Wide front rubber

S

sk15kev

Member
I have had my Z4 a couple of weeks, as posted on the noob sub section. http://www.z4-forum.com/forum/viewtopic.php?t=50180

Z4800.jpg


I have 245/40's on the front of the car, which others have said are very wrong, but I was wondering if they are okay to leave on or should I get them swapped pronto.

There is just enough room for them to turn in the arches and I actually quite like how they look.

What issues could keeping the 245s on cause me and any suggestion as to what I should do?
 
It'll obviously influence handling somewhat.

Not sure but usually the Z4 on 18" have staggered fitment wheels/tyres, so the front wheels are also narrower, but if yours are the same that will influence things too.

Overall rolling radius is altered

225/40 R18 = 200cm circumference
245/40 R18 = 205.2cm circumferance
= 2.6% larger

Maybe the car will compensate for that, but maybe stuff like ABS or ESP/TCS systems will be impacted in some ways.


Wider tyres might be more rigid = smaller optimal slip angle = sharper steering = possibly more over-steery.

But that is offset with maybe more rigid tyres = stiffer roll rate = more weight transfer = more under-steery.

Then also track width may be wider or narrower than ideal depending on the offset of the wheels to begin with (are they OEM or replica?!) that may influence overall handling balance and also possible arch rubbing or strut rubbing of the tyre.

Tyres are more expensive.

More drag = worse fuel consumption.



Now, if any of these things are good or bad for you is another matter, but they are certainly things to be aware of.

Personally I'd run the right width for all the reasons above... though the mere fact these cars ran stupid run flat RE050 tyres which are horrendous to start with means any tyre that isn't RFT even if it's the wrong size by a long way is probably miles better overall hehe :D

Dave
 
Thanks for the replies.

There is no stagger now, so they are 245 Goodyear run flats all around. The front tyres are both nearly new (including the alloys), so very reluctant to pay for new rims and rubber to reduce the size.
 
They look like five series MV2s - if they are genuine you will probably get a decent price for them as I imagine there are a lot of people wanting to give their 520d an M-Sport makeover.

If you sold them via eBay you might actually get enough money to buy a proper set of staggered MV2s off the forum. A new set of CSL reps would look good too.
 
There is no problem running 245/40 on the front. I do and have no issues at all. in theory the speedo will over read very slightly, however most speedos under read, therefore it will probably end up being closer to a true reading.

There is approximately a 16mm difference in overall diameter between the 225 and the 245, however to put this into context, running a new tyre until it has 2 mm left would reduce your wheel diameter by 12mm in tread wear alone!.
 
IMO - it's about aesthetics. From what you've said, you're not doing any damage to the car.

If you like 'em - keep 'em :thumbsup:
 
Marius said:
jimbo1958 said:
Wouldn't the OP's Insurance Company have to be informed?
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Don't be ridiculous

Sent from my LT18i using Tapatalk 2
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No need to be rude Marius....it was a serious question.

Anyone else? Would putting wider rubber on your car invalidate you Insurance if the company hadn't been notified?
 
By the book - it should be disclosed....

Insurance companies will take a different stance, depending on the company you're with, but it's likely to increase the premium. Some companies are quite relaxed as long as you're not fitting 20" spinners with gigantic tyres etc

Of course you could play the daft lad and claim you know nothing about it.... but in the event of a crash :|
 
Adamski said:
By the book - it should be disclosed....

Insurance companies will take a different stance, depending on the company you're with, but it's likely to increase the premium. Some companies are quite relaxed as long as you're not fitting 20" spinners with gigantic tyres etc

Of course you could play the daft lad and claim you know nothing about it.... but in the event of a crash :|
Click to expand...

Guessed that was the case before I googled it.
Seems a lot of companies are quite relaxed about fitting "Winter Tyres" as well but some require notification as well. http://www.abi.org.uk/Information/C...r_Tyres__The_Motor_Insurance_Committment.aspx
 
I think if I were underwriting that car I'd be a bit unhappy about the tyres. Yeah it might be fine for 10 years, but in an evasive manoeuvre is it gonna make the ESP not work optimally? Might the ABS be a bit iffy in extremes?

2.6% larger rolling rad means the ABS will think the tyre is going slower than it really is, so it'll trigger ABS sooner by 2.6%, which might mean 2.6% longer stopping distances!?

To an insurer the unknown is what they won't like.


I still tell mine about no RFT just so it's covered off, along with my optional sports seats and my modded exhaust. They don't care, but at least they know!

Dave
 
Mr Whippy said:
I think if I were underwriting that car I'd be a bit unhappy about the tyres. Yeah it might be fine for 10 years, but in an evasive manoeuvre is it gonna make the ESP not work optimally? Might the ABS be a bit iffy in extremes?

2.6% larger rolling rad means the ABS will think the tyre is going slower than it really is, so it'll trigger ABS sooner by 2.6%, which might mean 2.6% longer stopping distances!?

To an insurer the unknown is what they won't like.


I still tell mine about no RFT just so it's covered off, along with my optional sports seats and my modded exhaust. They don't care, but at least they know!

Dave
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A locked wheel is not a variable...It either is or it isn't. a slight increase in wheel diameter will have no effect whatsoever on your ABS, but could arguable improve your braking as the tyre footprint will be increased.
 
Interested in all your points.
Sorry been away for a few days, we had our third boy, so been a bit busy.

I took the car to a dealer on Saturday to have an error code read which turned out to be a split throttle intake hose seemingly. They also highlighted the tyre issue to me, so I think I am coming around to changing them. Any tips where to look for good deals (other than EBAY)?

thanks
 
itsa1975 said:
Mr Whippy said:
I think if I were underwriting that car I'd be a bit unhappy about the tyres. Yeah it might be fine for 10 years, but in an evasive manoeuvre is it gonna make the ESP not work optimally? Might the ABS be a bit iffy in extremes?

2.6% larger rolling rad means the ABS will think the tyre is going slower than it really is, so it'll trigger ABS sooner by 2.6%, which might mean 2.6% longer stopping distances!?

To an insurer the unknown is what they won't like.


I still tell mine about no RFT just so it's covered off, along with my optional sports seats and my modded exhaust. They don't care, but at least they know!

Dave
Click to expand...

A locked wheel is not a variable...It either is or it isn't. a slight increase in wheel diameter will have no effect whatsoever on your ABS, but could arguable improve your braking as the tyre footprint will be increased.
Click to expand...

ABS doesn't trigger when the wheel stops, it triggers it based on the threshold it determines is a tyre slipping beyond it's optimal slip ratio, and that is about 1g of deceleration, which it works out from the other wheel speeds too.

I'm pretty sure that is why ABS is less effective up hill than down hill because of the extra braking you get from gravity.

Though all systems will vary.

All that said, ESP will use wheel speeds to determine the yaw rate of the car too. Again maybe the system compensates for different sized wheels, then again maybe it doesn't.

Does your insurer know? Nope... they'll just add a weighting for unknown variables that hopefully you make them aware of.


Dave
 
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