What wheel backspacing works with the FFR IRS rear axle set-up?

[narly1]

For the purposes of wheel selection what backspace do I need with the IRS rear axle?

I do have some straight axle / backspace /wheel well clearance data already and want to know if I can just use that.

Thanks,

Earl

[progmgr1]

Earl,

I went with a 4" backspace (apparently I fouled up the measurements), and it wasn't enough. I'm still mulling over what to do about that...

Other builders have recommended everything from 4.5" to 5.75". Based on my experience, I suggest 4.5" is optimum. You'll still have 1"+ clearance between the sidewall and the body, and 10" wheels will still fit inside the full fenders.

Keith HR #894

[narly1]

....and 10" wheels will still fit inside the full fenders.

Keith HR #894

I'm going fenderless so wheel width is not an issue.

Can you confirm that you have the IRS rear axle set-up?

Thanks for sharing your findings,

Earl

[narly1]

A bit more information:

I'm trying to figure out how a wheel with a 7.07" backspacing might work on my Speedstar with the IRS rear axle option.

I know that for a Speedstar with solid rear axle and a wheel with 5.88" backspacing the minimum clearance comes out to about 1".

I'm a little skeptical about using any fitment data from the standard 33HR given the two different body types.

If I had the track widths of the two axles to compare I'd have a much better estimate of the clearance.

Earl

[progmgr1]

Sorry I wasn't clear. Yes, I have the 2015 Mustang IRS in a Gen 1 Hot Rod.

Keith HR #894

[narly1]

Sorry I wasn't clear. Yes, I have the 2015 Mustang IRS in a Gen 1 Hot Rod.

Keith HR #894

Good info, but the unknown is if the Speedstar body is any wider or narrower at the rear.

I have since been told that the straight and IRS rear axle set-ups share the same track width. Just confirming that from a second source now.

So once I know that for sure and taking into consideration what a fellow Speedstar builder's results were using a straight axle, I'll know what backspacing I can use.

Earl

[narly1]

FFR confirms that the rear axle track widths are the same for both versions at 59.25".

Earl

[FF33rod]

Earl,

I went with a 4" backspace (apparently I fouled up the measurements), and it wasn't enough. I'm still mulling over what to do about that...

Other builders have recommended everything from 4.5" to 5.75". Based on my experience, I suggest 4.5" is optimum. You'll still have 1"+ clearance between the sidewall and the body, and 10" wheels will still fit inside the full fenders.

Keith HR #894

It also depends on wheel width and diameter - 4.5 would not have worked with my 315s on 11" wheels (18 diameter), 5.5 is optimum for that IMO...

Steve

[progmgr1]

It's difficult for me to measure the track width right now. However, I did measure the distance between the body mounting points in the back: 41.75". Add twice the fiberglass thickness and ~0.2" to account for the typically used spacers (washers) and you get to the width of the body inside the wheel wells. How does that dimension compare to your Speedstar?

Keith HR #894

[narly1]

How does that dimension compare to your Speedstar?

Keith HR #894

I'm not there yet. All I have to go by is how a known wheel backspacing /width combination worked out on a Speedstar built with a straight axle.

The 33HR and Speedstar both use the same frame, my concern is more about a potential difference in the body widths at the rear wheel well. So I'm leery about using 33HR backspacing / width data to pull the trigger.

Given the luxury of time I'd just wait until I was at the point where I could measure off the car but in this case I can't. The wheels that I really like may be discontinued soon and I don't want to miss out. LOL.

Earl

[progmgr1]

You can always go with a larger backspace and use wheel spacers if it is too much...

Keith HR #894

[narly1]

That's where I'm at Keith. The rear wheels I'm looking at will need about a 1" spacer to clear the body.

Earl

[RoadRacer]

I have since been told that the straight and IRS rear axle set-ups share the same track width. Just confirming that from a second source now.

Perhaps I’m misunderstanding but there are a few different live axle widths available - I deliberately went for the widest rear to make wheel choice easier. So don’t use assume anyones live axle wheel choices will work for an IRS

mustang rear widths.jpeg

[narly1]

Perhaps I’m misunderstanding but there are a few different live axle widths available - I deliberately went for the widest rear to make wheel choice easier. So don’t use assume anyones live axle wheel choices will work for an IRS

mustang rear widths.jpeg

Jeez, now you have me scared.

I guess I'll hold off until everything's together and I can measure for myself to be 100% certain.

Earl