rim offset and adapters....

[cmoore]

i am aware of all the dimensions and bolt patterns/offsets etc...that cause difficulty for getting rims to fit on a Z. i foiund a nice used pair with both 4 bolt patterns.....my question is as there are several companies that make adapters (with separate lug extensions) as well as just the spacer plates. what does physics tell us is wrong with all this - or is there? presumably you are adding unsprung weight but that will typ be more than overcome by the lighter wheels.....so is there some excessive wheel brg. wear that will occurr? and if wheels are properly balanced why is this?

thanks in advance if anyone can help?

and as datsun owners, can't we lobby that they make decent rims that still fit. (panasport/revolution aside)

[Zvoiture]

I also would like the more mechanically inclined among us to weigh in on a serious discussion concerning extentions. At the ZFESTwest show, they were EVERYWHERE. Everyone is running wheels with tons of negative offset (I think I have that right--FWD-style, 'flush' wheels). Adaptors everywhere. Half inch, One inch, 1 1/2" is really common, and I saw some that HAD to be near 2". Let's say you have perfect adaptors--no slip, perfect alignment, hub-oriented or not...whatever--does it affect Wheel Bearing geometry? To me, it doesn't seem like it would matter, but I have heard smatterings of otherwise.

Anyone?

steve77

[2ManyZs]

I for one am totally against using spacers unless it's an absolute necessity. Now, a spacer that is 1/2 inch or less isn't going to affect the geometry to the point of a failure. But anything more than that is asking for trouble IMO.

Yes, you are applying a lot more stress to the hub and bearings if you use a spacer, how much depends on the thickness of the spacer. The leverage being to the studs,hubs, and bearings applied is being increased as you move the wheel further form the mounting flange. Not only that, but because the tire and wheel package are so large they increase the amount of contact patch, therefore the side loading of the tire that is being transferred into the bearings and hubs. You will also increase the stress on the entire suspension as well as the bearings. Most suspensions were designed to work with a certain amount of side loadings and the bearings, hubs and such were sized according to a certain size and placement of the wheel.

You may even affect the roll center of the suspension, and the bump steer when you use a wheel that will require a spacer.

I'm not an engineer, but there would be a formula somewhere that would give the exact difference in forces, it's all in the leverage.

If you have to use a spacer, use the the smallest one possible.

Think of it this way. If your arm were the spindle, and you were to have two tires mounted to your arm, which would put less stress on your arm. One that has the centerline of the wheel at your elbow, or one where the centerline of the wheel is at your wrist?

[pparaska]

Z2ManyZs wrote:

Yes, you are applying a lot more stress to the hub and bearings if you use a spacer, how much depends on the thickness of the spacer. The leverage being to the studs,hubs, and bearings applied is being increased as you move the wheel further form the mounting flange.

I disagree. If the center of the contact patch remains in the same place, the bearing and hub loads remain the same, all else being equal. The issue here is that that many of the wheels available on the market have more positive offset than what's required on the Z. The spacer just allows you to move the offset of the wheel to a place where it fits correctly on a Z (in other words, reduce the offset of the wheel).

If 20mm of positive offset (mounting surface 20mm towards the pretty side of the wheel from the 0 offset position) is what's needed on a Z (this is what I think is correct) and the wheel you want to use has 58mm of offset, then you'd need a 38mm (or about 1.5") spacer to come up with a combined wheel/spacer offset of 20mm positive.

Think of the spacer as an extension of the wheel mounting surface toward the center of the car.

Again, this would place the center of the contact patch in the same place as stock, and the hub and bearing loads would be the same as stock.

Not only that, but because the tire and wheel package are so large they increase the amount of contact patch, therefore the side loading of the tire that is being transferred into the bearings and hubs. You will also increase the stress on the entire suspension as well as the bearings. Most suspensions were designed to work with a certain amount of side loadings and the bearings, hubs and such were sized according to a certain size and placement of the wheel.

True, but there are about 30 years of experience of racers and enthusiasts that have not shown this to be a problem with the Z. Except maybe it has contributed to some stub axle failures. I'll let the racers (John Coffey, Keith Thomas, etc.) chime in on that one.

You may even affect the roll center of the suspension, and the bump steer when you use a wheel that will require a spacer.

Not if you keep the total offset of the package the same (center of contact patch in the same area), the tire outside diameter the same.

I'm not an engineer, but there would be a formula somewhere that would give the exact difference in forces, it's all in the leverage.

Shear loads go up linearly as you move the contact patch center point away from stock. Moments go up by the square of the change (roughly).

If you have to use a spacer, use the the smallest one possible.

I disagree. If the spacer needed is nearing 3/4 inch, I suggest going to a bolt on adapter spacer. This way the lug studs don't get over worked. Bolting an adapter to the hub is NOT a problem in any way, as long is it's a good design and material. The only drawback is that those expensive adapters are now only good for that offset wheel you've chosen and if you go to another wheel with another offset, things won't work for you. If the wheel had more offset, you could add a think spacer though.

Think of it this way. If your arm were the spindle, and you were to have two tires mounted to your arm, which would put less stress on your arm. One that has the centerline of the wheel at your elbow, or one where the centerline of the wheel is at your wrist?

Good point, but if the total offset of the wheel/spacer is the same as stock, there is no change in where the centerline force acts, and therefore no more stress on the hub/bearings/spindle, etc.

As with any change in wheels on a car, you must keep the center of the contact patch at or near stock or things like loads on these parts and suspension and steering geometry ARE effected, and usually in a "bad" direction.

[hmsports]

I'm going to have to agree with 2ManyZs. Stay away from spacers if at all possible.

They WILL load the bearings more and exert forces on the suspension that they were not designed for.

pparaska's point of the contact patch being the same is valid - it should not change the roll-center or bump-steer. But changing the track by moving the tires out will affect toe and ultimately, bump-steer.

As you move the mounting face farther away from the bearing and ball joint, regardless if the tire patch stays the same, you will be puting more stress on those components. Same principle as taking a longer grip on a wrench (spanner).

You could argue about physics and theory all day... but as a racer I wouldn't use them. I've know several guys who do and they are constantly replacing bearings -- I only do at the start of each season -- and my old ones still look fine.

For a daily driver, you would probably be ok. Especially since they might be your only choice to get the killer rims you want.

[pparaska]

I'm going to have to agree with 2ManyZs. Stay away from spacers if at all possible....

They WILL load the bearings more and exert forces on the suspension that they were not designed for.

[snip]

As you move the mounting face farther away from the bearing and ball joint, regardless if the tire patch stays the same, you will be puting more stress on those components. Same principle as taking a longer grip on a wrench (spanner).

I'm sorry, but that statement is just completely false. The center of the tire patch is all that matters. The wheel mount surface does not, as long as a spacer is used to move the center of the contact patch back to the stock location.

You could argue about physics and theory all day... but as a racer I wouldn't use them. I've know several guys who do and they are constantly replacing bearings -- I only do at the start of each season -- and my old ones still look fine.

Well, if we can't agree that physics is the only thing that matters here, then I don't think we can have a useful discussion. Physics is the only thing that DOES matter, as far as what loads, etc. are. A wheel spacer used to bring the total offset of the spacer and wheel to the stock location DOES NOT CHANGE THE LOADS ON THE BEARINGS/HUB/SPINDLE. Now if you put larger stickier wheels/tires on, you'll be adding more loads to the parts, no matter if you have spacers or not.

The mounting surface of the wheel has nothing to do with the loads the bearings and spindle see, if a spacer is used to bring the center of the contact patch to the stock location.

If you don't care about physics, then I'm not sure what yard stick you are using. The laws of physics DEFINE what happens in the phsical world.

Have the racers you've seen with spacers put on wheels/tires that have moved the center of the contact patch away from the stock location? I bet that's the reason their bearings are going.

Porsches have been using wheel spacers for decades without any problem. Of course, the factory engineered the setup, not some amatuer racers, so the PHYSICS are correct, and there aren't problems.

Pete Paraska - Mechanical Engineer, MSME

[hmsports]

Dude,

There's no reason to get testy over this -- no body is trying to discount anything about physics here -- or what you are saying.

We are all just adding our experiences and thoughts to the discussion.

Good day.

[pparaska]

I'm not being testy - sorry if I offended.

I was just defending my assertion that it's the center of the contact patch, not the wheel mounting surface position that matters.

Experiences are fine, but if all the info isn't there (as in where the contact patch center was on the cars that had spacers and bad bearings), then it's not worth anything as far as added information.

The devil is in the details.

[RickS]

I have to agree with pparaska on this one. Think about it, if you had a wheel that needed a spacer what would be the ideal solution? Correct offset in the first place, right?

Now remember that as I take it a step further.

Say, instead of adding a spacer we increase the thickness of the wheel where it mounts to the hub to make up the differance. So, now is it easier on the bearings than a spacer? - No; nothing changed.

Now, you take that same wheel with the thick center and remove unneeded material (outside) from that extra-thick center. What you end up with is a wheel with the correct offset but still nothing has changed.

One final example. This time, let's say you have 16"x8" stock OEM wheels with 3" of back-spacing. You buy some cool rims that are the same size (16"x8") but these have 5" of back-spacing so you use 2" (probably 1 1/2" would do it) spacer. Guess what? You now have the exact same offset and contact patch centerline as the stock wheel.

If you went with a 10" wide wheel instead it would put more load on the bearings than the 8" wheel wether you had the correct offset to make it fit or you used a spacer because the limitation is still interferance with your suspension and that forces you to put 2 more inches of tire outboard of the hubs bearing regardless. That 2 extra inches of leverage will find it's way to the hub bearings wether or not there is a spacer somewhere in-between.

Adding spacers usually goes along with wider rims and tires that increase the outboard size but not always. Simply stating that spacers cause more strain on bearings is incorrect. It should be said that spacers do often make it POSSIBLE to add wheels and tires that can cause more strain on bearings. On the flipside, you can go buy a rim with offset that allows a perfect fit but if it adds any tire further out than stock, more strain will be introduced to your bearings PERIOD.

[RickS]

I thought this needed a bit of clarification.

I'm sorry, but that statement is just completely false. The center of the tire patch is all that matters. The wheel mount surface does not, as long as a spacer is used to move the center of the contact patch back to the stock location.

Yes, that statement is false but not exactly for that reason.

If you had a 13"-wide wheel and a 5"-wide wheel with the exact same contact patch center, the 13"-wide wheel would put more load on the bearings. It is the amount of rim beyond the hubs bearing surface that matters.

________Hub_________6.5" of leverage

___Hub___2.5" of leverage

Moving the contact patch center does cause other problems though; specifically with steering effort. This is because as the contact patch moves away from the hubs (outboard) pivot point, you have to drag the tire across the pavement to turn. Adding more castor causes a similiar effect (though the added dynamic camber is usually worth it to a point).

This is why the new Corvettes use DEEP backspacing. It keeps the contact patch center line closer to the pivot point (good for steering) and less of the tire ouboard of the hubs bearing surface (good for bearings) as well. This also has the added benefit of allowing longer suspension arms for better camber and toe gain curves.

[Zed]

I'm in agreement with pparaska and RickS.

See the attached (poorly executed) illistration.

[Dan Baldwin]

Hey all.

If the spacers are well made (flat, parallel surfaces, correctly centered tapered mounting holes, hub-centric), then ideally the loads are exactly the same as long as the overall offset hasn't changed, as in Zed's illustrations. Still, they can be a PITA, as I witnessed a Z-guy having to uninstall and reinstall his at the track on Sunday/Monday. Also, you can't easily retorque the spacers to the hub. I wouldn't condemn their use, but they do introduce additional potential problem areas. Finally, FWD offset rims don't look nearly as cool as zero-offset rims on a Z.

Oh yeah, the shear load is the same no matter where the contact patch is laterally, and the moment changes linearly with the contact patch offset, not with the square.

Dan Baldwin

'71 240Z 3.1

COMSCC #7 SPB