Wider rear tyres okay?

[HansfishS3]

Hi all, just a quick question, I currently have 225/40/18s on the front, would it be okay to put 235/40/18s on the back? I know these cars love to understeer a lot and wider on the back wouldn't necessarily help, but will it put any strain on the haldex system? Or is it complete fine?

 

[<tuffty/>]

In a word... No... In reality you have to match the rolling radius front and rear within some percentage that I can't remember off hand else the abs/esp will complain... This has been discussed before... The search actually works rather well these days unlike the good old days

<tuffty>

 

[HansfishS3]

Oh right..but surely slighty wider tyres won't affect the rolling radius?

 

[HansfishS3]

Right calculated the rolling radius to change by 1.35%...............the car has handled fine okay so far, no complaints from the esp systems...all seems well, my fronts need to be changed very soon so I will stick another pair of 235s on

 

[basil brush]

Yes it will ! The aspect ratio ("40") is a percentage of the width so 40 of 235 is more than 40 of say 225 giving you a larger radius which at a given point of error will fool your haldex into thinking you have wheel slip due to differing wheel speeds .
Re do your calculations on 235/35's

 

[<tuffty/>]

Oh right..but surely slighty wider tyres won't affect the rolling radius?

Aspect ratio is a ratio of the tyre width... 40% of 235 is taller than 40% of 225

<tuffty/>

 

[basil brush]

Yay I beat yoda

 

[HansfishS3]

Yes it will ! The aspect ratio ("40") is a percentage of the width so 40 of 235 is more than 40 of say 225 giving you a larger radius which at a given point of error will fool your haldex into thinking you have wheel slip due to differing wheel speeds .
Re do your calculations on 235/35's

Doh! my bad...redoing on 235/35/17 gives a larger difference of rolling radius (2.5%)