[Johnny Kay]

Working to update the rear tires and stance of my 96'SS.
I am widening the stock 17"x8.5 to 17"x10.5" so I can safely install 315's
Currently running Mickey Thompson Street Comps all the way around on stock wheels - MTT-6275 - 275/40R17 - Mickey Thompson 6275 Mickey Thompson Street Comp Tires | Summit Racing
Updating the rear to MTT-6278 - 315/35R17 - Mickey Thompson 6278 Mickey Thompson Street Comp Tires | Summit Racing
From my measurements (see attached sketch) it should be close if I add a 1/2" spacer to the drivers side - Wheel Adapters, Wheel Spacers, Hub Rings for your car! | Motorsport Tech
If not I will notch and box the frame.
My question is . . . do my frame to fender measurement look representative to other 96'SS?
We all know nothing is square back there and cars are typically offset in some way . . . just looking to hear if my car is within the norm in comparison to any of yours.
Attaching pictures of my notes and my not to scale drawing for comment.
Thanks in advance for the input.
PS - in the process of updating the pair of stock wheels to 17"x10.5" rear wheels. If they turn out and test well I may run six 17"x10.5" wheels, refinish so they look factory new and sell 3 pairs to help offset my costs - as you can imaging this is not a cheap process and in retrospect it might have been better to order custom wheels that look like our stock.
Direct message me if you might be interested in updated stock 17"x10.5" alloy at [email protected]

 

[91ss]

Don't forget the 6mm offset.

 

[jayoldschool]

My SS is much tighter to the lip on the right side than the left. Always assumed it was the body offset, but will measure the rotor to frame when I put the big wheels on soon.

 

[nsaness]

I can relate to any person who sketches and uses graph paper
I think your calculations are darn close.

Although I have a 94; I did similar, but not the same:
I installed aftermarket upper and lower control arms. I had Weldcraft widen my stock wheels to 11” wide. For some unknown reason, my axle is pretty well centered to the frame and to the body, so I am using a 1-1/16” spacer on both sides of the rear.
With that set up, the tires are about 5/8” away from both frame rails.
With that large spacer, you can see the tires are still within the fender. At worst case, you may want a little more spacer. The key seems to be that the smaller diameter of a 315-35-17 tire is forgiving as the fender shape tucks back in toward the bottom of the car. If someone increases the outside diameter of the tire, I assume that fender rub could become an issue with the larger wheel spacers.

 

[Johnny Kay]

Thank you!!! Exactly the feedback, pictures and ultimately the confirmation I was looking for. Can you share the details on your spacers please - design, brand, part number? I totally agree with your assessment on using the 25.7 shorter tire. I mocked up the 305 at 27.8” tall and didn’t think I could make it work without a getting into the fender or notching the frame. Thanks again.

 

[Fred Kiehl]

Before you start using offset spacers, get the body and axle centered on the frame. Once you have that, you have a symetrical starting point to adjust the wheel/tire spacing. Having offset spacing can cause some strange handling characteristics.

 

[Johnny Kay]

Thanks Fred.
Great suggestion.
Have you done this before on a B body and if so any lessons learned you might share?

 

[Fred Kiehl]

I have not straightened the body, but it is not that difficult. Loosen the mounting bolts, pry the body to where it is even distances to the frame on both sides, and tighten the bolts. The rear is probably pretty close, but if you want to make it even, and it is not, that would require adjustable upper arms (center the axle), or possibly a Panhard bar. The arm mounts may be off a little side to side, or the arms may be bent/different lengths. Measure everything, and measure them diagonally for square. The lower arms do not locate the axle side to side.

From your measurements (if they are of from the rear), the body is off by 1/8 inch to the left (move right), and the axle is off by 3/8 to the right (move left).

The tire is 10.8 at the tread, and somewhat wider at mid sidewall. The tread is not always perfectly proportioned around its circumference, and different tires of the same size can vary as well. Depending on the accuracy of your measurements the offsets of the body, frame and axle may be slightly different. Measure the body to frame without the wheels/tires on the car, and measure the distance from the wheel mounting flange to the frame for accurate placement. Put a straight edge on the flange, and measure from the plane of the hub to the frame to be accurate. If you want to do it right, you need to start with a symetrical car.

 

[jayoldschool]

How do you shift the axle to the left?

 

[96 Black]

I can relate to any person who sketches and uses graph paper
I think your calculations are darn close.

Although I have a 94; I did similar, but not the same:
I installed aftermarket upper and lower control arms. I had Weldcraft widen my stock wheels to 11” wide. For some unknown reason, my axle is pretty well centered to the frame and to the body, so I am using a 1-1/16” spacer on both sides of the rear.
With that set up, the tires are about 5/8” away from both frame rails.
With that large spacer, you can see the tires are still within the fender. At worst case, you may want a little more spacer. The key seems to be that the smaller diameter of a 315-35-17 tire is forgiving as the fender shape tucks back in toward the bottom of the car. If someone increases the outside diameter of the tire, I assume that fender rub could become an issue with the larger wheel spacers.

View attachment 204471

Yah graph paper FTW

[single pic above kept and used for reference]

For another reference, I had Weldcraft widen my stock wheels to 10", and with a pretty well centered axle and no spacers there's a fat 1/2" clear between rubber and frame at rest. Plenty clear at the lip and just a v. small shiny spot on one side inner well from the most aggressive turns. But my sig susp. setup likely limits twisting what might be otherwise. So, just remarking this pretty much matches up with your result using ~1" spacer with 1" wider wheel. ✔✔

As mentioned, the sidewall bulge of whatever particular tire gets chosen is as much a factor of final clearances as anything.

@nsaness,
My Vredestien 315/35-17s have been quite good overall - traction, quiet, wear... but they're NLA in that size, and over 15 years old so really need replacing. What are yours and do you recommend them? They look like NITTO 555 G2. Did you consider their 555RII or NT05, along with other brands in the mix as well?

 

[1slow96]

Mine were widened to 10" ,but 10.5" would have fit with 315/35 Mickey Thompsons.
Replacing wheel studs with longer versions from ARP will give you the freedom to add
spacers to one ,or both sides for aesthetics ,etc. I have used up to a 3/4" spacer on driver's
side. With a 5/8" on passenger side as the housing end is about 3/16" further out on this side.

 

[SSandman]

I have not straightened the body, but it is not that difficult. Loosen the mounting bolts, pry the body to where it is even distances to the frame on both sides, and tighten the bolts. The rear is probably pretty close, but if you want to make it even, and it is not, that would require adjustable upper arms (center the axle), or possibly a Panhard bar. The arm mounts may be off a little side to side, or the arms may be bent/different lengths. Measure everything, and measure them diagonally for square. The lower arms do not locate the axle side to side.

From your measurements (if they are of from the rear), the body is off by 1/8 inch to the left (move right), and the axle is off by 3/8 to the right (move left).

The tire is 10.8 at the tread, and somewhat wider at mid sidewall. The tread is not always perfectly proportioned around its circumference, and different tires of the same size can vary as well. Depending on the accuracy of your measurements the offsets of the body, frame and axle may be slightly different. Measure the body to frame without the wheels/tires on the car, and measure the distance from the wheel mounting flange to the frame for accurate placement. Put a straight edge on the flange, and measure from the plane of the hub to the frame to be accurate. If you want to do it right, you need to start with a symetrical car.

Just about every impala I have seen is like this, but I don't think it's the body. I had my chassis out, laser tram'd it before putting it back in, and centered the body as perfectly as could. I have adjustable control arms and thrust angle is spot on. I ended up having to run a thicker spacer on one side with my 345/40/17s but it only required 2.5MM difference in spacer widths IIRC to almost perfectly center that side in the wheel well. I don't know any adverse handling characteristics with the 2.5mm change, but I also don't take turns hard with this wheel and tire combo since it's my drag radial setup.

The average B body I have seen is usually much worse than 2.5mm though usually more like 1/4 inch difference side to side, so it's definitely worth attempting to center everything up as much as possible and correcting the control arm mounting holes if your car had the incorrect hole locations mentioned in the TSB years ago. There usually isn't much room for the body to move in the rear though, the pinch molding of the body ends up pretty close to the chassis on either side. Still, if my car is any indication, it seems like even after taking all the efforts to center things out, you will still be left with a slight offset. 2.5mm doesn't sound like much, but it is pretty visible when your wheels are right out to the edge of the wheel well and could be the difference between rubbing and not rubbing during normal use

I still would love to know what causes this, if it's the way the body is built or if it's where the control arm mounting points are located on the rear differential and chassis. I wish I went crazy with taking measurements when I had the chassis out but not really motivated enough to do it now on my back on jack stands LOL

 

[1slow96]

The drivetrain is not perfectly centered to begin with. Design requires more room on driver's side
for steering ,brakes ,etc. Not sure if opposite is true on cars made right hand drive. Handling seems
improved as rear track widens. Front end geometry ,alignment ,scrub radius ,etc. can make a huge
difference. Any diagrams that include rear do so with intersection points ,angle(s) to an imaginary
straight line representing rear end housing. Hard to see how moving either/both rear tires outward
could introduce negative handling characteristics. Nor have I been able to find any published research
that indicates this.

 

[nsaness]

Thank you!!! Exactly the feedback, pictures and ultimately the confirmation I was looking for. Can you share the details on your spacers please - design, brand, part number? I totally agree with your assessment on using the 25.7 shorter tire. I mocked up the 305 at 27.8” tall and didn’t think I could make it work without a getting into the fender or notching the frame. Thanks again.

These are the spacer/adapters I used:

https://www.motorsport-tech.com/4DCGI/updateprice_car

I liked that they had a centering hub.

 

[1slow96]

An example of a 3/4" spacer. Though shown on left front in this photo.

 

[nsaness]

Yah graph paper FTW

[single pic above kept and used for reference]

For another reference, I had Weldcraft widen my stock wheels to 10", and with a pretty well centered axle and no spacers there's a fat 1/2" clear between rubber and frame at rest. Plenty clear at the lip and just a v. small shiny spot on one side inner well from the most aggressive turns. But my sig susp. setup likely limits twisting what might be otherwise. So, just remarking this pretty much matches up with your result using ~1" spacer with 1" wider wheel. ✔✔

As mentioned, the sidewall bulge of whatever particular tire gets chosen is as much a factor of final clearances as anything.

@nsaness,
My Vredestien 315/35-17s have been quite good overall - traction, quiet, wear... but they're NLA in that size, and over 15 years old so really need replacing. What are yours and do you recommend them? They look like NITTO 555 G2. Did you consider their 555RII or NT05, along with other brands in the mix as well?

You are correct, I bought the Nitto 555 G2 as a 315 place holder. I wanted to work on fitting a 335 (and this is why I widened to 11"), so I bought a reasonably priced 315 until I could get the car running. Truth be told, 335's have even fewer options than the 315's so I may be forced to stay at 315...
I don't care about tire wear at all, but I am looking for dry and wet traction. I thought the 555 G2's would have less grip than some of the other options, but it would be nice to have a set of true "3 season tires" for road trips, shows, and other non-drag racing activities. I will buy drag radials as well, (as I hope to have a car fast enough to gain an advantage from sticky tires).
In any case, I would love to comment on the 555 G2's, but so far, my particular Nitto's have only needed to maintain grip on the back of a flatbed 🤢 I hope to be able to provide feedback in the coming weeks/months....

 

[96 Black]

An example of a 3/4" spacer. Though shown on left front in this photo.
View attachment 204476

I note your road course work in your sig. But enthused to hear (and see?) what wheel/tire fitment compels that much spacer on the front?

 

[1slow96]

Thicker spacers allow tighter turns w/o hitting sway bar. Currently have a 5/8" spacer on driver's side. With 5/16" on passenger side. Tire/wheels are equal distant in relationship to wheelhouse. Wider wheelbase seem to offer more stability, but have not studied those effects nearly as thoroughly as the measurements themselves.

 

[Fred Kiehl]

A wider track will always make the car feel better in a corner. The other side of that is where the centerline of the tire is. If the centerline of the tire is outboard of the original track, it adds substantial tension load to the steering linkage. This will cause greater wear on the steering parts. If you want to widen your track 1/2 inch on each side, put drop spindles on it. It will automatically be 1/2 inch wider on each side. If you are now using spacers, you can remove them (or most of them) and keep the same track you have. The steering linkage will be more highly stressed with stock tires and wheels, so it will not change with the combination you currently have. If you want to go back to the "stock" offset with the drop spindles you need a 18 mm offset rim. If you upset the side to side "symmetry" you can have some variations in handling from side to side. You may be able to compensate in real time by adjusting your speed, but it will make a difference. It affects spring rate and shock damping.