I'm collecting parts to do the large bolt pattern disc brake conversion on my 68 Dart. When using the 73 spindles, UCA's , and LBJ's does this increase the track width over the stock 68 parts? If so, by how much?
Disc brake conversion question
- Thread starter 440 Mike
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71scamp78
Well-Known Member
I don't believe it changes any of the geometry
I have heard that it changes the track, yes, but not sure how much. Not a huge amount, at any rate.
Yes, using the BBP spindles and rotors widens the track width. I believe the difference is close to 3/8", so it can be important if your wheels/tires are already close to the fenders.
It's per side. According to this TSB from the factory the track width change was more than that, it lists the "typical" track with for 67-72 as 57.74 and the track with for 73+ as 59.15". That's a total change of 1.4", so .7" per side. The problem is that the track width also includes the backspacing on the wheel and the tire width, not to mention the brakes. Cars with KH disks, 9" drums, or 10" drums may have all been a little different. In the 1971 dealership book the Demon is listed as being different than the Dart by .1", and there's really no reason for that since the suspension is the same. Different wheel package? Anyway, the wheels and tire packages changed between 67-72 and 73+ as well, so, just going from the track width numbers doesn't necessarily tell you the whole story. Even a 1/4" change in backspacing gives a 1/2" difference in the track width. And really, it's kind of a moot point since you can't run the same rims anyway because of the SBP to BBP change.
Anyway, that was really confusing. I can tell you what will fit for rims and tires on a BBP combo. I actually tracked a bunch of combo's from the wheel and tire thread, and I run 275's on the front of my Duster, so I have pretty good idea of what the limits are. What size wheels and tires do you want to run?
Anyway, that was really confusing. I can tell you what will fit for rims and tires on a BBP combo. I actually tracked a bunch of combo's from the wheel and tire thread, and I run 275's on the front of my Duster, so I have pretty good idea of what the limits are. What size wheels and tires do you want to run?
A 215/70/15 is 26.9" tall. Usually anything around 26" tall or more starts to cause interference at the front lower corner of the fender. A 215/65/15 would the tallest I would suggest. I had to max out the adjustment on the lower fender to bumper brace on my car to push out the lower corner of the fender when I was running 225/60/15's on 15x7's with 4.25" of backspace, and they're only 25.6" tall. The factory front tires for these cars were only between 24 and 25.3" tall depending on the options.
For backspace, with a 6" wide rim as long as you've got 3.75" of backspace it should work, 4" would be better to improve the clearance at the front lower corner. You can run a lot more than that too, but you probably won't find a 15x6" with much more backspacing than that anyway. Max backspacing in the front for a 15" rim of any width is in the neighborhood of 4.75", maybe a little less. Any more than that and the rim hits the upper ball joint, brakes, etc.
For backspace, with a 6" wide rim as long as you've got 3.75" of backspace it should work, 4" would be better to improve the clearance at the front lower corner. You can run a lot more than that too, but you probably won't find a 15x6" with much more backspacing than that anyway. Max backspacing in the front for a 15" rim of any width is in the neighborhood of 4.75", maybe a little less. Any more than that and the rim hits the upper ball joint, brakes, etc.
ir3333
Well-Known Member
yes..with stock wheels they will stick out about 3/4" and odd as it sounds
it is noticeable.You will lose turning radius as well and i have cut 3/16" or so
off the ball joint bump stops to help a little.
it is noticeable.You will lose turning radius as well and i have cut 3/16" or so
off the ball joint bump stops to help a little.
I've switched a couple cars now and I don't think the actual change is anything close to 3/4". At least it wasn't on my GT, but that may have something to do with the brakes it had. The problem is that the diagram doesn't explain what the "typical" track width actually represents as far as brake and wheel packages, because both of those are tied into how the track width is measured. And if you look at the specs for individual cars prior to 1973, none of them are actually listed in the individual factory specs as having a 57.74" track width, so, I'm not exactly sure where that number came from other than the TSB. The '71 Dart is actually listed at 57.4", as I mentioned above, with a '71 Demon being measured at 57.5". There's no reason for that difference as far as the suspension is concerned, so, I wouldn't use the specs alone as gospel. And for each year only one track width is listed, even though it likely was different between the different brake packages. Based on what I've seen as far as wheel and tire combo's on SBP vs BBP cars, I think the difference is usually closer to 3/8", maybe a 1/2" at the most.
As for the turning radius, widening the track width actually makes room for a tighter turning radius. If the same diameter tire is now further away from the frame, it can be angled further before it hits anything. The difference there is the steering stops on the lower ball joints. But, having measured some of those I've found that the size of the steering stops is actually pretty different depending on the manufacturer of the lower ball joint. So, any change in the turning radius has more to do with the manufacturer of the lower ball joint, not the conversion or the change in track width itself.
As for the turning radius, widening the track width actually makes room for a tighter turning radius. If the same diameter tire is now further away from the frame, it can be angled further before it hits anything. The difference there is the steering stops on the lower ball joints. But, having measured some of those I've found that the size of the steering stops is actually pretty different depending on the manufacturer of the lower ball joint. So, any change in the turning radius has more to do with the manufacturer of the lower ball joint, not the conversion or the change in track width itself.
Much thanks to 72bluNblu and all who replied.
I feel pretty confident now that a 6 inch wheel with 4" backspace will place the tire where I want it. I just need to decide if I'm going to trim the front fender at the leading edge to fit a 27" diameter tire.
I feel pretty confident now that a 6 inch wheel with 4" backspace will place the tire where I want it. I just need to decide if I'm going to trim the front fender at the leading edge to fit a 27" diameter tire.
Plumcrazyreindeer
Skintback back and haulin a$$
Hey FABO fellows. This seems like a place to ask the question I need answered. I have a 70 duster with SBP front brakes. Does anyone sell a decent kit to swap to BBP?
There are a few kits that are “decent”, it depends on what you’re looking for and what your intended use is.
Personally, I think the best kit for that comes from DoctorDiff. He sells kits that include everything and use the later 73+ Mopar disk design, so you’re using factory designed reproduction parts. You can pick up replacements for pretty much all the components at most parts stores. And pretty much all stock parts, so, no real aftermarket fit issues. They're not the cheapest, but for a street car I think the simplicity of using later factory parts far exceeds any minor performance upgrade from an aftermarket brake. Especially if you're just running normal-ish sized BFG TA's on 14 or 15" rims.
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