MG Sports Cars

Here's what I'm getting at. Walmart carries one 17" tire in stock. It's a 245/45-17. That tire has a 4.3" sidewall and is about as close as we're going to get to a 50 series. It can be mounted on an 8 or 9" wheel and has close to 9" of tread. It's 20mm narrower than my tires but a 25.7" diameter. That's probably a good size for the rear. On the 17" wheel it gives plenty of room for suspension links and can tolerate 6 or 7" of backspacing which lets us get the upright away from the frame rail. Tire rack shows 98 tires in that size starting at $63 for the Sumitomo HTRZ.

A compliment to that might be a 215/50-15 which has about the same sidewall height and a 2" smaller diameter but Tire Rack only carries two of those. A 205/55-15 has a 4.4" sidewall so that's real close and they show 17 tires in that size starting at $66 for the Fuzion ZRi.

Or, we could go to a 55 series on the back. The 205 works pretty well there and will fit without much fuss but there are only 5 of them and all above $130. There are 33 in the 215/55-17 and 70 in 225/55-17 starting at $65 for the Avon Tech M500. That tire is 26-3/4" tall so that's about all we can do in a 55 series.

Jim

We need to have some sort of consensus here. It sounds to me like the majority is leaning towards tires in the 205 to 245 range so that's why I picked those sizes to play with, plus seeing the 17's at wallmart.

I don't think I'd put too much weight on how many tires Tire Rack lists in a given size... because that criteria would seem to give preference to a size that's popular for all sorts of vehicles (from sports cars to trucks) and for all sorts of tire types (from summer/performance tires to studded snow tires). That doesn't seem helpful to me. The real issue is whether suitable high-performance tires will remain available for the chosen wheel size. If there's one just good reasonably-economical high-enough-performance tire available for the chosen wheels, that's enough.

The BF Goodrich g-Force Sport is a pretty okay and pretty popular tire. Although it's not the first tire I'd chose for my own car, it IS one performance tire that's available in a whole lot of different sizes. So, I took a minute and pulled up the Tire Rack specs chart for this specific tire model: [www.tirerack.com]

Here's a quick summary:
205/55-15 ... $77 ... 20 pounds
215/50-15 ... $86 ... 21 pounds
215/55-17 ... not available (but if it were, it'd weigh about 25 pounds)
225/55-17 ... $94 ... 27 pounds
245/45-17 ... $122 ... 25 pounds

Or, to put it another way: 17" tires cost about $20 more and weigh about five pounds and 25% more than "comparable" 15" tires.

Good points Curtis. True there is a penalty for 17" tires. But they allow us to get the rear suspension upright out away from the frame rail as much as 3 inches further by using a wheel with lots of backspace, and this is also my argument for wider rubber in the back. Wider rubber means more potential backspace, which means more room for the upright. No need to go overboard of course, I can trim the "bell" for clearance also. That may bring other limitations naturally. (I also agree with Carl's point that we're trying to couple 500 ft/lbs of torque to the ground. I don't care how sticky a 225/whatever-15 is, it won't handle that job. Maybe no tire we can fit under there can, but in my opinion that's not any excuse for not making some attempt.)

The LCA outer pivot will not fit inside a 15" wheel. So with that we're limited to about a 3-1/2" backspace, a severely trimmed bell, and will still probably have to space the rim away from the inside by lots more than we'd like. A 16" rim could be made to work, but as we see that's a risky proposition.

Anyway, having once had my tires obsoleted already, I do think the number of tires offered by various manufacturers can be taken as an indicator of future availability. 70 or 90 tires in a performance size is going to mean a pretty good selection of summer performance tires available, as opposed to a list of five or 2 or 1, or now in my case, zero. It is also going to mean more choices on price, as I've seen a direct and solid relationship between the number and the lowest cost tire. So if I can buy a good ultra performance summer tire in a 245/45-17 for $63 I think there's no cost penalty there.

There is more weight, as noted. How exactly tire weight plays out as unsprung weight though is a little less than crystal clear. For one thing the tire is the initial absorber of shocks and impacts. Regardless, I think the better geometry makes it a fair trade off in the rear (only!) but in the front I see no such reason to pay that penalty and I am in complete agreement with Curtis on that end of the car.

Jim

Carl, I know you know way more than me about this stuff so maybe you can explain what's behind this...

"You'd have hard time convincing me to bolt a 2" spacer on the front hub of my car."

I've yet to have anyone give me a good explanation for what the difference between a 7" wide wheel with +40 offset (which are pretty common these days) with a ~2" bolt on type spacer (effectively resulting in -8mm offset) and a 7" wide zero (or slightly negative) offset wheel?

Now, I get that you wouldn't put a spacer on a wheel that would result in a heavily negative offset.

And I get that you wouldn't want to use a spacer of the type that slips over the lugs between the wheel & hub for more than about 1/4" (if at all). But, that's not what I was referring to. I was referring to this type... [www.skulte.com]. Seems like once bolted on, a size that results in roughly 0 offset, would put the same stresses on the axles, bearings, etc as a +/- 0 offset wheel which is what a lot of us have. Unless the concern is related to lug nuts coming loose which seems unlikely?? What am I missing?

Another trip to Walmart, another 17" tire. 235/55-17. Nothing like walking through the tire store and actually looking at them. Oddly enough, many of the new 55's visually look thinner than my old 50's. Guess it depends on the tire. Anyway, that tire is 27.18" tall so it's pushing things a bit and would be a tight fit. Very available though (not a surprise) with 50 to choose from at tire rack. The Pirelli P Zero is $91. For a tire with an appearance more in keeping with the traditional look and maximum rubber on the 17" wheel, that one's worth considering.

Is there *ANYBODY* other than me that thinks a 17" wheel on the back is a good idea?

I saw a lot of 16's. It will be awhile before they go away. We could take a chance on them for rear tires and still make the suspension fit, though it'd be tight. I'd rather have more room to work with but it looks like it could be done though it may take some trickery. They are most common as 60 series although 55's are about 3/4 as common. The 225/60-16 is 26.6" tall and should fit rather easily with the flares I would think. These take about a 7" rim which means a maximum backspacing of about 5". Not wonderful but I could work with it. The bad news is that we can't get wheels to fit from Weld.

Opinions please?

At this point I have to assume that I'm the only person who sees a 17" wheel on the back as being a good thing. If this is the case, we may as well use Steve's Jag wheels, put the car together, and then if Weld wants to give us a set of G-Force wheels just spec them out the same as the Jag wheels. Although that approach may sound like it simplifies things (and in terms of wheel and tire selection maybe it does) I want to go on record here in saying it could cost us 2-3" of rear suspension travel in compression. Even if I cut the bells, the U-joints have to go somewhere.

Jim

Rob,

I'm not a suspension guru, just a student.

IMO, The reason you haven't received a good explanation for your above example is that with the spacer and wheels that you referenced their is no difference other than the mechanical coupling.

My problem is with using a spacer plus the wheel offset. This with disturb the suspension geometry. It will put more strain on the spindle (front geometry)& the bearings plus ruin the akerman angle. Probably okay for street use, but I lean on my car pretty hard in the corners. I already worry if I'm gonna break something. No need to aggravate it with spacers (regular spacers scare me).

I asked Ted years ago how wide he could go with his front end without screwing up the geometry ( 'cause I was fixated on the wider Miata). He said two inches. He has built & installed quite few 2" wider front ends since then. Wish I had one.

Ok Carl, sounds like we're of the same school of thought... aside from concerns with mechanical coupling, spacers that bring a heavily positive wheel back close to 0 offset isn't bad for suspension geometry, spindle, bearings, etc. Using spacers in a way that results in a significantly negative offset is bad those & using regular slip on spacers is questionable if not bad (certainly anything over about 1/4" is bad).

RE: [Ted] has built & installed quite few 2" wider front ends since then. Wish I had one.

The front end width combined with discussion of 17" wheels was behind my comment about spacers... because I have Teds IFS & had him do it 2" wider than stock. With my 15x7's with 3.5" backspace (approx 12mm negative offset -- which is about as negative as I wanted to go), and 205/55/15's, I barely extend beyond the stock fender, let alone the Omni flares (see pic).

So, given the front suspension on this car is stock width or 2" narrower and Jim was planning for flares on all four corners, it's going to be hard to fill the front wheel wells. I'm not sure it can be done without spacers or really negative offset wheels. And, I think it will be even more difficult with 17" wheels since they almost all have very positive offsets and from having looked a lot in the past, it'll be really hard to find the same wheel that comes in 17" with a big positive offset (e.g., in the range of 5" backspace discussed above) that also comes in a 17" or 15" with zero or close to it offset. If they are available, finding them on the cheap will be really unlikely unless we luck into a sponsor that has that option & donates.

This all just brings me back to the need to pick the wheels -- factoring in the IRS design, the front end width, and whether/where flares will be used -- before the IRS design & width is locked down -- it all has to work together.

Rob




Edited 1 time(s). Last edit at 03/05/2008 01:01PM by rficalora.

"So we should be able to handle up to a 5" sidewall ..."

Okay, make me get my calculator out.

Using the extreme, with a 5" side wall (also known as section height), on the rear with an aspect ratio of 50 the width would be 10"(254mm), 40 series would be 12.5" (317.5mm). Rim sizes, using the 90% rule would be 9" & 11 1/4".

A 285/40-17 on a 9-10" rim on the rear would probably be the most we could go. I think a 275 would be good.

On the front, I think we should be more conservative, maybe a 205/65-15 or a 215/60-15 (225 at the most) on a 7" rim.

All,
I am preety well lost on the suspension discussion. I don not usually mess with that with my engine swaps. I just usually use the stock stuff.
I am not against the 17 in. wheels.
I like a small wheel with a large sidewall tire on my street cars. I just like the look. I know the lower profile tires are much higher performance. Since this car IS higher performance, the 17 sound good.
Will too much tire strain the front end too much and cause breakage?

Steve

Steve, larger diameter wheels and larger tires shouldn't add any extra stress to the suspension unless the offset is quite a bit different from stock. There will be some additional loading during cornering because of the larger contact area, but the weight should be within a range that you shouldn't notice any difference. If the wheels and tires are considerably heavier than the original steel wheels and 14" tires we may have some control issues from the front shocks because of the extra unsprung weight, but the alloy wheels should offset most of the added weight of the tires.
As I've already said, I'm more of a fan of the traditional wheel / tire packages rather than the newer large diameter rims and narrow sidewall tires, but with the older style sizes getting harder to find we may have to move into the present day. It would be nice if we could find some stock steel wheels with the width and offset we need such as the Chevy Rally wheels so we could go ahead and do the mockups and then get a replacement alloy in the same size and offset later. I took a look at the American Racing site and saw a wheel that would be neat if for no other reason than the name, they call it the "Roadmaster". Another wheel I liked was the Salt Flat Special. Either would look good on the car.





Edited 1 time(s). Last edit at 03/05/2008 02:44PM by V6 Midget.

It took me a while to figure out what Rob was saying but I think I understand now. Sounds like you have a +1" spindle, a zero wheel (7 divided by 2 equals your 3-1/2" backspace) and a 205 tire that sticks out past the fender a bit. With a stock front end it'd clear fine, right? So you're seeing problems with using a wheel with lots of backspace on the front of the car. I agree, and if that's what we were planning that is where this business about spacers got started. In a case such as that I think the billet bolt on spacers would be OK, but would rather avoid them. Probably the best way to do that is as you suggested, find a sponsor such as Weld who can give us a wheel in both a big offset 17" diameter wide rim and a zero offset 15" moderate width rim. I agree that is the course we should pursue if nobody has any serious objections. We haven't heard back from Curtis on this and he was pretty vocal about not needing the big tires, so Curtis, do you remain unconvinced, or does the need for suspension room sway your opinion a bit? I'd rather we were more or less in agreement before I go back to Greg with sizes and to see if we can get some info on the appearance of the wheel.

I like Carl's recommendations. I think they satisfy the conflicting demands well and will look good on the car. The tire size selections are relatively common which should be a good thing. It remains to be seen if we can fit the 275's under the Omni flares but if not we do have an alternative. I also like Bill's suggestion that we use cheap beater wheels for the build and we can probably find something usable on ebay if nobody can donate anything in the sizes we're looking for. Those are some pretty wheels you found Bill, I especially like the top one but maybe it's just because I like spinners. Let's keep those in mind, but for now I'd like to stick by Greg and give him a chance to follow through for us. We've not even told him exactly what we're looking for yet.

And to that end, if we assume that we can get no more backspace than say, 2 inches less than the wheel width, shall we specify, for the front, a 15 x 7 zero offset wheel (3-1/2" backspace) and for the rear a 17 x 10 with a 7" backspace? The 10" would have a 3" frontspace, which is 1/2" less than the front wheels at 3-1/2" so they should be pretty close in appearance. We can juggle the backspacing and width a bit if we need to.

So the question is: Does anybody have serious objections to these sizes? Speak now,... Please! I would like to send Greg an email tomorrow afternoon.

Jim



Edited 1 time(s). Last edit at 03/05/2008 11:15PM by BlownMGB-V8.

Works for me & you got what I was saying. One thing to be aware of... when you talk about backspace, you include the wheel lips. But when you talk about wheel widths, the lips aren't included. So a 15x7 wheel is 7" wide between the insides of the lips (where the tire bead mates to the wheel). The lips are typically 1/2" wide each so a 15x7 wheel is actually 8" wide -- 7" inside to inside of the lips + two 1/2" lips. That means a zero offset 15x7 wheel has a backspace of 4". My 15x7's with 3.5" backspace actually have a 1/2" negative offset. Backspace is measured in inches; offset in mm. 25.4mm to 1" so my 1/2" negative offset are about 13mm negative offset.

When you talk to Weld about wheels for the front I'd suggest you shoot for 15x7 4" backspace (zero offset) or even 4.5" backspace (+13mm offset). Without flares on the front & with a stock width front end I'm pretty sure the 4" backspace will just barely fit; 4.5" should give a little breathing room. If you're going to use flares on the front, you'd probably want to go with 3.5" backspace to push them out a little. I may have a pic where I measured my combination, if I do I'll post it (on my wife's PC right now so don't have access to the pics). I also drew a pic that explains backspace, offset, etc. & will post that later too.

Here are the pics I mentioned this morning I'd post...

As you can see, with 3.5" backspace 15x7 wheels & 205/55/15 tires & and stock width front end (which would move wheel/tire in 1" per side compared to this pic since my front end is 2" wider than stock), the tires probably wouldn't fit under the stock fender. A 4" backspace would likely fit ok. Or 3.5" with a small flare would work well. One other thing to consider, as you shift the wheel inboard (e.g., 4" backspace instead of 3.5) you need to be sure the wheel will fit over your brakes (some wheel barrels taper as they approach the hub) and that you don't start having other clearance problems with front suspension or headers.


This 2nd picture is the diagram I drew to help share the relationship between wheels, MG body, flares, axle width, etc.

Thanks Rob, that really helps to clarify things. One other item though since I just put the wheels back on the front of my car and was looking at it. It has 14" wheels and a (based on your clarification) 3-1/2" backspacing, which I suppose gives about 13mm *negative* offset. (Negative is when the wheels move outwards?)

The rim is pretty close to the steering knuckle. In a 14" diameter I doubt you could use a 4" backspace with that wheel. What would really help is for someone with 15" wheels and the stock suspension to check the steering knuckle clearance to let me know if a 4 or 4-1/2" backspace is going to clear it.

Jim

RE: Negative is when the wheels move outwards... That's right...

RE: steering knuckle clearance... are you referring to where the tie rod ends attach to the steering levers on the back side of the king pins? If so & you don't get a quick answer (e.g., no one happens to have 4 or 4.5" backspace wheels sitting around) you can check it pretty easy... Just hold a straight edge across the front hubs on the flat part. Mark a spot 7.5" from the center of the hub; that'll reference close to where the inner part of the wheel barrel will be (stated wheel diameter, e.g. 15" on a 15x7 wheel, is the diameter at the inner edge of the tire bead). Clamp another piece of wood to that spot at 90 degrees so it sticks in 4" (or 4.5", etc.) so you can see whether/where you'll have clearance problems. If it's close anywhere (e.g., around the steering linkage, shocks, etc.) you'll need to have the wheels you'll be using as the barrels are shaped different on different wheels. Some taper more than others as they approach the hub mounting surface.

I've attached another pic that helps visualize the taper... Credit where credit's due; I found this pic on www.buicks.net :)

Rob

According to your directions the 15" wheels should be no problem and just about any reasonable backspacing should work. Since Ted's making us a swaybar we shouldn't have to worry about that, which leaves our headers. We'll be making those so hopefully we can avoid the tires with them.

Jim

I just sent Greg an email about the wheel sizes and such. He won't get it until Tuesday since he's out of town for a trade show but I hope to hear back around the end of next week. I suggested that in order to display their products sooner we could use a different wheel for this summer's show, so we'll see what he thinks. So the next step will be to find some tires. I'll get on that next week.

Jim

Great diagrams, Rob!

Jim, you want to borrow my 305/50-15s from my Camaro? :)

Sure, Carl, wait until we decide on 17's and then offer them!

I got a response from Greg. Looks like Weld is in, provided their 2009 budget will cover it. I broached the idea of some set-up wheels. Now for the tires. Might as well see what Goodyear has to say. So far our success rate is pretty hot. I think once the weather breaks I'll go see Terry Kesselring to talk about paint. In the meantime, keep an eye peeled for a set of cheap back wheels just in case.

I sent off a check yesterday for the pistons which Kyle will ship to Dale. Steve, I went ahead and paid for them so you don't need to be concerned about a treasury pay-out. Cost me $69.76 including shipping and money order so I guess you can put that in the donations column if you like. Only way we're going to get pistons for less is if someone gave them to us. (Wait a minute,... I think I just did!) These are .040" over from a crate motor, Kyle thinks about 9 or 10 compression. They should serve us well. Let's see, 4.3125 + .040 works out to 464 cu.in., marginal but acceptable ;-)

Jim

Anyone live in/near Witchita KS? There's an Omni for sale on eBay that's there. 99 cent starting bid, no reserve, & 0 bids so far. Guy says he'll take $100 for the whole thing as a buy it now which puts the flares at about $25 each (a little less than I paid)... but someone would need to pick it up, cut them off, & then sell/scrap what's left.