One of the most common and misunderstood questions we get here at The GMR is “what is the WMS measurement?”
Before we can go into what the definition is we must first understand what the WMS (wheel mount surface) measurement is Not! Commonly it is confused with “axle flange” width, and sometimes with “housing flange” width. WMS is also not the overall track-width of the tires from the outside of one tire to the other. The WMS (wheel mount surface) is exactly just that, the surface where the wheel actually touches when on the rear end. This surface can vary with the different styles of rear end setups so below I have displayed what each one is and where the WMS is factored into the equation.
The Traditional Flanged Axle Rear End (semi-floater)
Out of all the different rear end setups the Flanged Axle is by far the most confusing rear end when it comes to measuring the WMS. In the diagram above you see a few different measurements but the most important is the 3A measurement. This will give you the axle flange to axle flange overall width. This is still not the WMS, you are missing the rotor hats.
Above is a picture of a Wilwood rotor hat, this piece is what holds your rotor on your axle. It slides over your wheel studs and is located between the axle flange and wheel. This is where the variable comes into play, these rotor hats will vary in thickness from 0.200 to 0.360 of an inch each. It depends on the manufacture of the brake package you are going to utilize. So, to achieve the correct WMS measurement you need to take measurement 3A and add the thickness of one rotor hat multiplied by two . An example of this would be a 56.25″ measurement for 3A (axle flange to axle flange) plus 0.25″x2 (rotor hat thickness of .25″) which will give you a WMS of 56.75 inches.
If you have a desired WMS and your looking for measurement 3A (axle flange to axle flange) then you need to take your desired WMS and subtract the rotor hat thickness multiplied by two. En example of this would be if you wanted a 56.75″ WMS and you have a 0.25″ rotor hat then you will have 56.75-(0.25×2)= 56.25 inches.
When you are ordering your rear end or suspension setup you need to make sure that your suspension manufacture gives you the correct numbers. Far too often we get inaccurate information about rear end widths because manufactures often give out the 3A measurement as the WMS when it clearly is not. When this is incorrectly given out you have the possibly that your wheel / tire combo will not work. I often see cars where if the wheels were placed an 1/8″ to either side they would be rubbing, not acceptable. The other situation you do not want to get into is using wheel spacers, most of the time it is “ok” strength wise***, but more importantly its just tacky. Just be sure that you are getting the correct information from your suspension manufacture, and be sure to specify which numbers they are referring to.
***I’m referring to wheel spacers that are less then 3/16″… anything along the lines of one or two plus inches will be dangerous on the track and highly NOT recommended under ANY circumstances… and again.. its just TACKY! ***
The GMR Full Floater Rear End (and most other manufactures)
With the GMR Full Floater the measuring and complexity of the measurements are simple. This setup does not utilize a rotor hat, so there are no variables to factor in depending on the brake components manufacture. All GMR hubs mount the rotor off the back of the hub body with the use of a rotor adapter.
The wheel mounts to the end of the full floater assembly, directly on the drive plate. This makes the 3A measurement in the diagram above the actual WMS measurement. The other added benefit is that your WMS will never change even if you switch out brake kits / manufactures. When ordering a full floater the only measurement you need to give is the WMS, The GMR or your builder will be able to cut the housing and place everything accordingly without confusion.
The GMR C6-Hybrid Live Axle Rear End
One of the newest rear end options for the pro-touring market is the C6-Hybrid setup. This setup basically mounts a rear corvette hub / brake package to a live axle rear end. Although new, The GMR expects to see this become a replacement for the semi-floater in the years to come. The GMR also has a very specific way of measuring the WMS on this Hybrid rear end setup.
Measurement 3A is the actual WMS measurement but with an exception, we factor this in as the “stock” rotor hat from GM. This is because the setup was designed around stock components, thus why we factor in the stock rotor hat thickness into measurement 3A. When you are NOT going to run a stock brake setup then you need to figure out if your selected brake manufactures kit changes the wheel mount surface. Typically they will either increase it or reduce by about an 1/8″. This number needs to be factored into your 3A measurement.
For example, if you decide to run a big brake kit for the rear of a Z06 that increases the wheel mount surface by 1/8″ per side then you need to factor that into the 3A number. To get the accurate WMS you will have to double the 1/8″ increase and subtract that from your desired wheel mount surface number. When ordering brakes, most manufactures will be able to tell you how much change from stock the package will have.
Remember if you have any questions not answered by the above please contact GMR.