Wheel Offset advice needed

[SWoll]

Hello all.

Have bought a new car and a couple of the wheels are a bit scuffed up. Rather than getting them repaired I'm considering keeping them to one side and fitting with winter tyres as we've recently moved out in the sticks in North Yorkshire so could come in very handy.

Looking at replacement summer wheels for the current almost new continental tyres and whilst width, bolt spacing and centre bore are all fine I'm struggling with offsets.

The current wheels are 21" , 9.5J, ET35 running 265/45 tyres. Seeing numerous wheels with offsets of 20-30mm, so would they fit and what would the consequences be?

Thanks for any replies.

[cuprabob]

Assuming the same width of wheel, a wheel with ET20 will reduce the clearance to the wheel arch by 15mm and ET30 by 5mm. You need to check if there is enough clearance to ensure there is no rubbing.

Put the details into this site.

https://www.willtheyfit.com/

[Rotary Potato]

SWoll said:

Hello all.

Have bought a new car and a couple of the wheels are a bit scuffed up. Rather than getting them repaired I'm considering keeping them to one side and fitting with winter tyres as we've recently moved out in the sticks in North Yorkshire so could come in very handy.

Looking at replacement summer wheels for the current almost new continental tyres and whilst width, bolt spacing and centre bore are all fine I'm struggling with offsets.

The current wheels are 21" , 9.5J, ET35 running 265/45 tyres. Seeing numerous wheels with offsets of 20-30mm, so would they fit and what would the consequences be?

Thanks for any replies.

So offset is all about how far the wheel is tucked in or sticking out of the wheel arch. Lower offsets (or negative) are more sticky-out, and higher offsets are more tucked in. So moving from ET35 to ET30 (if you weren't aware, offsets are measured in mm) will cause the wheels to stick out an additional 5mm, and moving from ET35 to ET20 would cause the wheels to stick out a further 15mm.

As for 'will they fit', that will depend entirely on the car. My guess would be that the vast majority of cars have enough wiggle room to support a 5mm change in offset. And a good number will have enough room to allow a 15mm change in offset. But that is just a guess. You can get a good idea on whether they are likely to fit or not (usually when decreasing offset like you are considering, the failure point will be the outside edge of the tyre rubbing on the inside lip of the wheel arch, usually at the top) by measuring how much of a gap you currently have. But it's not exact as you cannot easily replicate dynamic conditions statically. When the car is on maximum roll in a loaded corner, with steering angle applied, it'll have a different gap between the wheel and the arch compared to when it's sat stationary on a driveway. I'd say your best best on 'will they fit' will be to ask on an owners club forum/FB page, and try and find someone who's fitted wheels with a similar offset to those you're considering and ask about their experiences.

Assuming they do fit, you will have 2 direct consequences. The first is that people in certain car scenes will think you are a little cooler for having 'flush' fitting wheels (or maybe even 'hella flush' if you are really pushing the boundaries of fitment ). The other consequence is that your wheel bearings will be taking slightly more load due to the increase in fulcrum my moving the wheels slightly further out. Unless you happen to know the engineer who designed and specified your car's wheel bearings, it'll be difficult to say exactly what impact that has, but it won't be positive. There's no way of confirming if that negative impact will notably shorten the life of your wheel bearings.

If this were me, I would not sweat a 5mm change for either fitting, or for shortening the life of my wheel bearings. There's no science behind me not sweating it ... just a combination of experiences and gut feel. It all comes down to your appetite for risk, and that may be different to mine.

[SWoll]

Thanks RP. My understanding was that going from ET35 to ET30 or ET20 would push the wheels out and give more clearance to the calipers etc. so thanks for confirming. Appears to be a lot of confusion online on this topic.

So have seen some ET30 wheels I like, but at the moment can only seem to find them in 9J rather than 9.5J as with the current wheels. Tyres I wat to swap are 265 which appears to be the widest you can go on a 9J rim, but again with what potential consequences (they're 45 profile)? I actually don't mind them being slightly pushed out on the rim as would offer more protection to the alloy but what are the downsides?


ETA - Ah, so the change in wheel width totally screws up the offset of course and means the difference is 11.3mm and the wheel itsef will actually sit further into the arch.

Need to find 9.5J ET30-35 or asking for trouble aren't I?

Edited by SWoll on Thursday 3rd April 11:08

[Rotary Potato]

SWoll said:

Thanks RP. My understanding was that going from ET35 to ET30 or ET20 would push the wheels out and give more clearance to the calipers etc. so thanks for confirming. Appears to be a lot of confusion online on this topic.

So have seen some ET30 wheels I like, but at the moment can only seem to find them in 9J rather than 9.5J as with the current wheels. Tyres I wat to swap are 265 which appears to be the widest you can go on a 9J rim, but again with what potential consequences (they're 45 profile)? I actually don't mind them being slightly pushed out on the rim as would offer more protection to the alloy but what are the downsides?


ETA - Ah, so the change in wheel width totally screws up the offset of course and means the difference is 11.3mm and the wheel itsef will actually sit further into the arch.

Need to find 9.5J ET30-35 or asking for trouble aren't I?

Edited by SWoll on Thursday 3rd April 11:08

I'm afraid I don't have a lot of experience of how different tyre sizes sit on different width wheels, so going to leave that bit to others.

In terms of your maths, I'm afraid you're a little out.

So ET is measuring how far the mounting face of the wheel is from the vertical centreline of the wheel. An ET of 0 means the mounting face sits dead centre on a vertical line down the centre of the wheel. Negative ETs mean the mounting face is nearer the inside lip and positive ETs mean the mounting face is nearer the outside lip.

So if the ET did not change, a move from 9.5" to 9" would shrink the width by 12.7mm. This means the inside lip would be 6.35mm further away from the suspension upright, and the outer lip would be 6.35mm further away (tucked in) from the wheel arch.

If we then drop the ET by 5mm, we are effectively moving that wheel 5mm outwards. Therefore the inside lip of the wheel would be 11.35mm further away from the suspension upright compared to a 9.5J ET35. The outer lip (the face you see when the wheel is on the car) would now be 1.35mm further in to the wheel arch (tucked in) compared to a 9.5J ET35.

If you could visually spot a wheel 1.35mm further into an arch, you have a better eye than me!

[Rotary Potato]

I'll also add that things like caliper clearance are determined as much by the style and design of the wheel as by it's offset and width.



Here is a random alloy wheel picture I've taken off a google search. I've chosen it purely to illustrate a point.

Those spokes are about 1-2cm deep, and so clear the brake calipers on 99% of cars.

Imagine the exact same wheel, with the exact same width and offset, but those spokes (not the hub, just the spokes after they come out from the hub) are now 15cm deep, sticking further into, not out of, the wheel - obviously that's ridiculous, I'm massively over exaggerating to make the point easier to visualise. Those 15cm deep spokes will foul on the calipers of 99% of cars despite the offset and width not changing.

So just because one wheel of offset X and width Y clears the calipers on your car, it doesn't automatically mean that every wheel of offset X and width Y will. Obviously, wheel manufacturers are aware that there is 'stuff' they need to avoid near the wheel, and so do give an amount of clearance for that 'stuff', but it isn't necessarily the same from manufacturer to manufacturer. I would say that when you're making relatively small changes (like you are talking about above) the chances of you getting tripped up by this are remarkably small, but they are not zero. The only way to guarantee a wheel will fit on your car is to fit it to your car (or find someone with an identical car who has fitted identical wheels and ask their experience).

Offset and width can help determine where the wheel will sit in the wheel arch, thus helping you to avoid suspension components and the lip of the wheel arch ... but it won't give you a guarantee that it will clear your brakes.

[Panamax]

Rotary Potato said:

I'll also add that things like caliper clearance are determined as much by the style and design of the wheel as by it's offset and width..

^^^ Yes, until you actually try the fitting you can never be sure, unless you know someone has fitted identical wheels to an identical car.

Sometimes calliper clearance can be rescued by fitting a narrow spacer. More of a shim than a spacer.

[ShortBeardy]

^ This is by far the safest route - a good scour of forums and owners clubs to get an idea of what has previously been used.

note that aside from loading wheel bearings changing offset a significant amount can also impact steering feel and *I think* self centering