed sensor errors and even keep the insurers happy as they are OEM sizes

Zitat von shiggsy;106530

Isn't the front wheel speed compared to the rear wheel speed to detect wheel spin?

I think if your front and rear tyre diameters differ by more than 3% you will have problems. I did this once and my ASC T used to cut in under moderately hard acceleration and cut the power to the rear wheels.

true.
but that is the ABS sensors, and they do not matter for the speedometer.
That sensor sits inside the diff

Unfortunately, tyres are only produced in discrete sizes - it is often impossible (or very expensive) to find tyres exactly the correct size. However, as long as you manage to keep the rolling diameter to within 3% it will cause no problems with the ASC (and ASC+T) system. Here's a table that shows the tyres sizes that can be used (although not complete) for the E31:

Zitat von philR;106518

I have started to look into replacement tyres and would appreciate some advice.


I have 18” wheels and using a tyre calculator the tyres size required to give a match as close as possible to the original 17” wheels are 235 40 18 front and 265 35 18 rears.


I assume the size of the front tyres will not affect the speedo reading therefore is there any advantage in fitting 245 40 18 tyres?

Thanks.


Phil

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The two sizes you quoted (235 40 18 front and 265 35 18 rears) work very well and are available as Goodyear F1, which have a good rim protector built-in and stick to the road like glue. It's also the size combination used on the e39, 5-series, so it's kind of BMW approved. (I have these fitted on my car). Note that not all possible sizes are actually available from different brands and some sizes can be considerably more costly, if not that common.

The important match between 245/40x18 and 285/35x18 is the 0.5% rotation speed difference - no issue for ABS/Traction

Narrower variants such as 235/40x18 and 265/35x18 have a 0.4% rotational speed difference - again no issue.

1 or 2 percent difference in speedo reading/gearing between these options will have a tiny effect.
I guess the reasons BMW chose 245/40x18 and 285/35x18 as an option for the Csi was to make it more "meaty" than the 840 and add more contact area for cornering?

Note that the speedo overestimates by about 5 - 7% anyway, so a few % more or less won't make a difference. Important thing is that the front and rear have roughly the same diameters.

Larger tyres also put more stress on the front suspension and steering linkages, so bear that in mind too.

If you are considering 285 rear, then the combination used on the Alpina B12 5.7 might be a good guide.

More than 285, there may be issues with rubbing on the wheel arch, although I think some have managed to use these larger sizes without problems. It will also depend somewhat on the wheel offsets.

The biggest problems I found with 285 rear tyres was aquaplaning on standing water. It is extremely upsetting to see the ASC light come on while you are travelling at 80 MPH!

The 235/265 combination I found to be the most satisfactory for road-holding, economy and road noise. I've presently got 245/275, they look meaty, but don't provide any additional safety.

Zitat von Timm;106557

The biggest problems I found with 285 rear tyres was aquaplaning on standing water. It is extremely upsetting to see the ASC light come on while you are travelling at 80 MPH!

I wonder if ist is a question of 275/285 or of the tyres, especially the brand/model...

Could be Harry, but they were Toyo Proxes-S with their famous swept tread pattern - still, just because it looks pretty doesn't mean it moves water out of the way!

I think the regulations the authorities operate with in Norway is that the diameter must be within 5% of the stock dimension.