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Discussion Starter · #1 ·
Hi,

Can anybody tell me why people say wheel spacers are bad?

If the answer has something to do with the wheel studs, then does anybody know what the concern is in particular? Is it the tension load from lateral tire forces, or the bending load from accelleration/braking?


I am in a spot where I may have to use them, I'd like to understand the problem thoroughly so I can make an educated decision.

Thanks!

Steve
 

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Try a search. There's been discussion on this before.
 

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Wheel spacers that are properly built and utilized are legal and common in many racing venues. I've seen C Prepared Mustangs with 12" wide wheels and slicks using wheel spacers at SCCA Solo2 events, is your use more strenuous than that?:)

See the Maximum Motorsports website for some good ones and some info on how to use them. Ther's more to it, but that's a good start.

I must say, I am curious as to why you "may have to use them"?

Most people voluntarily use them to obtain a desired effect, wider track, proper back spacing, and such.
 

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If you're using a good quality hubcentric spacer, the mounting surfaces are cleaned up, and all the lugnuts involved are properly torqued, you should be okay. The clamping (friction) loads between hub & spacer and spacer & wheel surfaces, from proper torque, are what resist outside loading, not the studs themselves. If a CP mustang running Goodyear race slicks at Nationals can't tear off a wheel, then you'll probably be fine.
 

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Discussion Starter · #5 ·
> Wheel spacers that are properly built and utilized are legal and common in many racing venues. I've seen C Prepared Mustangs with 12" wide wheels and slicks using wheel spacers at SCCA Solo2 events, is your use more strenuous than that?

It depends on what the concern is. If laterial load is what makes spacers unsafe, then clearly no. But that is not the only source of stress they see. I am planning to use them in an unorthadox way.

This is why I'm asking if anybody knows what the actual technical concern is, rather than if people think they are safe when properly used.


> I must say, I am curious as to why you "may have to use them"? Most people voluntarily use them to obtain a desired effect, wider track, proper back spacing, and such.

I just built a new rearend and switched to a typical griggs suspension + coilovers. I did not know where the coilovers were going to go when i narrowed the rearend, so now when I compress the driver side rear suspension fully the the brake banjo is 1/16" from touching the spring (would not touch if not for the coilovers).

I'm using single piston calipers so as the pads wear the caliper body will move inwards and cause interferance.

The two things I'm concering is milling the caliper to move the banjo in, or putting a spacer between the axle and rotor to move the caliper out.

Obviously this is not how they're intended to be used, so before I decide if this is safe I have to understand what the failure mode is.

Anybody have a clue?
 

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There is nothing wrong with using wheel spacers. Coleman sells high quality steel spacers in 1/8" which I have used on race cars for 10 years with no problems.
 

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We've used wheel spacers on our 95 cobra for five race seasons and three Indy Vancouver (drivers tend to get a little more 'gutsy' with TV coverage) events with no problems. As mentioned above, properly cleaned and torqued should result in a problem free application.
 

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my buddies 1989 Porsche Speedster which has Turbo flaired rear fenders but standard size rear wheels came with spacers from porsche factory.
 

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>It depends on what the concern is. If laterial load is what makes spacers unsafe, then clearly no. But that is not the only source of stress they see. I am planning to use them in an unorthadox way.

This is why I'm asking if anybody knows what the actual technical concern is, rather than if people think they are safe when properly used.

I just built a new rearend and switched to a typical griggs suspension + coilovers. I did not know where the coilovers were going to go when i narrowed the rearend, so now when I compress the driver side rear suspension fully the the brake banjo is 1/16" from touching the spring (would not touch if not for the coilovers).

I'm using single piston calipers so as the pads wear the caliper body will move inwards and cause interferance.

The two things I'm concering is milling the caliper to move the banjo in, or putting a spacer between the axle and rotor to move the caliper out.

Obviously this is not how they're intended to be used, so before I decide if this is safe I have to understand what the failure mode is.

Anybody have a clue?

Gotcha.

The only failures I have seen were oddball stuff. One guy who attached the spacer to the wheel with small screws (the lugs were too short.) Another one involved a "universal" spacer with large slotted lug holes that let everything move around until it broke the lugs. Maybe someone else has more data.

As long as you use a quality spacer that's not too thick, I don't see where you'd have a problem. You want full engagement of the lug nut threads (some people say 6 threads are enough; I'd go with longer lugs if necessary) and the steel spacers Cosmo mentions just seem like a better idea.

Sorry I don't have anything more concrete.
 

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Discussion Starter · #10 ·
Looking back I was a bit demanding to ask only for the reasons why they fail and not personal experience. If there is no such reason, then anybody who's being honest wont be able to give an answer, so my question was unfair.

I have concluded that the reason their failure has become a legend was failures due to improper use.

Thanks everyone (especially rfloz) for your responses. Off I go to bolt on some spacers. ;)
 

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Spacers add a bending moment to the stud. Since most spacers are not intimately attached to the wheel or the hub they float (meaning they don’t transfer load, disregarding friction due to joint clamp up) and add eccentricity to the joint. The thicker the spacer the more bending leverage applied to the stud under load.

Basically the technical reason people don’t have too much problems running various spacers is that the studs have enough strength margin to react the additional bending load due to the presence of the spacer even when combined with shear with a properly torqued stud.

Now if the spacer is attached to the hub or wheel with attachments that are equivalent in strength/strain to the studs then you can consider the spacer either part of the hub or wheel and joint eccentricity doesn't play a role.
 

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I think that bending would only be an issue if wheel offset was not taken into consideration; moreover the scrub radius would also be affected without such change.

HellrotRed said:
Spacers add a bending moment to the stud. Since most spacers are not intimately attached to the wheel or the hub they float (meaning they don’t transfer load, disregarding friction due to joint clamp up) and add eccentricity to the joint. The thicker the spacer the more bending leverage applied to the stud under load.

Basically the technical reason people don’t have too much problems running various spacers is that the studs have enough strength margin to react the additional bending load due to the presence of the spacer even when combined with shear with a properly torqued stud.

Now if the spacer is attached to the hub or wheel with attachments that are equivalent in strength/strain to the studs then you can consider the spacer either part of the hub or wheel and joint eccentricity doesn't play a role.
 

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Discussion Starter · #13 ·
Hellrot,

Your post confuses me. If the bending load is the issue, then it does not matter whether the spacer is attached or not. Rather the bending load would be determined by how far out on the stud the load is applied.

Since the studs are centered in the wheel's hole (and therefore do not touch the wheel at that point), the load must be applied at the nut (or at the mating surfaces between the axle flange and wheel, if the friction is truely high enough).

So if it doesnt matter whether the spacer is intergral to the wheel or not, then adding a spacer would be no different than adding a wheel with a thick flange.

I'm just thikning out loud here.. please anybody feel free to speak up.
 

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88Copcar said:
I think that bending would only be an issue if wheel offset was not taken into consideration; moreover the scrub radius would also be affected without such change.
I believe your thinking only in terms of cornering and loads due to suspension deflection. The studs also have to transfer torque from accel and braking. The addition of the bolt bending moment I am speaking off results from the torque.

Edit

Here is an exchange from another board that delves into the details more. One point of view is similiar to what I commented on by discounting friction. The other point of view considers friction. Kinda of depends on what assumptions (conservative or less conservative) you want to make to ensure you have an adequate margin for strength.
 

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No, that would interfere with the caliper mounts. Spacers go on just before you put the wheel on, on the outside of the rotor. (Usually longer studs are needed also for proper lug engagement)

Also, keep in mind that spacers directly affect your suspension's scrub radius. Not an issue on non-steering wheels (our solid rear axles), but if you're increasing the scrub the most notable result is increased steering effort, which obviously is undesirable. Then again, if you're matching a spacer with a wheel with a larger backspacing, the two can cancel eachother to maintain original SR, or concievably tune it.

Continuing this further requires discussion of KPI and other topics not really related to this thread, but these are good things to keep in mind. For more tech, www.corner-carvers.com
 

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Rant on.

Properly designed and used wheel spacers DO NOT create any change in bending load on a wheel stud. This isn't a matter of opinion; it's a function of physics.

The job of a wheel stud is to apply a clamping load to hold the wheel to the hub. When the lug nuts are tightened, the wheel studs elastically stretch like very very stiff springs. In an optimum situation, you tighten the lug nuts until the stud is at about 90% of its elastic limit. This will give you the greatest possible force holding the wheel to the hub.

There is friction between the wheel face and hub face. This is measured by a term called coefficient of friction. Coefficient of friction is the ratio of normal force at the intersection of two surfaces to the lateral force required to slip the bodies relative to one another. Good street tires have a Cf of 0.9. This means that if there is a 100lbs vertical force applied to the tire, it can generate 90lbs of cornering force before it slides.

This is why the amount of clamping force at a joint is important. The more clamping force, the more torque will be required before the two bodies can slip relative to one another. Unless the two bodies slip, there can't be any bending load on the stud. The stud is under maximum stress when it is torqued to 90% of its yield strength and the car is sitting still. When the car starts moving the stress applied to the stud stays the same or goes down, unless one of a couple of things happen.

One, the vertical component of any external force applied to the wheel is so great that all of the clamping load between the wheel and hub drops to zero. At this point the stud is loaded in bending and shear and may break or yield.

Two, some portion of the clamped joint, wheel or hub is flexible and allows the tension load on the stud to drop to zero. A general rule in mechanical engineering is that in all clamped joints the joint itself needs to be 10 times stiffer than the fasteners used to clamp the joint.

Inserting a wheel spacer between the hub and wheel changes nothing about these physics. The hub center of a hubcentric spacer does not hold the spacer on the hub and reduce the chance of stud failure. There is never any load on the lip of the wheel spacer. For there to be a load on the lip, the friction force in the joint must have been completely overcome.

Increasing the length of the wheel studs to use a wider wheel spacer only changes one thing. It increases the amount of tension force that is removed from wheel studs that are not at the top of the hub. This effect is not very large within the range of real wheel spacer widths.

Most wheel stud failures are from under tightened lug nuts (not enough clamping force) or over tightened lug nuts (the stud has been stretched past it's elastic limit, so the clamping force goes away). If the wheel spacer or wheel is not thick enough it can be too flexible and allow the studs to fail by loading them in bending.

Rant off.
 

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... now one could argue that putting the wheel further away from the spindle increases the torque on the spindle itself...

...but this again only increases the tension loads on the studs, not the bending loads.

It also can be said that spacers (or any other form of increased offset) affect wheel bearing life, which is plausable but acceptable.
 

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RagingGrandpa said:
... now one could argue that putting the wheel further away from the spindle increases the torque on the spindle itself...

...but this again only increases the tension loads on the studs, not the bending loads.

It also can be said that spacers (or any other form of increased offset) affect wheel bearing life, which is plausable but acceptable.
Careful there, Jack may rant at us again :)
 
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