Do I have this right? IRS backspacing measurements

[Humdinger]

Before I start getting the body fitted I figured it would be easier to get the measurements I need to pick a set of wheels and tires. I have a set of wheels picked out and I'm planning on big/little style so here are my selections:

Front:
15x4 with 1.75" backspacing 0 offset

Rear:
15x8 with 4.5" backspacing 0 offset
15x8 with 5.5" backspacing +25mm offset
15x10 with 4.5" backspacing -25mm offset
15x10 with 5.5" backspacing 0 offset

I took some measurements of the places I thought the rubbing would take place first... I'm wanting to tuck as much of a big tire as I can. Here's some pics... (Note: use the measurement on the inside of the square!)

Bump stop topside

Plate under torsion

This one has one end of the square sitting at 7.5 inches in the center of the mounting surface (simulating the distance from hub on a 15" wheel)and the other end measuring the backspacing...

I know the tire will rub before the wheel but am I measuring this right? I buying the wheels and tires new so I really wanna do it right the first time! I get backspace measuring but the offset thrown in confuses me a little bit so I figured I'd ask first!

[jsturtlebuggy]

I ran Flat 4 BRM wheels 15x6.5in with a 4 1/8in back spacing along with a 31x10.50x15 tire on the rear.
Tires was very close to trailing arm bolts. About a fingers width. Never showed any signs of rubbing.
Tires really should be one of the last things to buy, because if project ends up sitting for any length of time you end up buying another set.
Tires will rot just sitting even in cool dark place.

If you plan on using over a 30in tall tire with a lot of back spacing on wheel you are going to have to change out the spring plates to the 1961-1968 short torsion bars and spring plates to keep from rubbing.
_________________
Joseph
Fair Oaks/Orangevale, CA
Elrod Motorsports
Motion Tire Motorsports
Having fun with Dune Buggies since 1970
Into Volkswagens since 1960

[kustoms]

What kind of body do you have? The fenders play a role as well. Zero back spacing on the front will push the tires out past most fenders. Buggy rear fenders are not the same for every buggy type.
_________________
Google this.
"Bread and Circuses"

And wondering what the heck happen to my Samba post count?!?
Tom Thompson
Tijuana Kustoms Inc. (TM)
1-516-338-8746
www.tjkustoms.com

[Saint]

And what brand rear tire were you running? I have a set of 31s and they rub the torsion end.
_________________
PLM.....

[NJ John]

Button head bolts buy you a little space.
_________________
1973 standard, yellow, lowered, 3” narrowed front, 1600 blo-thru turbo w/single dell
11.41 et buggy. Long gone
Let’s go O’s! Let’s go O’s!

[jhoefer]

[mark tucker]

my tires rubed the torsion bar end cap(short bars,long would not work) I cant recall the back spacing 12x15 douglass wheels.I think it was 3.5" back spacing.

[EVfun]

This will in part depend on how big and how little. I ran a pretty aggressive looking package for years, I replaced the tires a couple years ago just because they where old enough to drink.

I ran P235/70R15 rear and P135R15 up front. That is about 4.5 inch difference in tire height and about 4 inches difference in width. My rims are 15x7 with zero offset rear and 15x5 with a +1.5 inch (38mm) offset front. It looked right on my Manx style buggy and the wheels look pretty nice on my Berry Mini-T buggy now. The current tires are P215/70R15 rear and P145/65R15 front. I am running long '67 axles so I have a similar wider rear track width to your IRS frame.

I had the chrome steel wheels made for me in 1990. The wheels where offered by Discount Tire, with about a 2 week wait since they where special order. They are a lot like the white spoke wheels often seen on Baja Bugs, but chrome plated after a stamped center is welded into place with the chosen offset. The front offset is as much as they could do, I initially wanted +2 inches.

If you are not set on a wheel you might see what custom made offerings are still available. I would recommend you concentrate about offset, not backspace, because the wheels could be measured at the lip or at the inside tire seat. Which way they choose will effect backspace, but not offset. I recommend you pick as much positive offset as you can up front so the tire twists more about it axis and swing front and back less. This is just a fender clearance thing as some Manx style buggies don't have a lot of clearance to the front of the front fender. Of course, style may dictate otherwise.

You could put a set of stock wheels and tires on, then measure from them to all points, then measure the stock offset. Make sure to watch the suspension travel too. Real tall rear tires could hit the body on suspension compression and up front you have to watch both where they go when turning and where upward suspension travel will put them when turning. It looks like you are measuring well, you know, measure 20 times and rub none.
_________________

[Humdinger]

Wow that's a lot of responses in such a short time! Lol I'm on my phone so bear with me on answering everything!
"Tucking" was probably a bad way to put it... I would like to have larger rears and have as much as possible under the fender. I don't plan on slamming it, maybe just lower it a little bit. I like to drag race every once in a while so slamming it would only hurt me.
This will strictly be a street only buggy... I have a 4runner for offroad fun so I'm planning on running a 26 or 28 inch tire in back. I still haven't made up my mind on that...
My body is called a Sandman aka Hotrod Buggy... They were made in Texas for a little while and have been made for a long time in the Netherlands. I'm not lowering it a lot so if they stick out a little it's not that big of a deal. I put the body on the other day to see how much I need to shorten the tunnel and the rear fender measured 30 inches from the ground with factory beetle wheels and tires. I'm probably going to shorten it tomorrow so ill take some more measurements when I get the body on.

Here's a shot of the wheels:

[mark tucker]

I have a set of those with new 255-60-15 bfg,s on them, I got them for my bug but never put them on, there just sitten in my laundry room out of the way(out of my way) there way shorter than the 19.5wide x33tallx15 mickey thompson street tires I used to run.then I went down to 16x31.5x15 hoosiers dirtrack tires(block tread is awassom on the street& rain too!!)