What offset should a aftermarket alloy 8j x17in pcd100 with standard STI tyre size of 225/45 be to fit? I am looking to replace the standard STI blobeye alloys with Rota g force.

Will ET45 fit. I know the standard alloy is 7.5j not sure what the offset is but will changing to a 8j width impact anything?

Regards.

 

et 45 will be fine

 

Thanks for the replay, does the width of the new alloy have any impact?

 

Not at that size, I have a wide track which has an additional 10mm width track but the same size body, im running an 8.75 width wheel on an et 30 and only had to do minor work to the arch for them to fit, yours will be fine.

 

Thanks mate. I am finding out the correct offset today but what is there a safe offset range for alloys, if such a thing exists?

Regards.

 

Not to my knowledge, but your max offset will obviously be dictated by the width of the alloy and what width tyre you want to use on the car.

You need to look at all factors. I wouldn't want to go any lower than et30 to be honest as then you are into the realms of messing around with the arches quite a lot and stupid stretch on the tyres, if you have the car lowered and then its even more of a ball ache.

But your setup should be fine.

 

Thanks again, I have found out that the et is 48. So would this be a problem using standard 225/45 17in tyres?

 

nope should be fine

 

The Widetrack blobeye is 10mm wider track at the rear only, and has more inner arch clearance than the none widetrack blobeye at the rear, that's why it has the extra arch covers.

The Widetrack blobeye had the same offset wheel as none widetrack (ET53), but is 12.7mm wider (8" wheel as opposed to 7.5"), so the outer edge of the wheel is 6.35mm further outboard, add in the extra 5mm track per side and the widetrack blobeye wheel sits 11.35mm more outboard the car than the none widetrack version.

With regards to your wheel, you are going from an ET53 7.5" wheel to an ET48 8" wheel. That means your outside edge of the wheel will move 6.35mm (half the width increase 12.7/2)+5mm (change in offset more outboard 53-48) = 11.35mm more outboard.

So measure your current wheel clearance, if you don't have a spare 11.35mm you will have to modify the arch for clearance.

 

Thanks John that was an excellent answer I will measure it.

Regards.

 

John, I missed out on the 8j rota's alloy. But I was wondering as I can't seem to calculate this.

I now have available a alloy which is 7.5j which is the same as the standard alloy but the et is 35. Will this cause an issue and does the offset affect the cars handling?

Is there anyway you can tell if the alloy clears the brembo caplier from the alloy size and measuremwnts?

Regards.

 

If using the same width wheel all you need do to check side movement is calculate the difference in offset.

ET53 is stock, ET35 is your proposed wheel, so just do the maths, 53-35=18.

That means the outer edge of the wheel will sit 18mm further outboard of the current position.

With regards to brake clearance, that something that will change from wheel design to wheel design, I cant tell you if it will clear or not.

changing offset does alter the cars handling as you are changing the scrub radius of the tyre/strut assembly. That has implications for stability under braking and acceleration as the load forces are resolved via the scrub axis.

I havnt plotted the scrub radius on my car, so cant tell you off the top of my head what effect this will have in terms of improving or otherwise the cars handling, i should plot that really.

You tend to find the wider track gained from lower ET wheels is a bigger bonus than the loss in stability from changing the scrub radius on a track based car, but on the road you may find the opposite is the case as you need less track width in reality as the corner loads aren't so high, and for the road, stability is far more important.

If the scrub radius changes from positive to negative with the offset change (if it's a positive scrub radius as designed) then you will find the car follows the road surface more and it becomes a bitch to drive fast, especially on rougher surfaces like country B roads.

The group N wheels are all ET49, so i would think that is the sweet spot for the limit on offset change for rough road driving, but it would be best to plot this properly to understand what the effect is.

 

Very good reading