Experience with bump stop spacers/shims/packers?

So after getting back from the Challenge I found this fender rub damage:

It wore the inner shoulders of the front tires down enough to expose cords. The only rubbing I'd ever noticed while driving was at full lock when turning in parking lots since installing the Porsche GT3 brake cooling scoops in the front. I think what's happening here is rubbing from 2 different causes, some from steering angle which I've circled in yellow, and some which I think is from compression travel which I've circled in red. I suspect the travel-related damage happened on the way to the Challenge, specifically some awful stuff I hit on the highways around Windsor, ON and in Kentucky.

To address the lock angle issue before storing the car for winter, I installed the Verus rack limiter kit with the minimum 1/3rd shim setup for a hint less lock angle, and trimmed the GT3 scoops where it looked like they were contacting the tires.

To address the travel issue I've been thinking about adding bump stop spacer shims (sometimes called packers) to the shocks, these are for the type of bump stop that runs on the shock piston. There are two types available, aluminum types like this:

And plastic types like this:

The aluminum type looks like it would be less likely to pop out of place but the plastic type should be less likely to cause any wear to the shock piston. If I assume the stock bump stop setup would prevent the stock tires from hitting anything, I should add spacers equal to the difference in radius between the stock and current wheels, which is a hefty half-inch. Then I've also got camber plates which will cause the wheel to move closer to the shock tower as it travels upward...and camber bolts which move the top of the tire closer to the shock tower at all times.

Any experience with these? Will they pop out and/or wreck my shocks?

No experience, but for ~1/2in thickness a 2-piece shaft collar might work and definitely will stay in place.  IDK what shaft size you need exactly, some Konis are 22mm.  McMaster has shaft collars for ~$10 each, Amazon happens to have a set of 4x for $18.

"Sinoblu 22mm Bore Double Split Shaft Collar, Aluminum Locking Collars Clamping, 22mm Bore Size, 45mm OD, 12mm Thickness, with M5 Set Screw (Pack of 4)"

Personally, I would add more stop rather than a solid shim- as a bonus, a longer bumpstop should also feel nicer when you do end up using it.  The stock bumpstops on just about every other Subaru model are longer, so there should be lots of options out there to try.

¯\_(ツ)_/¯ said:

Personally, I would add more stop rather than a solid shim- as a bonus, a longer bumpstop should also feel nicer when you do end up using it.  The stock bumpstops on just about every other Subaru model are longer, so there should be lots of options out there to try.

I think this is the best option. It won't damage anything, putting collars on the shocks may have unintended consequences.  

¯\_(ツ)_/¯ said:

Personally, I would add more stop rather than a solid shim- as a bonus, a longer bumpstop should also feel nicer when you do end up using it.  The stock bumpstops on just about every other Subaru model are longer, so there should be lots of options out there to try.

Yeah I'd been thinking I would try the shims first to experiment with how much length is needed, and once that's worked out I'd replace the original bump stops with ones of the correct length. This should also have to save me from pulling the strut clear of the top hats more than once, which is a pretty big pain due to the spring setup and camber bolts.

Now that you mention it, another option could be to cut up a bump stop to make my own spacers which should more accurately approximate an extended bump stop and eliminate any concerns with putting harder materials around the shock shaft...

Oapfu said:

No experience, but for ~1/2in thickness a 2-piece shaft collar might work and definitely will stay in place.  IDK what shaft size you need exactly, some Konis are 22mm.  McMaster has shaft collars for ~$10 each, Amazon happens to have a set of 4x for $18.

"Sinoblu 22mm Bore Double Split Shaft Collar, Aluminum Locking Collars Clamping, 22mm Bore Size, 45mm OD, 12mm Thickness, with M5 Set Screw (Pack of 4)"

Putting these around the top of the shaft just under the top hat/camber plate could work too, that part of the shaft can't actually end up inside the shock anyway...

We use spacers on some of our shocks to allow for the use of oversize tires. They're plastic, I wouldn't use metal. We get them from Fox Racing but the circle track shops have them.

You don't need as much spacer as the change in tire radius unless your motion ratio is 1.0. It might be close, but chances are you'll need just a bit less. Looking at the picture, though, it looks like the red section is vertical and not horizontal. In that case, you'd be looking at wheel spacers.

The red section is kind of diagonal to the ground. On one hand spacers could buy a lot of space while leaving me with more compression travel, but on the other hand in the longer term I'm planning to switch to a significantly wider wheel & tire setup so the "just don't have any tire that far inward" approach would definitely interfere with those plans.

I suspect the camber plates are what allowed this contact to happen since there were no issues at all last year with just the camber bolts. It's running close to -3deg now vs. -1.5deg with the bolts alone, and the plates move the entire path of the wheel under compression inward while the bolts didn't.

I run the plastic style packers on my NC Miata autocross/trackday car.  I use them to limit travel to keep the 245s out of the fenders.  They are pretty painless in operation, and I haven't had issues with them coming loose over the last many years - and I pull them out when I take the 245s off after autocross, then pop them back on when I swap wheels for the next event.

I pulled my springs, mounted wheels, then cycled the suspension to see where I had contact, and added packers until no rub.  I made a note of how many front/rear for each tire size I run and keep that in my track kit.

Measure your shock shaft, so you get the correct size packers.  My Koni yellows took a different size than my Flyin Miata/Fox coilovers.  Easy to fit, fish them between the spring coils and snap into place.  Usually can wiggle them off too, but if your fingers are tired just use needle nose pliers.

A strut's going to have a big burly shaft. You'll definitely want to confirm diameter if you're getting parts from circle track shops that use shocks. I'll bet these would be pretty easy to cut with a laser...

Swapping them out with a tire change is commitment!

How much can you moosh the inner fenders?

In the spot that matters most, exactly none - the tire wore a hole in the liner and then took the paint off the frame directly behind it. I rattle-canned some paint back onto it to keep rust at bay and make any future contact easy to see.

It looks like I may need to make spacers rather than buying them, I can't find any clip-on types for shafts over 16mm and it looks like these front shock pistons are much thicker than that, closer to 21mm. I've found that Delrin and HDPE (AKA cutting board plastic) are the best materials for these.

Gameboy, send me some measurements and i can cut you some bits. I have delrin and abs on the shelf

So the front shock shafts are 22mm diameter, might want to make the spacer IDs 1-2mm bigger so they don't stick to the shaft.

The outer diameter of the shock body is 40mm so that can work as an OD, even 36-38mm should be fine.

Slot width: You may have an equation or lookup table for this in-house already if not, maybe start with 16mm and I can widen them if necessary.

Thickness: If I can get 4x 1/4" thick and 2x 1/8" thick (or just 10x 1/8" thick if that's easier) then I can try any amount from 1/8" to 5/8" spacing in 1/8" increments. If 1/8" is too thin, 6x 1/4" could work. Let me know where to Paypal you for shipping at least

All those measurements are easily doable. I will make up the parts and send you an email to get shipping info in the next few days.

Once you figure out what you like for stackup, add some tape around the stack for extra retention.

World's first set of bump stop shims for stock Toyobaru front struts?



Testing starts when the snow and salt are gone!

In reply to GameboyRMH :

You would probably be suprised at the circuitous route those took to get to your door. Thats after they showed back up at my door after they first got shipped.

I threw in a few spares since i had the material chucked up. Happy holidays.

rkammerer said:

I run the plastic style packers on my NC Miata autocross/trackday car.  I use them to limit travel to keep the 245s out of the fenders.  They are pretty painless in operation, and I haven't had issues with them coming loose over the last many years - and I pull them out when I take the 245s off after autocross, then pop them back on when I swap wheels for the next event.

I pulled my springs, mounted wheels, then cycled the suspension to see where I had contact, and added packers until no rub.  I made a note of how many front/rear for each tire size I run and keep that in my track kit.

Measure your shock shaft, so you get the correct size packers.  My Koni yellows took a different size than my Flyin Miata/Fox coilovers.  Easy to fit, fish them between the spring coils and snap into place.  Usually can wiggle them off too, but if your fingers are tired just use needle nose pliers.

How do you account for how much the bump stops will compress before fender contact? Just curious if you make a decent assumption on how much the foam will compress or if you aren't using bump stops at all. About to do the same.

So I started work on the car today, all I did was file down the sharp edges on the shims and they snapped right on like a polished production part! I agonized for a long time on whether to start with 3/8" or 1/2", I ended up starting with 1/2", I figure this will take up the difference in stock vs. current wheel radius plus a hint extra to help with the more inward wheel path.

Still waiting for the first rain after the last snow to put the car back on the street.

Olemiss540 said:

rkammerer said:

I run the plastic style packers on my NC Miata autocross/trackday car.  I use them to limit travel to keep the 245s out of the fenders.  They are pretty painless in operation, and I haven't had issues with them coming loose over the last many years - and I pull them out when I take the 245s off after autocross, then pop them back on when I swap wheels for the next event.

I pulled my springs, mounted wheels, then cycled the suspension to see where I had contact, and added packers until no rub.  I made a note of how many front/rear for each tire size I run and keep that in my track kit.

Measure your shock shaft, so you get the correct size packers.  My Koni yellows took a different size than my Flyin Miata/Fox coilovers.  Easy to fit, fish them between the spring coils and snap into place.  Usually can wiggle them off too, but if your fingers are tired just use needle nose pliers.

How do you account for how much the bump stops will compress before fender contact? Just curious if you make a decent assumption on how much the foam will compress or if you aren't using bump stops at all. About to do the same.

I've seen a rule of thumb used for the conical polyurethane foam bump stops used on modern cars, that they will compress to 40% of their free length, but the only way to be sure is to test them in a press. For plastic shims, they'll compress approximately none so the calculation is easy.

Had plenty of rain today so took the Toyobaru to give a friend a ride to an appointment at lunch, no noticeable changes which are just what I was hoping for, even when I hit a pothole. I think the contact was only happening under very severe compression, I can think of a few when I heard a scraping noise which I thought was underbody panels hitting the ground, but that turned out to be the tires hitting the fender liners.

Updating again, these got a proper testing last weekend on the construction zones of the 401, plus a jump on the highway which seems to be a normal feature...no signs of contact or telltale scraping noises This was on a drive to an autocross, course was super wet so handling changes weren't so easy to discern, but no negative effects are noticeable so far.

Glad they are working as expected.

Just updating this thread with the latest setup, after driving to the Challenge in late May and possibly hitting some more awful highway irregularities, I found the first hint of contact since installing the shims at 1/2", a little stripe of concave wear on the inside tread edge of the front tires, which were pretty low on tread already at that point. Having discovered this just after putting on a fresh set of costly new tires I immediately popped in another 1/8" shim bringing the total up to 5/8." Still no handling issues found even when aggressively hopping chicane curbs at the FIRM (that's a move that will teach you about the value of good damping!) plus a couple more autocrosses since then. Keeping a close eye on the new tires for any hints of contact.

Thought I would update this thread again after linking to it, still no signs of contact on the new tires but I know I am still getting a hint with a combination of lots of steering angle and compression, I can hear some rubbing when taking hard tight turns, like 180deg turnaround cones in autocross or the hilltop turn 13/14 on Mosport DDT, I actually first heard it on the big right-hand turnaround at the far side of the 2023 Challenge autocross course. I've found some paint removed from the area circled in red in the original post but the contact is minor enough that the tires aren't getting any meaningful wear from it.

The trouble is now that it doesn't rub in compression in a straight line or when making tight turns near static ride height, it only rubs with a combination of lots of steering and compression. I'm planning to get some coilovers in the next year or two that are in the ballpark of 50% harder than the current lowering springs, around 325F/400R (note the Toyobaru's rear motion ratio is lower so a harder spring rate is needed for a similar wheel rate), which I'm sure would eliminate the contact in a somewhat stochastic way, and that might actually be the best approach to fixing the remaining contact.