SR4 Rear Wheel Offsets
Discussion
I reckon the offsets of the wider rear wheels we've got may be the wrong ones.
Now that we've got the correct size tyres fitted*, the top edges of the tyres look uncomfortably close to the glassfibre return at the top of the arch, particularly as they sit slightly underneath them rather than fully inside them. Surely the edges of the tyres for say the first 50mm or so of the sidewall from the edge down should sit inside the edge of that return?
Our wider rear rims are Compomotive CXR 1310 10 x 13 and (the most important thing) the offset is ET-20. Anyone know what the recommended offset is or could someone possibly check measure theirs? (Not that measuring offsets is that easy, but some alloys have the offsets 'stamped' on them)
Thanks.
Now that we've got the correct size tyres fitted*, the top edges of the tyres look uncomfortably close to the glassfibre return at the top of the arch, particularly as they sit slightly underneath them rather than fully inside them. Surely the edges of the tyres for say the first 50mm or so of the sidewall from the edge down should sit inside the edge of that return?
Our wider rear rims are Compomotive CXR 1310 10 x 13 and (the most important thing) the offset is ET-20. Anyone know what the recommended offset is or could someone possibly check measure theirs? (Not that measuring offsets is that easy, but some alloys have the offsets 'stamped' on them)
Thanks.
- We only recently discovered that our SR4 was running on rear slicks that were 'slightly' under diameter: 545mm overall diameter, whereas if they are Dunlops that they should be their size 230/570/13? Having those under diameter tyres on didn't seem to cause any problems (to my snail trackday pace!) but I like things to be right.
I make them 20mm offset too on a 240mm rim width. I measured from inside rim into hub face as 100mm so 240/2 less 100 equals 20mm offset to me but as you say, not the easiest thing to measure with tyre sidewalls getting in the way. Tyres are 205/570/13 rather than 230 though, that could be part of the problem?
Nick
Nick
nick997 said:
I make them 20mm offset too on a 240mm rim width. I measured from inside rim into hub face as 100mm so 240/2 less 100 equals 20mm offset to me but as you say, not the easiest thing to measure with tyre sidewalls getting in the way. Tyres are 205/570/13 rather than 230 though, that could be part of the problem?
Nick
Thanks for your sterling efforts Nick: What you have come up with seems to tie in (sort of) with what Radical told me today. Nick
According to them, with the standard width wheels (i.e. 7" - as they are usually called - fronts and 9" - as they are usually called - rears; I think these inches widths equate to near enough the dimension inside face of rim to inside face of rim) the front and rear offsets should both be ET20; I think what they actually mean (meant) is ET-20, as the rims are almost certainly negative (not a positive offset as Radical's "ET20" would mean) offset, meaning there is a greater dimension from the mounting face to the outer rim edge than there is from the mounting face to the inner rim edge.
By the way, your dimensions mean that you have the standard 9" rears (not the wider 10" rears that I think - make that now know - I have the wrong offset on), in which event, as I understand it, the 205/570/13s are indeed the correct Dunlop tyres for these. If you have the 7" fronts (as you should have with the 9" rears), the front tyres should be 175/535/13 if they are Dunlops. BTW, the 7" rims will have an overall rim width measuring about 195mm.
With the 10" rears, a idfferent and wider tyre is reommended: 230/570/13s (which actually start out life with actual dimensions of 575mm o/a diameter, 280mm o/a width and 235mm tread width) are the correct Dunlop tyres for these and the fronts that go with these are 8" rims, for which 190/535/13 are the correct Dunlop tyres.
So I'm now absolutely certain that the 10" rims and Michelin 23/57-13s (which actually start out life with actual dimensions of 574mm o/a diameter, 279mm o/a width and 235mm tread width)are absolutely spot on for a Radical SR4 running the wider alloy option. My only problem is that the offset of my pair are definitely the wrong bloody ones for an SR4 running those wider rims and the tyres to suit. Round things!!!!!!!!!!!!!
The best way to try to confirm offsets is take the wheel off and see whether the ET is stamped/cast on the inside of the wheel; certainly are on my narrower/standard 7" front and 9" rear Compomotive CXRs (they are stamped on the mounting face), which I understand were the actual types fitted by Radical themselves when mine was made in 2004 (Chassis Number 21).
Would it be possible for you (and others) to check and see if yours are ET marked and let me know what you've got? I remain to be convinced that the standard 7" fronts should also be ET20 (or rather ET-20!)
jbreakell said:
Guys,
Do you Know what the offsets are for the front wheels??? or if they are offset at all???
James; see my reply to Nick above; I like to think we are getting there; if you are on 7" fronts, according to Radical (by my interpretation) they should be ET-20 ......... but I suspect (but only suspect at the mo) that it may actually be ET0. Do you Know what the offsets are for the front wheels??? or if they are offset at all???
Different again for the wider 8" fronts.
All should be clarified in the next few days...............we live in hope.
Could you possibly check for ETs confirmation on your wheels as described in my reply to Nick?
Cheers
You are correct that my fronts are 175 wide so I must be running 7" and 9" rims. I also believe this is correct from the Biduro series the car was raced in, think this size was mandatory. Also correct on negative offset as I understand who offset works.
Unfortunately I can't help on the offset stamp as the wheels are 3 piece split rim Image wheels so the offset can be changed by using different outer and inners.
I'm fairly sure that the fronts are zero offset - last night I checked back over some emails that I sent when selling my centrelocks and think the fronts were measured as zero offset, will check emails and wheels tonight for you.
Nick
Unfortunately I can't help on the offset stamp as the wheels are 3 piece split rim Image wheels so the offset can be changed by using different outer and inners.
I'm fairly sure that the fronts are zero offset - last night I checked back over some emails that I sent when selling my centrelocks and think the fronts were measured as zero offset, will check emails and wheels tonight for you.
Nick
nick997 said:
Fronts are zero offset.
Nick
Cheers Nick ............. my suspicions confirmed; it looks to have varied a touch over the years, which is probably understandable because certainly Compomotive CXRs have evolved and are still evolving in detail over the years: I have two sets of Compomotive CXR 7" fronts and 9" rears; on the fronts, what I am pretty sure (from the manufacture dates) was the original factory fitted set, the fronts are very slightly positive offset (ET6), whereas the later separately bought set are (like yours) 'no offset' i.e. ET0. What they definitely aren't is what Radical told me i.e. ET20. If they actually meant ET-20, meaning (as they said) the same offset on the 7" fronts as the 9" rears, ET-20 on the front might just fit, but it would be bit tighter to the wheel arch than I would ideally like to see. Nick
What is unfortunately now 100% certain is that my 10" wide rears are hopelessly the wrong offset, utterly incapable of having the negative offsets professionally reduced down to effect a cure, as this would result in ridculously tight clearances to the calipers. I am still scratching my chin as to which way to go, but truth be told, I'd recommend anyone with an SR4 thinking about going from 7" to 8" fronts and from 9" to 10" rears not to; there is barely room to do so; my 8" fronts are spot on, but they will chip the inner flange (on the lower wishbone) when being backed into the pits by helpers who inadvertently (and why wouldn't they) use full lock; there is a cure for this chipping i.e. using lock reducers on the steering arms but it significantly reduces the lock making manouvreing a right old pain; as said to me by one of my contacts at Peterborough.
Progress report ........... or should it be lack of progress report?
Just test fitted one of my new rear 1310 CXRs ET-7. This offset is fine; much better than the ET-20 that I had on before; with those, the wheels definitely came out too far, so much so that at the ROC Trackday, I had to run the car higher than I'd really want to and even then, the edges of the tyre still rubbed a tiny bit on full compression. So ET-7 is fine; from choice, for a little bit more clearance, I'd probably have ET0, but with 1310 Compo CXRs, that isn't an option; ET-7 is the smallest negative ET they can do with a 10" CXR.
So why do I say "lack of progress"? Because as the inner rim sits that bit further in, it means than rather than standing clear of it, the rim now passes over the bolt fixing the top wishbone. Yep, you've guessed it, not enough clearance twixt inner rim and bolt top (contact enough to lock the torque sensing diff), so I have effing managed to cosmetically mark my brand new wheel, which to an anorak like me is a bit ferkin annoyin.
I now have to frig around and find and acquire (or have turned) a bolt with a slightly less tall head to let the wheel pass over that bolt with a couple of mm clearance.
Seems I am the pathfinder on this ..... taking today's annoyances into account with the clipping that comes about with 8" fronts when full locking in and out of the pits, my advice remains that > if you are thinking about going from 7" to 8" fronts and 9" to 10" rears on an SR4, forget it, get a life and just concentrate on improving your driving technique.
Just test fitted one of my new rear 1310 CXRs ET-7. This offset is fine; much better than the ET-20 that I had on before; with those, the wheels definitely came out too far, so much so that at the ROC Trackday, I had to run the car higher than I'd really want to and even then, the edges of the tyre still rubbed a tiny bit on full compression. So ET-7 is fine; from choice, for a little bit more clearance, I'd probably have ET0, but with 1310 Compo CXRs, that isn't an option; ET-7 is the smallest negative ET they can do with a 10" CXR.
So why do I say "lack of progress"? Because as the inner rim sits that bit further in, it means than rather than standing clear of it, the rim now passes over the bolt fixing the top wishbone. Yep, you've guessed it, not enough clearance twixt inner rim and bolt top (contact enough to lock the torque sensing diff), so I have effing managed to cosmetically mark my brand new wheel, which to an anorak like me is a bit ferkin annoyin.
I now have to frig around and find and acquire (or have turned) a bolt with a slightly less tall head to let the wheel pass over that bolt with a couple of mm clearance.
Seems I am the pathfinder on this ..... taking today's annoyances into account with the clipping that comes about with 8" fronts when full locking in and out of the pits, my advice remains that > if you are thinking about going from 7" to 8" fronts and 9" to 10" rears on an SR4, forget it, get a life and just concentrate on improving your driving technique.
Edited by splitpin on Saturday 16th May 18:27
yellowrad said:
I had the same problem with the wheel hitting the top bolt. Radical stock an alternative bolt that almost sits flat and will solve your problem,
Jon
Thanks JTJon
That's the trouble when you're trying to do things on your own!: With another pair of hands, I'd have taken off the rear bodywork, then I'd have seen the problem whilst offering up, before marking the bloody wheel. Others seem to manage it OK, but I find it a right bogger to get the rear bodywork off solo. Oh well, you live and learn and at least there's a solution.
Kind Regards
splitpin said:
That's the trouble when you're trying to do things on your own!: With another pair of hands, I'd have taken off the rear bodywork, then I'd have seen the problem whilst offering up, before marking the bloody wheel. Others seem to manage it OK, but I find it a right bogger to get the rear bodywork off solo. Oh well, you live and learn and at least there's a solution.
I do it by lifting the rear wing supports to break the rear bodywork free from the car and then sort of rolling the bodywork over the rear wheels to the point where you would rest it to undo the electrics. At that stage you can move to the side of the car and continue the rolling backwards motion until the bodywork touches the ground. Then you can lift it backwards from the car by stepping into the gap between wing and car.It helps that my bodywork is not pristine!
Nick
Edited by nick997 on Sunday 17th May 20:46
Prawn said:
Hi Trevor
Can you tell me the offset of your front CXRs
Hi JonCan you tell me the offset of your front CXRs
I have CXR 1380 ET3s on the front of my car and they seem fine: If I understand correctly, Radical say ET4s. BTW, ET12 on a CXR 1380 would put the outer edge of a 8" rim in exactly the same position as the outer edge of the standard ET0 7" rim, but to my mind the track looks a bit mean on ET0 7" rims. ET3 CXR 1380s bring the outside face about 10mm further out compared to ET0 CXR 1370s. I think you are safe to order ET3 or ET4 CXR 1380s and let's be fair there is beggar all difference.
The only issue you will have with the 8" wide front rims is that on full lock, the inner rim edge (where it changes from the horizontal part to the vertical inner flange) will 'clip' (and bugger up the paint) on the trailing edge of the bottom wishbone. It most likely doesn't happen in normal driving (where one never uses full lock), but it will happen if someone puts on full lock to say turn into or be pushed reversed into a pit garage or trailer and also, it could well occur if one had a spin and the steering wheel got spun/wrenched out of one's hands.
To avoid this happening, apparently you can fit these, but they do significantly restrict lock apparently >
http://secure2.cyberware.co.uk/~rad-shop/acatalog/...
Important to note that (unlike the rear wheels) all the offsets given above are postive offsets.
BTW, it has to be said that my dealings with Compo were not entirely satisfactory; ping me an email if you want to know more about that: I replied yesterday to your (works?) email, so you'll have mine when you get to read it.
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