06 - Determining the Proper Width Cradle

How to Determine the Proper Width Cradle

Our Corvette Interface Adapter Kits are all the same - for any application. All you need to choose is the Front Kit, the Rear Kit or Both Kits.

We base all of our width dimensions on the Wheel Mounting Surface to Wheel Mounting Surface measurement. (WMS to WMS)

The different widths are achieved by removing a section from the center of the Corvette lower cradle, then welding it back together and bracing it.

By narrowing the lower cradle to change the WMS to WMS dimension, the only other important dimension that changes is the distance between 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.

If you're planning on using a wheel / tire combination that is different in any way, (height, width or offset) from what you now have - this is the easiest way to determine the WMS to WMS dimension without too much trouble.

I always recommend is that the builders determine what wheel / tire combination that they're going to use and order them before determining the WMS to WMS dimension. Once they arrive, look them over, then get them mounted.

I also 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 or the lips inside 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 on the outside of both tires, then measure from the outside of one tire's bulge to the outside of the other tire's bulge. This is for reference later on.

If the axle and / or control arms have been removed and there is enough room to place the wheel / tire up in the wheel wells, you're in luck. Jack up the car and place them in the proper place, making sure that there is sufficient room at the wheel well lip, as well as the inside of the fender. (1/2" to 3/4" per side.)

Then lower the car to its proper ride height, check all of the clearances again and take the WMS to WMS dimension from the inside of one wheel (where the wheel bolts to the brake rotor) to the inside of the other wheel (where the other wheel bolts to the brake rotor.) That's it!

If there is no way to place the wheels / tires within the wheel wells, you can still get an accurate WMS to WMS dimension without placing the wheels / tires inside the wheel wells.

Take a pair of 'plumb-bobs' on strings and tape them to the inside of the fender and let them 'drape' over the inside of the lip in the wheel well opening.
Make sure that they are about where the highest spot on the wheel well opening is - and about where the axle centerline will be.
Next, measure between the plum-bob lines to get your overall width.
Now, deduct a total of 1" to 1-1/2" (for the additional 1/2" to 3/4" per side clearance that was mentioned earlier.)

At this point, you can take the two 2x4s and place them against the widest part of the tire's bulge on the outside of both tires, then measure from the outside of one tire's bulge to the outside of the other tire's bulge, (just to see if the measurement is close to the one you got earlier.)
Once all is well and the measurements are double-checked, you can measure from wheel to the other wheel (at their wheel mounting pads to get your WMS to WMS dimension.)

If your measurements come up between a couple widths on the charts below, it's always better to go a little too narrow than a little too wide.

As much as I don't like using wheel spacers, a good set of tight-fitting 1/4" to 3/8" spacers

sometimes come in very handy for that final fit. Remember, if you're too wide, the cradle and axles might need to be narrowed a little - and that is too much like work...

Finally, the last step! Look at the charts below and see if your lower cradle

will need to be narrowed - and by how much. That's it!

Below the two charts is a lot of information if you want to clarify the instructions above,

(or have trouble falling asleep)

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 do not include the cradles and are all identical.

The different WMS to WMS measurements are attained by cutting out a portion of the cradle, then welding it back together.

Typically, the cradles may be narrowed to 12 different WMS to WMS widths. (Stock width and 1,2,3,4,5,6,7,8,9,10, 11, and 12-inches narrower.

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 in your project 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.

Confused? I know I am -

but once you get started on the actual car, you'll see that this makes a lot more sense.

Remember, you can always call us and we can figure it out together.