Wheel Offsets

C

chrisyork

Active Member
I've now done the trial fitting of my modified SD1 Vitesse 15" wheels to the car and so can report on offsets!

And the news is that they are comfortably clear of both the unmodified D post double skin section and of the boot side portion of the wheel arch. !!!!~~HOORAY~~!!!!

I'm afraid there will be quite a long wait before I can report on whether cornering loads shift the suspension around enough to give any rubbing problems! Bearing in mind that Denovo's were 205 section and the amount of clearance, I don't expect any issues though.

These wheels are fitted with 205/65 X 15 94V Pirelli P6000 Powergy tyres. These tyres are correct for rolling radius but you should note that they have a marginally narrower tread than many tyres of this section.

I have measured up the modified wheels and get a dimension across the outside of the wheel of 190mm and a depth from the inner rim edge of the wheel to the contact face to the hub of 148mm. By my calculation that gives an offset of 53mm into the car. The figure for the offset is unaffected by the rim size (it is relative to the centre line of the rim) so this dimension could be quoted when buying any width rim to give an exact central location within the wheel arch.

I don't reckon it would be wise to try going above 205 section. You might just get away with 215, but it would need a trial fitting first and for the wheels to be absolutely exact to this offset.

The wheels were produced by first carefully measuring the exact offset of the unmodified wheels. Each wheel was then individually machined to give the quoted offset by skimming the raised "boss" that seats against the hub. Between the five wheels we had (three from a car set of five, and two odd individuals which replaced wheels from the set which had damaged stud holes - looked like they had been run with the nuts loose and the whole wheel had been moving on the hub - that's V8 power for you!) THREE DIFFERENT OFFSETS!! Clearly Leyland quality control left something to be desired! Hence the individual machining which took off around 3.5mm.

For comparison I have also measured the Jensen Interceptor alloys originally intended for the car and sometimes seen fitted to P6's. Their offset came out at 30mm - so they would need around an INCH taking off to achieve the same offset. I really couldn't contemplate that much strength loss! Equally it illustrates the level of butchery required to the D post to allow them clearance when unmodified.

If you were really committed to the Jensen's, I reckon you could lose 5mm from the wheel, perhaps 5 mm by skimming the face of the hub, leaving around 15mm to gain by single skinning the double skin section of the D post. That sounds just feasible, but rather a lot of effort! Another possible dodge would be to shorten the half shafts very slightly to pull the De Dion tube in slightly narrower - best to try on a scrap one first though as I'm not sure how much tolerence there is inside the tube slider.

I don't have an offset figure for the standard steel wheels, but note that they probably don't run exactly central in the wheel arch. Nor do I have one for either the P5 (15") or P6 (14") Rostyles. Anyone out there with any minus their tyres - could you measure them up and post please.

I do know that the P5 Rostyle was a bit too far out for 205's, 195's are really about the limit. In any case the rim width is really too narrow for 205's. One possible solution if someone really wanted the P5 Rostyles would be to have different rims fitted to the wheel centres. This would be quite easy to do at time of rechroming and I know of at least one firm that does this sort of thing (albeit for lorry wheels).

I hope that adds to the fund of knowledge! Lets see some more data for the various types of steel wheel and also for those Jeep Cherokee wheels?

Chris
 
Interesting, the advert for the aftermarket (not genuine Jeep) wheels says offset 36mm ebay auction 7989096095

However, you have to see these auction no 180054725321 , now they would require some SERIOUS modifications !
 
Chris - I admire your persistance and am glad you have atlast got the result you wanted but I have to question your statement that the offset of 53mm will remain constant irrespective of rim width.
Just marking the measurements out on paper shows otherwise. Your rim is 7.5 inch wide (190mm) - if you draw a line 8 inches long, mark the mid point and then mark an offset of 53mm the outside edge of the rim is now about 48mm from the offset mark rather than around 41mm on your wheel. Such a difference would likely have your tyre rubbing on the D post.
I have put a set of 6X14 inch diameter rims on my Rover that I found at an autojumble. They had the wrong 5 stud pattern but had a flat hub so I have had them redrilled to the Rover PCD and a small amount taken off the back of the hub to get wheel "back" into the wheel arch.
Although it is a 6 inch wide wheel the outside rim measurement is 7 inches. The distance from the rear face of the rim to the hub face is 5 inches (and hence the distance from the front face of the rim is 2 inches).
Giving an offset of 36mm. Having checked with the original wheel I have kept the same offset but just increased the width of the equally front and back of the hub mounting face ( I think I only gained half inch either side). As you say if you want to go wider you need to use up the spare space BEHIND the hub mounting face although I think you will then find you may have clearance problems with the tyre hitting the suspension arm on the front!!
I have fitted 197/70 tyres and they just clear the d post.

I hope this might be of use to any others out there thinking of going wide!!

PS - a bit of wood the correct lenth of the wheel diameter that you want plus a tape measure is all you need at an autojumble to find suitable wheels. Just lay the wood across the wheel and measure the distance from the front and back rims to the hub face. Apparently my wheels were for some some 80's ish Japanese car. I am well chuffed as they looks reasonably in period and I really don't think I'll se another Rover with them on!!.

Cheers

Barry
 
Hi Barry

My understanding of the term offset is the distance from the centre line of the rim to the "hub face" of the wheel. Thus a 53mm offset will always have the centre line of the rim in the same place irrespective of the rim width. The tyre will then sit in the centre of the mudguard between the boot wall and the D post. As you observe the distance from the edge of the rim to the D post (and boot wall) will vary according to how wide a rim is selected. Within reason, this doesn't matter. Remember the tyre is much wider than the rim, so it is the tyre that fouls the D post or boot wall, not the rim. What is therefore crucial is the width of the tyre then fitted. Hence the comment about 205 being OK but 215 probably being marginal and the unusually narrow tread for the quoted section of the selected Pirelli's.

The SD1 Vitesse wheels are 6.5J which is a suitable rim width for the 205 sections tyres fitted. The P5 Rostyles are 5J and the 205's are definitely too wide for them - I know - I tried and they look really "ballooned" out over the rims! The P5's have the wrong offset as well, so double pain ensues! They do look good though.

So:

1) get a wheel of the correct offset

2) fit it with a tyre of width as large as possible for the rim, up to 205 section.

3) check you've got the correct wheel diameter / tyre aspect ratio combination to give you the correct rolling diameter.

Chris
 
I was just trying to help.
I read your message to mean that as long as a wheel has a 53mm offset it will fit a P6.
On re reading it I realise you are actually saying "By my calculation that gives an offset of 53mm into the car. The figure for the offset is unaffected by the rim size (it is relative to the centre line of the rim) so this dimension could be quoted when buying any width rim to give an exact central location within the wheel arch." which is indeed true but not a lot of use when trying to fit wider rims to a P6 as the hub mounting face on the car is NOT in the centre of the wheel arch - hence you can add width at the back of the wheel (in respect to the mounting face) but you can add very little to the front of the wheel (in respect to the mounting face) as it will foul the D post.

Hence I just don't think offset is particularly relevant within reason - you just want to know the distance from the mounting face to the front of the rim.
 
I would say offset is very relevent.

Imagine the face of the wheel is free to move at any point within the wheel rim. Slide the rim outwards and this increases the track of the car giving you a "deep dish" look to the wheels. Slide it out too far and you are looking at fouling the wheelarches.
The Rim diameter does not make a difference, assuming you are able to get tyres in suitable profiles such the rolling radius of the wheel remains unaltered.
 
Hi Barry

I understand what your saying and really appreciate people taking an interest - not many offers so far from other people with offsets for the other candidate wheels!

For the rear wheel mudguard of the P6 there are two pinch points. The first is well known - to the front face of the wheel against the short rearward extension of the D post which lives under the leading edge of the rear wing and carries the mounting for the rear door shut rubber. The second is at the back of the wheel against the vertical side wall of the boot in approximately the same location. This latter comes into play before there is any issue with the de dion tube rear radius arm - presuming that you have the cranked V8 ones!

Remember that the tyre bulges out beyond the wheel rim width each side. Unless you've done something really strange the pinch points are against the tyre, not the rim. This bulge width is not determined by the width of the rim but by the section width of the tyre. The width of the rim does determine the performance of the tyre - a very narrow rim relative to the tyre width will promote the tyre "rolling" off the rim and give a very soggy and imprecise handling response - a very wide rim relative to the tyre width gives a very hard vertical ride because the tyre side walls are too vertical.

The issue is therefore not about the dimension from the wheel rim to the pinch points but about the dimension from the tyre to the pinch points. There is only an indirect relationship between the rim to pinch point dimension and the tyre to pinch point dimension.

205 section tyres seem to be about the maximum practical width that will fit evenly between the front and back pinch points with adequate clearance. (remember that the cars rear track changes with suspension movement thanks to the action of the de dion tube - the track depends on what angle the drive shafts are running at, wide at full compression, narrow at a high ride height) This section of tyre could be mounted on any rim width between (say) 51/2J and 7J. Irrespective of the rim width the overall width of the tyre is the same.

The trick to avoid fouling on the body is therefore about determining the position of the tyre. Hence the interest in where the centre line of the tyre runs. This is the only fixed point the tyre shares with the wheel. We can therefore abreviate the discussion to talk about the centre line of the wheel rim. With the 205 section tyre there is just adequate clearance at BOTH the front and the back of the tyre. A lesser section tyre could be moved out towards the D post but this would generate large clearance against the boot side wall, which is pointless.

The discussion on offset is therefore aimed at getting the tyre to run centrally between the D post and boot side wall, NOT solely about clearance to the D post.

Supposing you were unable to achieve a wheel with the 53mm offset. The tyre width nominally increases / decreases by 10mm with each 10 numeric change of section. Thus a 195 section is nominally 10 mm narrower than a 205 section. I emphasise nominal here! If the best you could achieve with your chosen wheel type was 48mm offset, then you would be 5mm closer to the D post and hence have to come down 10mm in tyre section to a 195. You would then have the same clearance to the D post as a 205 section on 53mm offset but have an extra 10mm clearance to the boot side wall! I emphasise this is a theoretical discussion - I'd recommend trialling with scrap tyres before buying your new ones!

The practical impact of this is particularely relevant to the popular choice of the P5 5J X 15" Rostyle. I know this wheel has significantly less than the desirable 53mm offset, but not the precise figure. Being 5J it will not in any case stand wearing a 205 section tyre; so the choice is between 195 section and 185 (standard V8) section, depending on what the wheel offset actually turns out to be. From observation of cars at shows etc, I strongly suspect the offset will turn out to be about 45mm, as my impression is that there is slightly less clearance to the D post with this wheel and 195's than I have with the 205's on 53mm offset.

As has been noted in earlier tyre topics, 205/65 X 15 is a particularely attractive tyre size to achieve because there is wide availability of good quality and high speed rated tyres in this size. In 195/70 X 15 and 185/75 X 15 the choice is restricted, although not as badly as it is with 185/80 X 14! The 205/65 X 15 is also an exact match for rolling diameter where the others are not so precise. Anybody know what the availability of 195/75 X 14 is like for P6 Rostyles?

I do hope the rest of you aren't finding this too anal. At the end of the day this discussion determines which pretty wheels you can enhance the appearance of your car with!

Chris
 
195/70 x 14 are fairly easy to get hold of, used on 800's and as far as I was aware were a near perfect match for the P6 but I'm talking 4 pot vehicles.

I would also point out that we are mostly talking about the rear wheels when in fact the offset is at least just as important at the front if not more so as it affects the steering geometry.
 
Why is it that Fords from the same period still look superb on silver 51/2 J steel wheels with/without hubcaps yet everyone wants alloys on their P6 ?
I'm keeping my P6 "period " (and it's cheaper)
 
Fords, as they are now were the cheap and cheerful cars of their day. Rover, at that time IMHO were only seccond to the class leaders at Jaguar. Every class leading car these days has alloys as standards, and big ones!
Guess its keeping up with the jones' again?

My car currently sports the original "S" wheels. The hubcaps are in a barn... along with the rest of the pannels. :(
 
Whilst looking for some other info via the search function (I've just e bayed the Jensen wheels) I came across a couple of offset dimensions for other wheels:

P6V8 standard steel: ET46
Jeep Grand Cherokee: ET37

The standard steels are quite close to my "ideal" of ET53. They only have to cope with 185 section so I guess they've been more generous with the clearance to the boot side wall.

I would have thought the Jeep wheels would have needed a fair bit of hammer work to the D post and or undersized (ie smaller rolling radius) tyres.

Chris
 
Thats interesting as Al fitted the jeep wheels with no problem, in fact he needed 5mm wheel spacers for the front. The rears look close but he didn't report any rubbing problems.
He was running 205/50/16 tyres.
 
Hmm.

Interesting.

Al's tyres are undersize - standard (and 205/65 X 15) is 651mm rolling diameter, the 205/50 X 16's are 611mm which takes him 20mm away from the key D post area.

Only real way to check is to go and have a look and measure them up, but I think he sold that car didn't he? Are you still on the forum Al?

As for the spacers at the front - well I could finish up there yet! - we haven't had the opportunity to try it on lock yet. I don't think his spacers were to restore the steering geometry, rather to improve clearances at the back of the wheel on lock.

I agree there is a theoretical problem with moving the front wheels in or out of the car, but I have run a fleet of Land Rovers and various small trucks with non standard offset wheels for many years without any visible effect. I think it really only comes into play if the difference is very large or if you spend all your motoring time driving on full lock (mind you it feels like I've been going in circles at times...).

Chris
 
Wow - this subject is getting well weird - I did promise myself to drop out of it but I really need to question this!
Chris - you say "Al's tyres are undersize - standard (and 205/65 X 15) is 651mm rolling diameter, the 205/50 X 16's are 611mm which takes him 20mm away from the key D post area.
" which appears to imply that reducing the diameter of the tyre (ie lower profile) reduces the width??!! THe width will still be 205 so not quite sure where the saving of 20mm comes in. I accept that a lower profile would probably have less bulge on the side wall - but not that much.
I have 6X14's with 195/70 tyres. They have an ET of 37 which means they have a distance from the hub to the seating face of the outside rim of 1.5 inches (the standard wheel dimension is one inch) they just clear the D post.
I understand the Jeep wheels are 7 inches wide so if the have an ET of 37mm then they have a distance from the hub to the seating face of the outside rim of 2 inches. Now the reason Al's didn't foul may be because the 2 inch increase in wheel diameter (ie from 14 inch to 16 inch) actually moves the wide spot of the wheel tyre combo (ie the tyre bulge) out along the wheel radius away from the centre of the wheel and therefore clears the D post that way. All I know is if my wheels were an inch wider with the same ET hence pushing the rim half inch further out towards the wheel arch they would be so hard up against the D post that they wouldn't go around!
 
What Chris is saying is that by reducing the profile you reduce the overall diameter, not the width, hence moving the tread surface 20mm further away from the D post, and I guess the inner arch.

The good thing with the jeep wheels is that there appears to be plenty of them changing hands on ebay all the time, so you could get some, try them and then sell them again if they don't fit ! (although you would need to get at least one tyre) There are some 17" ones on at the moment that I like, but I suspect they are going to be a big problem, being 7.5" wide for starters.

Don't know if this image helps but its the only shot I can find of the jeep wheel fitted on the rear of Al's P6 (hope he doesn't mind me using it)

jeep1.jpg
 
One of the traps I fell into when I bought the Jensen wheels to use on my P6 was seeing photo's of other P6's apparently running them successfully. Once I'd found the problems, I then did a bit more research and quickly established that these cars had been running substantially under diameter tyres. The effect of this is to draw the part of the tyre which fouls on the D post rearwards out of harms way. The key area of D post is only a very small triangular section that normaly lies at about 10 to 15 past aginst the tyre. Reducing the diameter of the tyre therefore helps as does increasing the ride height (most P6's tend to have sagged well below standard ride height).

As an aside, larger diameter wheels and lower profile tyres moving the tyre bulge out away from the wheel centre actually make things worse - it's the extreme edge of the tyre that fouls on the D post.

I agree 100% with Richard - the only real way to proceed is to trial your chosen wheel along with a scrap tyre in your chosen size. My experience teachs that you should do this with as little unrecoverable expenditure as possible and the Jeep wheels off e bay route certainly seem to fit that criterion. Don't do as I did and get your wheels refurbished and shod with new tyres before you try them!

The distance Barry quotes from the hub to the seating face of the outside rim is interesting but not directly relevant to this problem. You also need to know how much "bulge" of tyre you have beyond the seating face and that can't simply be inferred from the tyre size. It also depends on how wide the rim is. Hence why it is important to refer all measurements to the centre line of the wheel / tyre. This is the only true reference point that assists in judging the likely fouling.

Of course there's nothing wrong with running under diameter tyres. You ought to recalibrate the speedo to cope with the reduction in overall gearing though. The effect on the car will be to give it better acceleration at the expense of fussier (higher revs) high speed cruising and poorer petrol consumption. For many owners these are not a problem. It happens that I tend to do long high speed journeys where cruising capability and fuel consumption are very important to me. My focus has therefore been to increase the gearing from standard - not reduce it! That has meant I've had to go the extra mile to get wide tyres and original diameter together.

Best regards

Chris
 
Now then an interesting question, how easy would it be to have the hubs re-machined to accept either 5*120 or 5*130 PCD rims ?

I ask because then you get a huge choice of either BMW and Range Rover rims (5*120) or Porsche Rims (5*130)
 
One of the things I'm getting a dab hand at is popping out wheel studs. Having seen the hubs sans studs I can't see it having a significant effect on strength to simply drill a second set of stud holes at your chosen pcd halfway between the existing holes. If you were really nervous you could plug weld the existing holes and then give the front face a skim to take any heat distortion out. The skim off the front face would also help with the D post clearance!

How's that for a golden scenario!

Chris
 
Just been looking in the Haynes, and noticed that the hubs are a star shape, rather than round, so wouldn't it be a bit tricky to re-drill ?
 
You must log in or register to reply here.
Back
Top