SP12 Standard Offset

[Technoholic]

I know this has been asked before, but I can't seem to find a definitive answer. For the SP12 wheels that are supplied by ACT, the standard 17F/18R which I believe are 17x7 and 18x8, what is the standard offset to allow direct fitment onto the car? I know the PCD is 4x108 but can't seem to find confirmation of the offsets for front and back.

Anyone know for sure?

Thanks

[QBee]

Just been and looked at mine, but it is not written on them.
I always assumed it was ET25 front, ET33 rear.
Does the ACT website not say?

In case it is of any help, mine has always been fine with 215/45 17 tyres on the fronts and 235/40 18 on the rear, but only with the ride height set correctly, not lowered too much.
I tend to run track tyres on mine, which come up wider than the same size in road tyres, so you might get away with 225 and 245.
However, in Toyo R1R wet track tyres, I do NOT get away with 225/45 17, they rub on full lock on the back of the headlight access panels.

[Technoholic]

Thanks for looking. I have definitely heard ET25 for the fronts but I thought it was ET35 for the back, not 33, but this is why I was asking, because there seems to be a number of different numbers out there. No the ACT website doesnt say unfortunately.

[mgjc]

I have SP12's on mine and I have asked that very question to Tim at ATC. He confirmed that the wheels were designed to use the standard TVR offsets of Front (25) and Rear (32).

Hope that helps.

[ianwayne]

Standard rears are 7.5" wide. An offset of 32 or 35 is a very minor difference, but the fact the rears are 18 x 8" will make the inside edge 6mm further into the wheel arch and the outside edge 6mm further out if the offset is the same.

There is plenty of clearance inside the wheel arch (as seen by 245 tyres used in place of 225 not being a problem) but if you go too far outwards, you run the risk of the tyre fouling the bodywork edge. An offset of 30-32 would be best.

[QBee]

Don't forget which way offset (ET) works.

A positive offset number is the number of millimetres of air gap between the flat face of the inside of the wheel (the part that touches the wheel hub) and the centre line of the wheel. So all 4 stud TVR wheels have a positive offset number, but a deep dish wheel on some Japmobile could even have a minus offset.

So an ET 35 (35mm offset) rear wheel will be 10mm further into the wheel arch than a ET25 front wheel. Bigger numbers mean the wheel sits further in, not further out.

Hence it follows that is you are, like me for my track tyres, buying second hand Ford Focus or Fiesta fit 4 stud wheels with an ET of 38, which would therefore be too far into the wheel arch on a TVR, you are able to bring the offset down to ET 33 by inserting a 5mm spacer between the wheel and the hub. Don't go more than 5mm spacer without lengthening the studs.

The one thing you can never do is make a, say, ET 10 wheel, that is catching on the outer wheel arch at the rear, fit the car - it is impossible to do anything that will move the wheel inwards.

[ianwayne]

You're right, got my calculations reversed. I've toyed with different wheels on different cars over the years, the LOWER the offset number, the further out the wheel will sit. It's so annoying that the vast majority of 4 x 108 Ford alloys are ET38 or even ET48.

[Classic Chim]

QBee said:

Don't forget which way offset (ET) works.

A positive offset number is the number of millimetres of air gap between the flat face of the inside of the wheel (the part that touches the wheel hub) and the centre line of the wheel. So all 4 stud TVR wheels have a positive offset number, but a deep dish wheel on some Japmobile could even have a minus offset.

So an ET 35 (35mm offset) rear wheel will be 10mm further into the wheel arch than a ET25 front wheel. Bigger numbers mean the wheel sits further in, not further out.

Hence it follows that is you are, like me for my track tyres, buying second hand Ford Focus or Fiesta fit 4 stud wheels with an ET of 38, which would therefore be too far into the wheel arch on a TVR, you are able to bring the offset down to ET 33 by inserting a 5mm spacer between the wheel and the hub. Don't go more than 5mm spacer without lengthening the studs.

The one thing you can never do is make a, say, ET 10 wheel, that is catching on the outer wheel arch at the rear, fit the car - it is impossible to do anything that will move the wheel inwards.

Whenever you add these very informative math posts I find myself trying to do math,,, which soon gets my head hurting, but always come to your answer, which is always right,,, obviously

So I just decided to look up my Azev 17 x 7.5 wheels with I think and offset of 25 allround.

I can’t spare the time to confirm the offset, I’ve scrolled the internet for 20 mins and so far haven’t definitely confirmed if this offset is correct for my wheel yet. I’ll look later
Anyway taking your words into account and assuming ET 25 is the offset , my fronts sit where they should be but the rears must be sitting 5 mm further out than if the offset was ET 35.
Is that correct?

I’ve then added 5mm spacers allround so taking both wheels centre line further out and closer to wheel arch?
Does that sound correct,,,
Bugger it i’ll Get on the will it fit site later,,,
I like these posts,,,,

[ianwayne]

Offset (ET - "Einpress Tiefe" from the German) is measured as an amount in one direction only from the centreline of the wheel. Unless you have massively deep-dished wheels it will be a + number.

So if the offset of your rear wheels is ET25, then the mounting hub face is 10mm further outwards than a wheel with an offset of ET35 would be. By adding a 5mm spacer, you effectively have an offset of +20.


Not exactly 'Carlos Fandango' but the equivalent of 13mm spacers on a standard rear 7.5" wheel (my rear Imolas are ET33)

Edited by ianwayne on Wednesday 23 January 16:26

[markcoopers]

Classic Chim said:

Whenever you add these very informative math posts I find myself trying to do math,,, which soon gets my head hurting, but always come to your answer, which is always right,,, obviously

So I just decided to look up my Azev 17 x 7.5 wheels with I think and offset of 25 allround.

I can’t spare the time to confirm the offset, I’ve scrolled the internet for 20 mins and so far haven’t definitely confirmed if this offset is correct for my wheel yet. I’ll look later
Anyway taking your words into account and assuming ET 25 is the offset , my fronts sit where they should be but the rears must be sitting 5 mm further out than if the offset was ET 35.
Is that correct?

I’ve then added 5mm spacers allround so taking both wheels centre line further out and closer to wheel arch?
Does that sound correct,,,
Bugger it i’ll Get on the will it fit site later,,,
I like these posts,,,,

I thought the OEM fronts were 7" wide and rears 8" meaning the math is a bit more complicated.

7" wide +25 offset has the edges of the wheels 63.9mm outside the hub face and 113.9mm inwards to the chassis.
7.5" wide +25 offset has the edges of the wheels 70.25mm outside the hub face and 120.25mm inwards to the chassis.
8" wide +35 offset has the edges of the wheels 66.6mm outside the hub face and 136.6mm inwards to the chassis.

Adding a 5mm spacer to these adds 5mm to the outside number and takes 5mm from the inward number.

If i am correct with an OEM rear of 8" and +35 offset you were 66.6/136.6. If you are now 7.5" wide with +25offset and 5mm spacer you are 75.25/115.25 meaning your edges of the wheels stick out 8.65mm more but are 21.35mm further from the chassis. Tyre widths are different from wheels so all of this is of little help in working out if they will fit

[ianwayne]

OEM rears are 7.5" wide, fronts 7", as mentioned above.

[markcoopers]

In which case then.
7.5" with +35 offset is 60.25/130.25
7.5" with +25 offset is 70.25/120.35
7.5" with +25 offset and 5mm spacer is 75.25/115.25

So i make that 15mm more outboard on the spaced new wheels than the OEM fitment on the rears

the fronts at 7" +25 offset were 63.9/113.9

So i make that 11.35mm more outboard on the fronts.

[QBee]

In reply to Classic Chim without re-posting his post, if you have ET 25 wheels on the rear, you had wider track than intended by TVR by 8mm each side, and you have made this 13mm each side with the addition of spacers.

But if it doesn't catch on the wheel arches, who cares?

[Classic Chim]

I’ve been busy on faceache considering drag strip times so not checked wheel offset yet but assuming the Offset is ET25 allround the wider track would suggest the weight on the wheels is less as they are further out from the centre line of the car than the standard wheels.

Might be why the hard T1R I had on previously slipped allover the shop or it didn’t help.
It’s marginal and with 215x45x17 F and 235x45x17 R
they are wider than standard tyres on 15/16 wheels care of the 45 profile I think?
They are a big tyre, im sure overall it’s pretty close to what the track is on standard until I added the spacers.
The spacers do seem to help in giving the car a stable feel though. Fills the arches and I can still get at least two fingers up past the tyres inside arches.

Rainsport 3 in these sizes grip like st to a blanket so overall compared to old style tyres the car has much more basic grip anyway.

Thanks for the math chaps. It’s making sense.

With spacers fitted and Rainsport 3


iPhone pic if you click on it then click again it should be slightly less blurred.
You can see the wider track and more aggressive planted look. I think it looks the mutts nuts and I do have more grip on the rear now but I thought that was mostly the softer RS tyres.

Country lanes are a blast and the car feels so much safer. Interesting.

Edited by Classic Chim on Wednesday 23 January 22:18

Edited by Classic Chim on Wednesday 23 January 22:21

Edited by Classic Chim on Wednesday 23 January 22:30

[Technoholic]

all this maths is great and helping me understand offsets a bit better, but still doesnt answer my question for certain. But if im right, then even though the offset of the original SP12 might be ET35 on the rear, if I went for an ET32, 33 or 34 for example, that's not going to make a massive difference to anything right?

[Classic Chim]

Technoholic said:

all this maths is great and helping me understand offsets a bit better, but still doesnt answer my question for certain. But if im right, then even though the offset of the original SP12 might be ET35 on the rear, if I went for an ET32, 33 or 34 for example, that's not going to make a massive difference to anything right?

What size wheels will these be, and then what tyre size will you be using.
To answer your question basically no it won’t change things drastically.

Use Will it fit tyre size calculator online and add in the standard wheel/ tyre sizes and offsets front to rear then add your new preferred wheel/tyre size and offsets, you should then see how this effects the tyre position on the road compared to standard.

[QBee]

No, no noticeable difference Dev. I run all sorts of offsets without a noticeable difference - as long as it fits and doesn't catch, it's great.
Far more important than a slight variance on ET is two other things:

1. Some decent sticky rubber, not more than 5 years old and in a suitable width and suited to your intended use of the car, and

2. A full four wheel alignment of the car by someone who knows wtf he is doing with a Hunter machine, understands TVRs, and is willing to free off the rear toe adjusters if they are (as usual) siezed. A good TVR guy will also check all the suspension joints, dampers, springs etc over before he even starts the alignment, as any play in the joints just wastes your money.

I know i bang on about how good Mat Smith is in this respect, but everyone who has made the pilgrimage to Downham Market (30 miles north of Cambridge) for their four wheel alignment has come away delighted with their transformed car and really pleased to have watched the alignment in process and to have discussed their requirements with someone as knowledgeable as Mat. He races a Chimaera himself and won the TVR Championship with it though having a clear understanding of the finer points of how to put a TVR together, including refining the handling to suit. And he gives good value for money and services them too.

[Technoholic]

My car's just come back from Str8six where they have changed all the relevant suspension bushes that were worn, and checked it all out so I know it's good. They did call out that the rear adjusters are indeed seized solid, so they need freeing up as you say.

I think I will order some team dynamics pro race 1.2s without the edge writing, with offset of ET25/ET35 rear. Then I will arrange for a 4 wheel alignment, but travelling to Mat Smith may be possible, it's just quite far!

[markcoopers]

Mr Technologic,

3mm diff in offset will make chuff all difference if the wheel width is remaining the same at 7.5". Crack on, as the wheel rim will not contact the chassis.

However as if tyres will catch of not is a different thing all together as you may be fitting a 205 or 215 or even 235 width. Clearly at 30mm wider or 15mm either side more in this example that may catch.

Most tyre "will it fit" sites will have a comparison program where you add in the existing tyre sizes and the new and it tells you the difference in width, diameter, circumference and % speedo change. I may not have read what your proposed new tyre and existing ones are, but post up and i'll check the calcs for you as i did all this 2 months back myself when changing wheels and tyres to different sizes.

Cheers,

Mark

[Technoholic]

Ok thank you, this is the full specs of what I plan to fit.

Team Dynamics Pro Race 1.2 without writing

Front:

17x7 ET25
Tyres: 215/40R17

Rear:

18x8 ET35
Tyres: 245/35R18

Currently just running standard imolas