Front wheel backspacing w/ disc brake conversion

[PumiceMan]

I've searched the best I can in the FAQs and in this forum, but haven't quite found what I was looking for.

I've got a 69 GMC 1/2 ton longbed with manual drums all around, and 6 lug 15" wheels (I'm assuming they're 15x7?). I'm currently planning on buying a 6 lug front disc brake conversion kit with 2 1/2" drop spindles (keeping the stock height springs). I was told that my stock wheels will not fit, because they have too much backspacing (~4 1/2" on the stock wheels), and that I'd need to go to something with no more than 3 1/2" of backspacing. Is this only true for 15" wheels? Or only for stock wheels? Where does the interference occur?

If I understand correctly, the disc conversion moves the wheel mounting surfaces towards the outside of the truck (wider track). If I decrease backspacing too, they move farther still, especially if going to an 8" wheel. I was curious what kind of wheel sizes / backspacing combinations are on your front disc setups. If I am going to have to buy new wheels, I want to have some idea of backspacing required so I know what my options are. If I get aftermarket wheels, say 15-17 inches and 7 or 8 inches wide, what are my backspacing options? Likely manufacturers would be Cragar or Wheel Vintiques (classic looks).


Thanks for any info you can provide!

[jimmydean]

Not sure who said that, but my '69 has the stock 15" Ralleys and I put on a 6 lug disc with drop spindle on just fine.

The trick is there are stock rims that work and some that don't. It's not the amount of back space as much as the shape of the rim on the back side.

Image borrowed from GMC Pauls:

[PumiceMan]

I appreciate the reply, jimmydean. I spoke with someone at CPP Friday afternoon - that's who told me the stock wheel backspacing was too much, and that I needed a max of 3 1/2". After reading some of the posts today regarding their tech advice, I guess I shouldn't be surprised. Most advertisements for these kits have the line that says "may not work with factory drum brake wheels" (kinda vague) but GMCPauls is the first I've seen that describes what they mean.

Do you happen to know what the backspacing is on your stock rallys? Are they 7 or 8 inches wide? Just collecting all the info I can.

If anyone else has anything they want to add, please do. I'm still interested in getting wheel size & backspacing numbers on your drop front disk conversions. Pictures would be great too!

[cdowns]

RALLY WHEELS ARE NOT STOCK ON THESE TRUCKS rally wheels weren't available on these trucks(after 73 i think) so all the trucks you see with ralley wheels are later model disc brake type wheels// i ran dropped spindles on my 71 15X8 with 41/2 backspace with no problems they were rally wheels 5lug from wheel vintiques

[jimmydean]

So what wheels came stock? My truck was a forestry truck that the PO bought at auction and sold to me. I know the center caps on my truck are not stock, but I was told the wheels were.

[PumiceMan]

Thanks for chiming in, cdowns - that's just the information I was after...
Seems to me that with a 15x8 inch wheel up front, and the disc brake conversion, that 4 1/2 in BS would be about what you want to keep things looking good. What I hadn't gotten out of tech support was if a 15in wheel would clear everything (provided the wheel has the right shape inside).

You got pics? I'd love to see what it looks like...

If the weather wasn't so crappy, I'd go out and pull a wheel off to see what the inside looked like, but since I'm fairly sure it's a stocker (it's not a rally), it probably wouldn't work. I'm assuming there's no difference in clearance issues between a 5 lug & 6 lug setup, right? I'd expect all the parts to be the same...

Thanks again for your help.

[Longhorn Man]

stock wheels were all plain steel wheels. no rallys, nothing special, just steel booring ass wheels.

[powell72]

A factory 6-lug rally will clear your disc setup, rally's came on '73 and up trucks that were all discs. Your front track width will increase with disc spindles, and even more with drop spindles. You can see on my truck were the tires are sticking out further in the front with drop spindles, this is essentially what yours would look like with the same size wheels all the way around.



I solved the problem on my truck by using 6.5'' Van rally's on the front, most of the width is removed from the front, less offset.




This isn't something you can do since van rally's are all 5-lug, but you get the idea what I mean, I hope this helps and isn't too long winded.

[PumiceMan]

Drop disc spindles and stock height disc spindles result in different track widths? I wouldn't have guessed that. The reason I've asked so much about what will work is that I'm still deciding whether to order a 6 lug conversion (possibly cheaper in the long run, especially if I can use my stock wheels) or a 5 lug kit (if I end up having to buy new wheels anyway). If I go 5 lug though, I'll have to do something about the rearend (new axles, rearend swap, or get the stock axles machined).

Like the stance on your truck, powell72. That's basically what I'm after on mine.

What I'm leanin towards is just gettin the 6 lug kit, then I got wheels that I can try and see exactly how they fit, and see where adjustments need to be made (if any). Looking at them in the garage earlier, then seem closer to the picture on the right (the GMC Pauls pic in jimmydean's post).

I'm sure this is a topic that comes up often, and you all get tired of answering the same questions. Thanks for your patience!

[Fred T]

Here's a side shot of my 69. I put in the CPP drop spindles and 3" springs on the rear. Truck had 6 lug rallies when I bought it.

Wheels are 15x8, just under 4" backspacing. Truck has P25570R15 tires, which rub on the offset in the front fender well when going over a large bump. A slightly smaller tire will not have that problem.

[powell72]

If you're thinking about aftermarket wheels I agree with you that the cheapest way would be the 6-lug kit, fit a set of wheels, take measurements and order accordingly. Thanks for the compliment, and your question is a good one that will help others so don't worry about repeats.

[PumiceMan]

Thanks for the vote of confidence, powell72. Could you tell me how much the rear of your truck is lowered (and is it spring or leaf)?

Fred T, your truck looks like it's got leafs in the back from the picture. You really only have 3" drop springs? It looks like it sits pretty level. I had figured with the 2 1/2" drop spindles from CPP that you'd need at least 4" in back. You using the same size tires and wheels all around, right? Maybe I should start a poll and see what other people are using. I was also curious about your experience with CPP (since I was looking at the basically the same kit). I don't want to get the thread off subject, but if you've got any feedback you'd like to give (good or bad) maybe you could PM me. I'd appreciate it!

This site rocks!

[PumiceMan]

Just saw the 4/6 in your signature line powell - ignore my first question!

[Fred T]

Coils on the back. Yes, it is a 3" drop with the light-duty springs. Same tires/wheels all around.

As far as CPP, I bought through a local vendor, www.bowtiebits.com . I had two complaints with CPP. They take forever to ship. 3 days minimum to process an order. It ended up taking 2 weeks to get the parts. My other problem was with the calipers. Someone used a dull cutter for the countersink in the bleeder holes, and the bleeders wouldn't seat. They sent another pair of calipers, but it was another 2 week wait.

[powell72]

My truck is coil rear Pumice.

[PumiceMan]

Thanks for all the info, guys. I was thinking of getting the CPP stuff through Bowtie Bits also (no shipping costs!). Guess I'll do some more research with ECE, GMC Pauls, and others before I commit. Thanks again!