I have a Vauxhall Astra Mk4 which has ABS and Electronic Brake Distibution (EBD). It ran on 195/60 x15 tyres as standard. I always move rear tyres to front and put new tyres on the rear, as recommended (no opening of old debates, please).
As part of an "upgrade" I fitted 205/60 x15 on the rear which will soon be moved onto the front, with new matching 205/60 x15 on the rear.
For a while now, roughly co-inciding with the use of increased rolling radius tyres on the rear, the braking has felt strange. It feels as if the servo isn't always working and the ABS is trying to kick back quite often, even when braking gently.
The servo has been checked out ok, as well as the non-return vacuum valve.
It's just occurred to me that the EBD might be getting confused. The rear wheels will be rotating slightly slower because of the larger rolling radius. As EBD uses the ABS sensors to assess rotational speed and deceleration, is it getting confused? Presumably the system has to cope with small changes in rolling radius due to different rates of tyre wear but can't cope with this large a change.
Is my theory correct? If so the problem will go away when new tyres are fitted. If not, any suggestions?
|
|
205/60 x 15 are larger diameter than 195/60 x 15, 205/55 x 15 would be closer to the required rolling ratio.
|
I know 205/60 x15 are larger diameter, that's partly why I chose them - I didn't want to reduce the profile, that would further reduce the ride quality.
Thanks, but the question isn't about tyres, it's about brakes.
|
>>Is my theory correct?>>
Very possibly, though you won't know for sure until the new front tyres are fitted.
|
|
Electronics do strange things, I once had a 2.0ltr Vectra with a strange vibration, the garage put it on a ramp with the front wheels free so they could get them up to a reasonable speed so as to see it it was a drive shaft imbalance issue, they could not work out why the car would not rev beyond 2000rpm or so, I had to tell them to turn the traction control off, the rear wheels were not turning so the car thought it had an excessive case of wheelspin.
|
|
|
|
Thanks, but the question isn't about tyres, it's about brakes.
But your question did ask whether the tyres might be causing the problem ;o)
|
yes the tyres could cause the problem - and probably are.
As pointed out the new tyres are a larger diameter / circumference etc.
your brake system monitors the speed at which the wheels rotate. As the rears now rotate at a differenbt rate to the fronts the ecu thinks that something is slipping.
As an aside your speedo will also be incorrect following this change.
|
|
|
|
I do not think your braking will be effected by the tyre change as the abs only really reacts when a wheel is close to locking not under easy braking. On the issue of front to rear, you can do this but you should change them diagonally as the effect of pattern walking is inverted from front to rear as the front have been working most under acceleration and rears under braking fo the greatest wear has been in opposite directions and the tyre has developed a wear pattern ( walk ) and if you just put the rear on the front, same side, then you have reduced the total weighted foot print on the front and more so on the rear so you could get the back end breaking out in the first couple of 1000 miles until the feathering wears off. Just look carefully at a front and rear tyre side by side and you can identify which end the tyre was fitted to what. Don't believe me, then just rub your hand round the circumference of the front tyre in the middle of the tread, clockwise and anticlockwise and that feels different and and grips your hand by a noticeable different degree. As for putting the new ones on the rear, well let's not relly go there but me, no, oversteer easy out, understeer, kerb, bent chassis leg. But if you must then diagonally gives you best alternative. Still don't believe, ask someone north of the border, as I am, how uses winter tyre, M&S's and like and the heel and the toe of the tread pattern can show significant wear delts due to using the tyres and a dry road. Safe Motoring. Regards Peter
|
|
Different rolling radii-front to rear- will certainly confuse electronic braking and stability systems.These check for slipping and spinning by comparing the revolutions from each wheel;until you get matching tyres front to rear,switch them off if you can.
|
Thanks guys, the majority consensus seems to be that my theory is correct. I'll let you all know when I change the tyres again shortly as it will then have the same size all round.
The odometer is over-reads by 2.5% over a calibrated distance of about 200 miles on the M6. My selected size of tyre will alter the mph/1,000 rpm by about 2.0% so the speedo will become more accurate, perversely!
|
I now have 2 new tyres, so I'm running the same size all round. The brakes are wonderful again - the servo assistance is there and the ABS doesn't cut in during normal braking.
Different tyre sizes DOES confuse the EBD - QED.
It seems strange however that EBD will cope with front tyres wearing more than the rear tyres giving a 3mm difference in rolling radius, but won't cope with a size change which gives a 6mm difference.
|
|
Well I'll be, I really did not think it was the cause of all you problems. Cool Regards Peter
|
|
It learns(a little);it can cope with slow changes(wear)and very small % differences.
|
Absolutly spot on JC
Different size tyres will cause the EBD to think that the wheels are turning at different speeds and it will adjust the braking pressure at each axle to try to rectify the difference.
|
|
|
|
