tyre difference causing ABS Failure

[sockpuppet]

Hi there guys

Just got my two front tyres changed to some new ones that are more grippy (wet north east weather taking its toll on performance and whatnot)

the wheels are the standard 15" alloys. the front tyres are 195/50/15, but the back ones are 16, so theyre 10ml higher, meaning the nose is lower.

ive noticed a small performance increase and stuff like that due to being more aerodynamic. im only leaving the back tyres the same because my mechanic friend is looking for some kumho performance front 15 tyres (then ill put these new ones on the back so its 15 all round). however, i was out today and as soon as i hit 80mph (and 73000 on the clock, incidentally) the abs light came on and ABS Fail appeared on the screen.

could this be caused by the difference in tyre sizes from front to back? to test the ABS i did a hard stop and skidded about a metre from 30mph, ABS definately wasnt doing its job. Turned off the engine and back on, and the light and the warning have gone. However, theres been a rattling coming from one of my wheels for weeks now. Im thinking it might be the ABS ring but im no mechanic, just basically got no idea whats going on, and i cant afford to get the new rear tyres just yet.

Any help for advice appreciated.

[HimBigChief]

If the rattle's coming from one of the rear wheels, the brake shoe retaining pins have likely come through the backing plate (due to corrossion) and will be rattling around inside the drum causing allsorts of damage to the hub/ring assembly. Best get it checked and it's also something that I'm going to have to sort on mine at some point in the next 3 weeks.

[StevieboyTD]

the front tyres are 195/50/15,

my mechanic friend is looking for some kumho performance front 15 tyres

He won't have to look far, that size tyre is about 30 quid a corner for kumho stuff from camskill.co.uk

[mjb]

the wheels are the standard 15" alloys. the front tyres are 195/50/15, but the back ones are 16, so theyre 10ml higher, meaning the nose is lower.

...the abs light came on and ABS Fail appeared on the screen.

could this be caused by the difference in tyre sizes from front to back?

Yes. As far as the ABS ECU is concerned, your front wheels are travelling at a different speed to your rears so it can't work out what to do, throws up a fault and stops working. You'll get the same thing if you put any speed on the front wheels with them in the air

[Doggy]

[quote="mjb]
...You'll get the same thing if you put any speed on the front wheels with them in the air[/quote]

So if you get some 'air' over an unexpected humpback bridge for example, your faithful Pug will compliment your driving style by disabling the ABS just before the unexpected hairpin you've just spotted....

Brilliant.

[mjb]

Seems to be at high speed for a period of a good few seconds. If your wheels are off the road for more than a second or so at this kind of speed I think you're going to be looking more at the seatbelt pre-tensioners and pillar strength to save you than the ABS system

[steve_earwig]

You'll get the same thing if you put any speed on the front wheels with them in the air

My BX just used to engage the ABS, so no brakes when you hit the ground...

Isn't going down a tyre size as bad as going up, insurance-wise?

[sockpuppet]

i didnt think changing your tyre size affected insurance? ive never ever seen it asked anywhere, about what size tyres i have :S

im really gonna need to get these back tyres sorted out then. theres a bigger gap between tyre and wheel arch now, which is a bit annoying, makes me want to lower it by 10ml to balance it out but theres no way i can afford lowering springs.

if the rattling is in the front wheels, what might it mean?

also, the guy who recommended camskill.co.uk, cheers

[steve_earwig]

I'll bet they never asked if you had a jet fuel assist take off engine on your roof either

[mjb]

Wouldn't that be a rocket, not an engine?

[steve_earwig]

That'd be a RATO...