12 - How to Determine the Proper Width

How to Determine the Proper Width Corvette Interface Kit

The main concern in ordering a Corvette Interface Adapter Kit is to make sure that you've calculated the proper width Cradle for your project.

The best way to do this is to physically measure from one Wheel Mounting Surface (WMS) to the other on the same axle.

Our kits come in 13 different widths and they're all based on this WMS to WMS dimension.

The kits are narrowed by removing a section of the Corvette lower cradle, then bracing it and welding it back together.

By narrowing the cradle to change the WMS to WMS dimension, the only other dimensions that change are the widths of the frame rails and all of the suspension geometry remains intact.

Of course, with any change in the width, the half-shafts (axle shafts) will also have to be shortened, which will result in a slight geometric change in the axles, which will easily be handled by the CV joints – and will not effect the control arm geometry.

I always recommend that before removing the current rear wheels, take a look from behind the vehicle and see if the placement of the tires looks right to you. Don't just look at the tread, but look at the bulges on the sides of the tires also.

Are the tires too far inset? Do they stick out too far? Will they hit the fenders if the vehicle bounces up and down? Are they in or out about the same as the front tires?

If they look good to you, then take a couple 2x4s and place them against the widest part of the tire's bulge and measure from the outside of one tire's bulge to the outside bulge of the other tire.

Now jack up the vehicle, slide in a couple jack stands and remove the tires.

With the tires off of the vehicle, measure the rear axle from side to side at the Wheel Mounting Surfaces. This is the dimension you will need to order the kit.

To double-check your dimensions you can stand your tires up and move them apart until the wheels' mounting pads are the same width as your axle's WMS to WMS.

Then once again stand the 2x4s up against the widest part of the tire's bulge, and measure across to the widest part of the other tire's bulge and you should come up with the same measurement as you did when the tires were still mounted on the axle.

Below is a chart to show the corresponding frame widths for the 13 different WMS to WMS dimensions of the kits offered by Dobbertin Performance.

The top chart is for the REAR KITS

The bottom chart is for the FRONT KITS

You may notice that many websites refer to the 'Track' of a vehicle the

same way as they do the WMS to WMS dimension.

This is completely wrong!

The 'Track' is actually the dimension from the centerline of one wheel/tire to the centerline of the other wheel/tire on the same axle.

Never use the 'Track' of a vehicle to determine the WMS to WMS dimension of an axle.

The diagram below explains the difference:

Since the Track is measured from the wheel/tire centerline, (Wheel C/L) any positive or negative offset of the wheels will effect the track width, but the WMS to WMS dimension will stay constant.

(We're showing axles with drum brakes for simplicity.)

The Negative Offset Wheel moves the wheels out, and farther away from each other, therefore increasing the Track Width - and decreasing the Backspace. These wheels have more outside 'dish' on them.

The Zero Offset Wheel is the only instance where the Track and WMS to WMS dimension the same – but hardly any wheels have zero offset.

The Positive Offset Wheel moves the wheel farther onto the axle, and closer together, therefore decreasing the Track Width - and increasing the Backspace. These wheels have less outside 'dish' on them.

Keep in mind that the Corvette Interface kits may be ordered in 13 different WMS to WMS widths. If you intend to use the same wheel/tire combination after the conversion, then order a kit with the WMS to WMS dimension as close as the one you have now.

If your project vehicle is rather wide and you can get by with a 66.75" WMS to WMS, then you can use the standard width C5 or C6 cradle as well as the stock length axles.

However, if you will be changing your current wheel/tire combination, and you want to go with a wider set – and keep the outside dimensions the same – there are many ways to accomplish this. You can juggle the wheel widths, offsets and WMS to WMS dimensions to come up with the perfect wheel / tire combination for your vehicle.

In the illustration below the WMS to WMS dimensions and Wheel Offsets are different, but their combinations all add up to make the outside to outside wheel dimension the same.

The top axle shows a pair of wheels with Negative Offsets, (and short Backspaces) which yields more of a 'Dish' on the outside of the wheels. The WMS to WMS dimension has to be shorter to make up for the wheels' Negative Offsets.

The middle axle shows a pair of wheels with a little Positive Offset, (and longer Backspaces) which yields less of a 'Dish' on the outside of the wheels. The WMS to WMS dimension has to be longer to make up for the wheels' Positive Offsets.

The bottom axle shows a pair of wheels with a lot of Positive Offset, (and even longer Backspaces) which yields no 'Dish' on the outside of the wheel. The WMS to WMS dimension has to be even larger to make up for the wheels' Positive Offsets. This is the most common configuration for Independent Suspensions, as the stock Corvette WMS to WMS dimension is 66.75”, which is quite wide.

Corvette wheels, like the ones pictured at the bottom of the diagram above, are a natural choice for the Corvette Interface Adapter Kits and have plenty of positive offset. (A lot of backspacing.) This wheel is sold as an 18” x 9.5” with a 56mm Positive Offset (it actually 10.5-inches wide) with a 7.45” backspace, yielding only 3.05” of frontspace.

OK... So where does that place the Mounting Pad? (The Mounting Pad is the flat surface on the back of the wheel, where it bolts to the axle.)

It's rather confusing – and here's why:

A wheel advertised as a 18” x 9.5” actually measures about 19” x 10.5”. This is because the advertised measurements are taken from the Tire Mounting Surfaces – and not from its overall height and width.
Then there's the fact that wheels are measured in inches, and most Offsets are measured in millimeters.
To figure out where the Mounting Pad on the back of the wheel is, first convert the actual width from inches to millimeters. This means the 10.5” wheel is 266mm and the midpoint is 133mm. (1” = 25.4mm)

Now add the 56mm Offset to the 133mm and you'll find that the Mounting Pad is 189mm from the rear of the wheel - or 7.45". This is the Backspace.

Next, subtract the 56mm Offset from the 133mm and you'll find that the mounting pad is 77mm from the front of the wheel – or 3.05”. This is the Frontspace.
Now add the 7.45” and the 3.05” and you come up with 10.5”, which is the actual width of the wheel.
I've always liked using the Backspacing dimension. Backspacing is simple and in inches – and is the actual measurement from the back of the wheel to the Mounting Pad. So there is no messing around with converting the millimeters to inches, or finding the midpoint and adding or subtracting the Offsets. Just take the actual width of the wheel, subtract the Backspacing and you'll have the Frontspacing as well.
An easy way to remember which is the Negative and Positive Offsets is to think about installing the wheel on the axle. When the wheel first slides onto the axle it's in the Negative Offset section, then as it goes on farther, it passes by the Zero Offset position – and heads into the realm of Positive Offsets.

Now here's an example for a 1970 Chevelle:

A 1970 Chevelle's stock axle WMS to WMS width is 62.5”. That's a full 4.25” less than the width of the standard Corvette Interface Adapter Kit mounted on a full width cradle (66.75”) – so we'll look at four options to keep the outside of the rear wheels/tires very close to the same width.

Use the Adapter Kit with the standard width cradle, (66.75”) and choose wheels with the same width and 2.0” to 2.25” additional backspace, (more positive offsets.) This keeps the wheels the same distance apart and gives a little more room to install the independent rear suspension.

Use the Adapter Kit with a 2" narrower cradle, (64.75") and choose wheels with the same width and 1.0" to 1.25" additional backspace (more positive offsets)

Use the Adapter Kit with a 4” narrower cradle, (62.75”) and choose wheels with the same width and 0.0" to .25” additional backspace.
Use the Adapter Kit with a 6” narrower cradle, (60.75”) and choose wheels with the same width and .75" to 1.0" less backspace than the original wheels.
Of course, if you want wider wheels – simply add more width to the inside (more backspace) to the wheel and it won't effect the outside width of the tires at all.
Now, go and take something for your headache.

A few alternatives for measuring:

A simple but effective way to come up with your preliminary calculations is to make a pair of cardboard mock-ups of your wheel/tire cross-section – before buying the wheels and tires. Place them on the ground with the proper outside tire bulge to tire bulge dimension and you'll get a pretty accurate measurement of the WMS to WMS dimension from the mounting pads on the backside of the wheels.
Ordering a kit can be done with math – but I don't recommend it. It's really easy to skip a step or miscalculate something – then there's always a chance that the manufacturer's information is off.
Another way to minimize a mistake is consult a professional who sells wheels and tires for a living. There are several national high performance and specialty chains, such as Summit, Jegs and Tire Rack that work with these figures all day long – and they can most likely suggest the proper offsets, widths and diameters once you give them all of your information.
Once you have received the wheels and tires but before you mount them, double-check everything, because very few sellers will allow wheels or tire to be returned once they have been mounted.

Auto Salvage yards are still a great place to find the proper dimensions, as are car shows. After talking to the vehicle's owner, take a pair of yardsticks and hold them up against the outside of the tire bulges from side to side.

Next, measure from the yardstick (still on the tire bulge on one side) to the WMS inside the wheel on that same side. Then subtract double that amount, (for two wheels.) That should give you an accurate WMS to WMS dimension.

The chart below can give you an idea of the WMS to WMS dimensions on some popular US-built vehicles. I searched the Internet for these dimensions and only added them to the list if I saw the same dimension on three or more sites.

I still wouldn't trust these dimensions enough to order a kit or cradle though - because there's a good chance that some of them are based on Track Width instead of the WMS to WMS dimension.

Frame Design and Fabrication - Instructions