This keeps popping up in conversations and every time I wince when I hear some folks $.02.

Below is a very child-like picture of 2 wheel options. Go ahead and laugh, I did.

#1 wheel has proper offset so no spacer is needed.

#2 wheel has a hollow-spoke 996 twin-turbo wheel and requires a 3" spacer to maintain the identical EDIT* (backspace) as the #1 wheel.



My question to everyone is this:

Is there a difference in bearing stress between #1 and #2 ?

no, there is no more strain on the bearing from one rim or the other IF they both end up with the same offset. the spacer doesn't change the torque applied to the bearing unless the offset (or in otherwords the distance the outer rim ends up) is different than the other... doesn't matter how you got there it's the same leverage

mike

I would agree there is no more torque (parallel to the axle) on the bearing with 2 wheels of similar width, with different offsets with one being compensated by a spacer. However, there will be more torque with a wider wheel than with the standard width wheel. If you compare a 5" wheel to a 10" wheel with the bearing centered between either wheel, the wider wheel can exert roughly twice as much torque on the bearing. If you use a 2.5" spacer to accommodate a wider wheel and the bearing is now not centered in the wheel, but is now offset towards the back 2.5", the torque on the wheel bearing will be roughly 3 times stock. I (over) compensated on my build by using a larger bearing and moving that bearing to the center of the wheel. http://www.914world.com/bbs2/index.php?showtopic=86910 If I weren't thinking of running a high horsepower motor with these particular wheels I would consider either spacers or sectioning the original bearing off the control arm and spacing it out with some tube. This option would certainly have been a heck of a lot simpler than my approach and lead to less fabrication.

Yes. About 3" of difference ...
Andy

I dis-agree. The end of the lever is where the center plate of the wheel is located. That's usually the mounting surface.

Add a 3" spacer to the mounting surface and you'll have a 3" longer lever (+/- a few things i am ignoring here) ...
Andy

Thank you Mike and Wilhelm.

That's exactly my point.
It doesn't matter "how" the wheel connects to the exact same backspaced wheel of the same size. the leverage is exactly the same.


Roger

About 3 years ago when I added the fiberglass flares to the race car I started using 3/4 inch spacers on each side in front and 1 inch spacers on each side in the rear. I use Goodyear slicks and so far I have not had any bearing issues on either end in approximately 28 race weekends.

Sorry Andy, there is no difference. the 3" spacer is compensating for a different offset wheel but the distance from the mating surface of the hub and bearing centerline to wheel centerline is exactly the same in both pictures. Therefore no difference in bearing leverage.

Or am I off base?

Does picture #2..... RIM......give more support than picture #1 "outer" rim ,since the inner rim on #2 comes before the spacer, compared to if you were to hit a "stone" with the "outer" "lip" of the rim in picture #1? It seems that if you were to hit a "stone"on the Outer lip of Picture # 1, It would be a serious high pressure point, possibly doing more damage, than if a stone was run over by the rim in the same spot - on the outer face , in picture #2, because the adapter is centered in the rim, relieving it of some pressure. Am I right ?..........

Yes, you are. The end of the Lever is not your red circle, but the mounting surface where the wheel is mounted.
And that is about 3" further out in the second picture.

The wheel will transfer the load to the mounting surface and everything back from there to the bearing will act as a lever.

Andy

PS: Of course, nothing is ever that black/white. In the first pic, the wider rim on the outside puts additional leverage on the mounting surface under load. In the second pic, the wider rim on the inside puts additional leverage on the mounting surface when unloaded.

That would only be true at rest with no other forces acting on the wheels.

Do you care a lot about bearing load at rest?
Andy

Ok, I just changed my mind...

It seems to me that there would be no difference in the forces the bearing sees between the two setups.... but only if you make the assumption that the center-line of the wheels in both set-ups sit at the same distance from the bearing, and if you make the assumption that the forces are being directed onto the bearing from the center-line. As Andy pointed out, the actual forces are being communicated to the bearing from the point of connection at the spacer...not wheel center line.




of course this is just my $.02

But the offset from the centerline of the outer wheel to the mounting surface itself acts as a lever.

If the mounting surface would be right at the center of the wheel, that offset would be 0. Any offset other than zero will add leverage to the mounting surface ...
Andy

Ok, one more try ...

The dark blue box is the bearing, the green circle is where the wheel mounts. The forces acting on that mounting surface are about the same in both scenarios.
However, the red line indicates the lever acting on the actual bearing. That lever is much longer in the 2nd setup.

Andy

PS: You're right that the effective length of the overall lever is the same in both setups (distance from the bearing to the outer edge of the wheel). But as i said before, that only applies if the wheels are at rest. As soon as you get any kind of sideload on the wheels, there will be a difference in forces working on the bearing.

the only factor in the effective "leverage" that will change forces seen by the bearing is where the bearing resides in relation to the center line of the wheel. the location of the point where the rim attaches to the wheel center and how long of a mounting "hub" from the wheel edge will have no effect [as long as the wheel is ridgid enough to transfer the load], this is simple.



and the plane will take off in to flight!

lets try this.... you have a wheel that you can balance with a point of contact right at the wheel flange which is exactly at 0 offset, then you take another wheel that the offset is set toward the face of the rim by 3", now you can't balance the wheel at the flange, but if you add a 3" spacer you change the point of contact to exactly as the first wheel and it will balance same as the first.

IMO, If the tire is the same and is in the same location relative to the bearing, it will produce the same static load on the bearing (with spacer).

It may be a different mass and have different dynamics in motion, but in a steady state equilibrium, it is the same.

(at least that is what I learned from 2 engineering degrees and years of stress modeling nuclear power plant designs.)

As long as the hub, spacer, and wheel are all rigidly attached to one another (and all assumed to be rigid bodies themselves), there is no difference to the bearing loading.

Only problem I see with your drawing is that all the spacers I see bolt to the brakes then have bolts to the wheels. If you used long bolts that went thru a spacer then to the wheel I can see this causing extra load on the bearings over the same wheel with the proper offset.

Of course I got the wheel spacers from hell on my car with no problems so far. Plan to over some custom wheels before to long just not sure on the size yet.

wow, 3" spacer?! will you be using super long studs/bolts or studs/bolts at the
hub then another set of studs/bolts at the wheel?

Yes, that is correct. And, provided both wheels had the same offset, that difference would be directly proportional to the increase in torque seen by the bearing. It is my understanding, according to the original poster, that in our scenario the distance from the center line of each wheel to the hub is the same...thus the only difference being the offset (which would require more wheel/tire to extend past towards the control arm than stock). Now, I suppose it would be up for debate as to whether or not this scenario is actually what is happening. Does anyone with spacers have measurements of the distance from their hub to the center line of their wheels? What is the measurement for stock?




In this scenario I would argue that, though the line through which the force is acting on a macro scale is through the center of the wheel (which in this case is the same as w/o an offset wheel), if one zooms in the only path by which the force can be transmitted is via where the wheel connects with the spacer/hub. Thus the bearing would see an increase in torque. I've attempted to draw a quick force diagram to show what I mean

Click to view attachment

Hmm... the line representing the wheel on my diagram shouldn't extend past the hub

They will be the same. The lever moment does not end at the wheel mounting surface. That is an intermediate point. The force is exerted at the point of contact with the pavement. The 3" longer level caused by the spacer is offset by the 3" shorter lever offered by the wheel offset, assuming that the wheel is in the same relative position to the bearing.

The stress is relocated from the wheel in the first case, to the spacer/studs in the second case. This in itself is less desirable. But, the load on the bearing is the same.

I hear what your saying, but I can't convince myself that it works.

Lets try looking at it backwards. Instead of from the perspective of the tire, from the perspective of the suspension (The force off of the ground is really only a reactive force anyways). The force due to gravity on the car is distributed to the tires via the suspension. Thus there would be a force acting downwards at the point of the bearing due to the cars weight and consequently there would be a moment at the point where the wheel attaches to the spacer equal to

(the weight of the car)X(the distance from the bearing to the wheel)

Now, There then needs to be an equal and opposite force to prevent the car from falling through the ground . That force is the one pushing up from the wheel. Now, we can roughly assume that the force is evenly distributed along the wheel and is thus imparted through its midpoint. However, I can't conceive how that would be true of the hub/spacer moment arm. The only physical point at which the force can act is at the point where the wheel attaches to the spacer, and it would have to be equal and in the opposite direction as gravity. Thus increasing the torque on the bearing...am I missing something?

the bearing to wheel center point is the only reactive veriable to the horizontal "torque". this is assuming that the wheel is capable to carry the weight for the application, the structure of the wheel +spacer and the attaching method to carry the weight will not induce any "torque" or lever moment to the bearing, as long as the bearing to wheel center ratio is the same.

think counter balance.

i think he means 1 the same as 2 with the spacer


the load is extended to the farthest point of where it's being leveraged, at the outer contact of the wheel

the 3" spacer creates no more or less leverage (between 1 &2) IF it compensates to the same offset ending up with the same outer leverage point

Maybe I'm thinking too hard about this, but this is another way of looking at it:

Click to view attachment


I don't think the geometry of the wheel is as important as how the arm extending from the bearing to the wheel is supported. The weight of the car pushes down at one end and and must be supported by the wheel. Well. the only point at which the wheel can support the weight of the car is where the hub or a spacer connects to the wheel.

like this?

You've only got half the system there. You have to consider the whole system. The wheel itself exerts no force into the system. The pavement and the bearing are where the force is exerted.

Let's look at it this way... take the spacer out of the picture:
Click to view attachment
Which wheel exerts more force on the bearing?

The left one. The torque moment is greater. Adding a spacer to the right one makes the torque moment the same.

The leverage is the same but there is the added weight of the wheel center and the spacer farther out that also agrivates the terrible situation. Most people started with a 4.625 offset wheel that was 5.500 inches wide. You therefore see that the engineers only needed a small bearing to support the load especially with 100 hp max. Evidently they as most engineers do built in a safety margin so that the 914/6 could still use the same setup and go to 6.00 wheels with I think the same offset. The scary part is the stub axle is still expected to support the big wheels that are being used. A bad bearing is one thing a broke stub axle is really scary especially considering it won't brake setting still.

I haven't, but check out the 996 tt porsche and see what rear bearings they are using to get a idea of whats needed on 11 inch wheels. I do realize it's a tail dragger and a lot heavier too though.

My friend the fabber and welder would get at least one big wheel a month to repair that had cracked. These were not forged porsche wheels but the cast ones you see on the low riders the cracks were always on the inside and usually had silicone on them.

What does cracked wheels have to do with torque moments?

I agree that it's the center line of the wheel relative to the bearing that dictates the torque. Forget offset, forget backspacing, not important. If the centerline of the wheel is farther from the bearing, there is more stress on the bearing.

Here is my two cents;

As long as the offset of the tire from the bearing is the same, it doesn't matter (in regards to the bearing) if you are using spacers or not!

Think of the rim and spacer as an assembly that defines the offset of the tire in relationship to the bearing.

The bearing has no idea what that "assembly" is. It only sees the force applied to it from the hub. It doesn't know if it's just a one piece rim, or a rim with spacer. It just sees the torque applied to it, which is a function of the tire offset from bearing.

If SirAndy's comments were correct that there is a difference, would that difference go away if the spacer was welded to the rim?, making it a one piece item?



Wes Vann

Lets keep it simple:

The only way load will be increased is if the outboard edge of tire is moved outboard.

Mark

New morning, new perspective

I took a new look at this and realized that the spcer does indeed add torque, but not on the bearing. You're right, no change in wheel center line compared to bearing = no change in torque on the bearing. Instead the spacers increase the torque seen by the connection point at the wheel, no a big deal since the wheel was desiged for that offset anyways.

Simple as the distance between the bearing and center-line of wheel.

Tim's response to the 3" spacer on my thread. maybe he will chime in with a current response:

Oh, and Hi Troy. You said that earlier too.

Deja Vu

So are the final results in?



Do we all agree that the bearing load is about the same (minus the obvious added weight of the spacer) ?


Does anyone have any other considerations or thoughts that haven't been covered here?

i think it's mostly agreed the bearing load is the same, but how would the spacer weight have any effect on the load? maybe if it was hanging free weight... BUT most importantly here is... WHY THE HELL does this even interest me

There is a common misconception that two wheels of the same size and back space but with different offsets, have much greater bearing loads.
This is not true.

That's all I wanted to clarify.

Now I'll add the "beating the dead horse" smilie.......

oh shit... i can't help it...

they can't be the same width and backspacing and have different offsets. the offset is just another variance of the backspacing measurement. if the offset (+ or - measurement from center of rim) was different on the same width rim the backspacing (distance between the rear of the rim to back of mounting surface) would change with it.

maybe i'm misunderstanding you. are you refering to the different offset (without spacer)? but if so that would create a different backspacing.

sorry

Whoops.... I guess I was using wrong terminology.

So what is the term for distance from mounting surface of the hub (not the mounting surface of the wheel) to the rear wheel lip? That is the constant in my argument.

I thought that was backspace but backspace is the distance from the rear wheel lip to the mounting surface of the wheel.


Goody! I get to learn something new...

backspace is measured from the back othe rim to the back of the mounting surface of the wheel. the off set is the measured from the center of the rim to the back of the mounting surface of the wheel.

From tirerack.com website

http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types:

Zero Offset
The hub mounting surface is even with the centerline of the wheel.

Positive
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside.

*Backspacing, similar to offset, is the distance from the hub mounting surface to the inside lip of the wheel (measured in inches).

Clutch boy got the word. Leverage. Think back to hisckule fizics and the lever-arm math.

I haven't seen anyone mention scrub radious yet? That really changes the way a car can change dirrections.

is "scrub radious" the same or simular to "akerman angle?"

now that that is mentioned, the akerman angle would be affected with wider/more positive offset.

read this on scrub radius. http://www.hrsprings.com/technical/scrub_radius