rskrause

Got into a discussion about these last night with some knowledgable car guys. Specifically, we were talking about use of spacers on the rear wheels for drag racing. One point of view was that the use of thin (<1/2") spacers on the rear is ok. The other was that they are big no-no. Reasoning as follows.

Against: Spacers create additional shear loadings in the studs that can easily exceed the strength of the studs and lead to failure. When the wheel is bolted directly to the rotor there is one shear plane. With slippage at that plane, a bending load is induced in the stud which increases the tension load in the stud by a value proportional to the sine of the bend angle. The ending moment is applied at the stud root- the load is contained to the shear plane, and since the offset is nearly zero it is essentially a pure shear load.

If you use a spacer there are now two shear planes which makes slippage of the wheel with respect to the rotor more likely since there are two sets of surfaces to clamp. Instead of being applied at the the root of the stud, it is now applied away from the stud root. The bending load is proportional to the spacer thickness and the tension load increases proportional to the square of the sine of the bend angle.

For: The wheel and hub are locked together by friction. Assuming no flex in the spacer, then there is no shear load or bending load on the stud. For example, if you tighten the nut to the stud and place 50,000 pounds of tension on the stud and the coefficient of friction between the wheel and hub is 0.9, you can put 45,000 pounds of shear force (per stud) on the wheel to hub interface without inducing any bending force. So, with a good coefficient of friction between the hub-spacer-wheel interface, there is very little bending force on the studs.

The "for" argument makes sense to me. It implies that spacers should be rigid (ie steel and not aluminum) and have a fairly rough (not shiny) surface. This would apply to the mating surface of the rotor hub and the wheel mounting pad when no spacer is used as well. If the coefficient of friction is 0.9 at both the spacer-hub and spacer-wheel interface, you would decrease the shear force at which slippage would occur by just 9%.

This discussion also pointed out why you should check the lug nut torque. If the lugs are loose, there may be movement between the wheel and hub, which is a bad thing because it puts shear force on the studs.

Any comments? Are the principles substantially different for roundy-round racing and/or for the front wheels?

Rich Krause

OldSStroker

Wheel studs (or bolts) are designed to resist the shear load at the wheel/hub interface. If the stud is torqued correctly and stretched the appropriate amount, there is virtually no bending load from the drive torque.

When you add a spacer, you move the point of application of the load away from the hub face and therefore put the studs in bending as well as shear. Bending is bad for bolts. Obviously the thickness of the spacer is the moment arm. Some racing organizations allow specers up to maybe 1/4 inch thick, which is 40% of the diameter of the 5/8 studs they use.

IMO, spacers up to 40% of stud diameter is about all I think is safe. That's about 3/16 or 5mm on 1/2 inch studs. At 1/2 inch spacers, you are overloading the studs with sticky tires, big torque and lots of chassis bite.

Properly tightened wheel attachments, especially with alloy wheels with their flat mounting surfaces don't loosen. If wheels with spacers loosen with use, it's because the bending loads are yielding the studs.

If the coefficient of friction, etc arguement holds water, it should also work for 2 inch or more spacers, not just for 1/2 inch. I don't think so.

FWIW: With steel wheels (of OEM design) the mounting surface around each stud in cupped, so it acts like a Belleville spring when torqued and helps the clamping loads. Ever notice that torque specs for alloy wheels are usualy higher than they were for steel wheels, and that streel wheels rarely loosened in use?

Botom line: I'm on the "Against" team with maybe a slightly different explanation. If you must use a spacer, go to bigger diameter studs. If your lug nuts loosen in use, lose the spacers.

My $.02

Mikael

I have spacers for my 17x11 wheels making ~400rwhp w/ no problems. I think practice takes precedence over paper.


I also think you guys spend way, way, way too much time thinking about the sheer loads and coeff. of friction on wheel studs when using spacers.


Maybe you should bring notecards with topics like "BEER" and maybe "GIRLS" and discuss that.


Just a thought.

rskrause

What's beer and girls precious?*

*Apologies to JRR Tolkien

Rich Krause

Lonnie Pavtis

I do believe that the type of spacer/adapter that bolts on to the hub, & the wheel then bolts to the spacer, is of questionable safety. These are rather thick & could induce bending loads.

Now, if thin wheel spacers are unsafe due to bending loads, explain how the rotor being bolted between the hub & wheel is any different. I would like to see spacers that are bolt pattern specific & also fit the hub tightly. Remember that IROC's & TA's in the late 80's had a wheel spacer from the factory on either the front or rear wheel (can't remember which it was).

rskrause

Lonnie: the thoughts I had were similar with respect to the rotor. Isn't the rotor in effect a very thick spacer?

But with respect to the idea of the rotor fitting the hub tightly, I was wondering how you meant. Do you mean fit the lugs tightly? At first thought, that made sense to me too. But after giving it some more thought, I not sure how important that would be. The holes in the rotor have some play relative to the studs, right? But it doesn't move relative to the studs, presumably due to the friction between the hub-rotor-wheel assembly.

Rich Krause

OldSStroker

I see it as just the opposite: the original studs hold the bolt-on spacer tightly to the hub as if it were a wheel with no spacer. The additional studs in the bolt-on spacer then hold the wheel as if it were being attached to the hub. This is a legitimate way to space wheels out from the hub. FWIW: Check out how front wheels are mounted on a 1 ton dually pickup.

The problem is that you need over an inch of spacer thickness to allow adequate support of the new studs, and clearance for the nuts. So, from 1 inch on out, this is the way to go. From 3/16 to 1 inch is the problem.



It's thicker spacers (1/2 inch was mentioned) when added to the slip-on rotor, which is maybe 10 mm thick, which push the limits of bending.

If a spacer is deep enough so that the "hub-centric" piloting of the wheel is lost, the spacer should fit the hub snugly, and have a concentric hub to locate the wheel. It shouldn't fit the bolts tightly because then it fights the hub for centering.

FWIW, the late 80's 16 x 8 TA cross-lace and the equivalent IROC wheels had different front and rear offsets, in order to achieve the necessary clearances. We have some of the wheels, but I never noticed OEM spacers. If you try to put those "rears" on the front, you might run into interference without spacers. Some folks run "fronts" all the way around for maximum track width.


My highly-opinionated $.02.

WS6 TA

Third gen f-bodies that came with 16" wheels and iron drum brakes came with spacers on the rear wheels (I think they were 3/16"). If they had aluminum drums or disks the hubs on the brake parts were thicker and didn't need spacers.


The stock 16" fronts had 0 offset, the rears had 16mm, and the 15's had 1/2" on the front and back wheels.

chevyguy3

last year i saw 3 cars shear all thier lugs and lose a tire at the track. all 3 of them had wheel spacers. 2 had 1/2" studs, not sure what the other was

fwiw i use 5/8 studs and no spacers

in regards to the 3rd gen example, keep in mind when the factory did that im sure they didnt have 3500+ lbs cars launching on slicks with big power in mind

WS6 TA

FWIW, I’ve seen 2 cars shear lug nuts without spacers in the last year…

Personally, I fall on the side of “spacers are bad” basically due to the bending loads argument, but I wonder how bad they really are if they fit tightly around the lugs like a brake rotor hat does…

rskrause

That's enough to give me food for thought!

Rich Krause

OldSStroker

Do NHRA and IHRA have tech rules governing spacer use?

Eric Bryant

My take on spacers is that nearly all of the failures associated with them are due to a loss of preload on the studs, either by deformation of the material or by loss of material through fretting. Maybe I'm oversimplifying things too much.

Injuneer

I've seen a few parts designs and pit practices that boggle my overly conservative engineering mind.... and I'd guess that at least half those concerns have proven correct when parts were damaged or failed due to improper use or improper design. Sometimes a little reflection on what you are doing and how you are doing it pays off in the end.

Rich is looking at over 700rwHP, and very sticky tires... what works (at least for now... ) on a 400rwHP car might not work well on his. I don't like spacers of any kind, at least not on the "power" end of the car. I do use thin spacers on the front for brake clearance, but I don't believe there will be a lot of load on the studs until I start pulling the front wheels........ and dropping it in total shock!

Lonnie Pavtis

OldSStroker. The thick bolt-on spacers I routinely see are aluminum. This was my criteria for concern. Yes, the cast iron spacers on 1-ton duallies are strong & rightfully so being made from 1" thick cast iron with (8) 5/8" studs tolding them together. They are also approx 4" thick spacers .... not really comparable to our cars.

Rich, The rotor got my attention as they often have substantially larger holes than the studs. In fact, the Baer type of rotor on the Cobras, has nearly an 1/8" larger hole than the stud.

I do not think the bending moment is much of an issue here (on a large truck with side loads from turns etc it can be)as we are not talking heavy loads (3500# car / 20 studs) as much as the effect of a loose wheel not providing enough clamping force to resist the shear force on the bolts (600ft.lb. engine x 2.5:1 converter mult., 2.48 low gear, 4.1 rear gear = 15,252 ft.lb. at the wheels)

Has anyone compared the area of 5 wheel studs to the area of the axle shaft at the wheel bearing? How many people here have broken an axle?

Also hub piloted mounting would be a definite strength advantage, but next to none of the aftermarket race wheels fall in this category either.

All in all, I would venture to say that 90% of these failures are directly related to loose wheels subjecting the studs to shear(retorque often to maintain proper preload), repeated tire changing (how many of you think of your studs wearing out) or overtightening (tightening past yield strength) via impact use.

For example, I often drive a Chevy 4x4 Tk (500hp 427, 4.56 gears) with 35x12.5 tires . You would be amazed on how often the studs need retorqued with aluminum wheels (I have also cracked 2 Weld wheels where the bolt in center meets the outer ring... 5400lb empty & a lot of bending stress due to tire height) First 50 miles (approx 1/4 turn), after 150 (approx 1/8 turn), recheck at 500 & occasionally a few will have noticible movement. After this, they rarely move. How many of you guys recheck your wheel 3 times & also use a torque wrench? It makes a big difference. Impact wrenches are for removing wheels, not installing them in my opinion. If you bolt on slicks every weekend, replace your studs frequently.

Food for thought..........