No one seemed to know much about the bumpstops and how hard or soft they were, so I decided to buy a scale and started testing some of the bumpstops that I have. I have found a pretty good method using a generic bathroom scale (good to 300lbs.), a hydraulic press and some 2x8 blocks of wood to spread out the pressure. This is about as accurate as I can get even though we only need a good ballpark figure for these bumpstops. I decided to squish them in 1/8" increments and record the pounds of pressure. The results where quite an eye opener for me. Here are the stops I have to work with:

1. the medium 40mm foam one from my front Koni SA shocks #70.34.54.000.0
3. the Ground Control front from their adjustable perch kit
2. the front factory orange unit (from 99 T/A)
4. the rear factory yellow tapered unit (from 99 T/A)
5. the longer 55mm version of the first Koni #70.34.53.000.0

Keep in mind that you have to plot the pressure over the distance traveled. The length makes a big difference, too. You'll notice I didn't measure the GC past 3/4" while the (shorter) Koni, I measured to 1". The reason is because the GC was much shorter than the (shorter) Koni and the pressure was starting to ramp up very quickly.

First up is the (shorter) Koni which measured 1 1/2" long (40mm).
How much squished - pounds of pressure
1/8 55
1/4 75
3/8 90
1/2 105
5/8 110
3/4 150
7/8 200
1" 300

Next is the Ground Control unit. It measured 1 1/16" long.
1/8 50
1/4 70
3/8 95
1/2 140
5/8 220
3/4 290

Next was the factory front piece which was right at 1 1/8" long
1/8 210 pounds!
1/4 290

Whoa! I knew the stock piece was a lot stiffer but this surprised me. So at 1/8 inch of compression it is about 4 times stiffer and it doesn't look progressive at all. (I actually broke the scale and had to go buy another one. I retested all the pieces again with the new scale.)

Next is the factory rear which is 2 5/8" tall
1/8 45
1/4 65
3/8 95
1/2 125
5/8 150
3/4 200
7/8 260

This one surprised me, too. I thought it would be much stiffer than this. Everyone seems to mention how firm they feel and that they can snap the rear loose in a fast corner. I wonder if it's the first 1/8" or so going from 0 to 45lbs that is too sudden or maybe it's that it ramps up too quickly in the middle area. The only why I can think to lower the rates would be to drill some holes through the side. Where, what size and how many will make it softer in the areas you want. I imagine there is a science involved in figuring that out, also.

Last is the longer Koni bumpstop, I recently ordered, which is 2 3/8" tall (55mm).
1/8 30
1/4 45
3/8 60
1/2 70
5/8 85
3/4 100
7/8 110
1" 145
1 1/8 195
1 1/4 260

This unit is the same as the Koni bumpstop I tested as #1, just a little longer. The increased length makes it more progressive and quite a bit softer initially. If I where to chop it down to match the height of the shorter Koni I'm sure it would match it's rates. It feels like the same material and softness, just shorter.

If I had the time I would plot out the curves in a graph, but due to not having a web host, I wouldn't be able to post it anyway. The main thing here is to look at the slope and how the pressure ramps up.

Even if most have no use for this info, I hope someone will find it interesting and perhaps usefull.

Jason

Was recently able to get some pics hosted. Enjoy.




Ground Control also has 4 aftermarket bumpstops that can be bought. I recieved a set from Jon Aadland. They are the "soft, progressive 10-14mm shock" ones. One was full height and the other was one that Jon had cut down. I measured them both and here are the results.



The longer one is 2 1/8" tall
1/8 25
1/4 35
3/8 45
1/2 60
5/8 70
3/4 85
7/8 110
1" 135
1 1/8 160
1 1/4 210
1 3/8 300

The shorter one is 1 5/8" tall.
1/8 20
1/4 35
3/8 45
1/2 55
5/8 75
3/4 100
7/8 140
1" 200
1 1/8 300

Link to Ride Height and Suspension Travel Specs

crosslink to FFRAX

 

Just a bump in case someone needs the info.

 

Or a bumpstop even.


Cool info though.

 

Good info thanks for the work.

 

Added a new bumpstop to the chart. One is stock length and one is cut down.

 

Another bump for those that might need the info.

 

This should be a sticky.

 

This is handy, especially for all the people in the Wheels and Tires section who are quick to recommend removing the rear bump stops to fit 315mm tires because "they don't really do anything anyway".

Thanks Jason!


-Mike

 

There's no doubt the stock rear bumpstops get used. I've seen a lot of stock vehicles where the rears are practically falling apart due to use. I run 315's out back and I also cut off the stock mounting tab for extra clearance. I used zip ties on the shock to measure compression travel for normal driving as well as through dips and then measured the shock's total compression travel and found that the axle will contact the exhaust pipe before bottoming in the shock. There are more details in my "Stock Shock and Suspension Travel Specs" thread linked at the bottom.
Anyway, at stock ride height I found I could get away without bumpstops, but once you lower the ride height, you will definately need them. The big problem with the stock rears is that they are too stiff and limit usefull suspension travel. I would recommend cutting about 1/2" in length off of them and then cutting a taper back on it so that it is more progressive in rate. 1/2" of extra travel can make a big difference.

On my car, I measured out how much travel I had and then fitted the best bumpstop I had onto my shock body. I'm running the Koni SA rear shocks and this is what I came up with on my car.


Once I lowered the rear to about 26 1/4" or 26 1/2" (I can't remember right now) there is was only about 1/2" between the bumpstop and the shock body, but that's OK. I'm running a stock spring modded for 170lb linear rate and have the Koni shocks adjusted where I want them. Even though the Koni 55mm bumpstop gets used all the time, it's so smooth and progressive I really can't feel it. The rearend just soaks up the bumps with ease and that's the main goal.

 

I work for a company that makes these products, we were actually the OEM supplier for the rear bumpstops. I don't have much to add to the above information but all the bumpers shown are made out of a product referred to a microcellular urethane. It is basically a fancy form of foam rubber.

With at least the rear bumpstops, they do get used, but more often than not, they don't dissolve from use but rather the metal they are bonded to rusts out and then the bumper seperates from the mount because of the rust.

The only thing I can really add here is that the ones that are MCU (foam, cell appearance, not solid like poly bushings) use a material that is very linear in response until the cells are completely compressed, then the stiffness shoots up.

If you are trying to do tuning then the MCU is actually very machineable. You can cut or drill on it to obtain the rates you want, and it is not prone to fatigue cracking like rubber or poly, so long as you keep it working only in compression.

 

Koni calls theirs "cellular polyurethene", but microcellular urethane is good to know. The stock rears and the Koni's do look very similar. The stock front ones are a type of foam, but are a lot denser. Do you know if they are also MCU?

On those rear one I've seen falling apart, I'm not sure of actual cause but they were still intact at the mounting point, but the tapered end was all beat up like it had taken multiple poundings over a long time.

The progressive nature also seems to be mainly in the shaping. More of a taper will make it more progressive. I was thinking a few holes through the side of a stock rear bumpstop would soften it up a little and make things better. It's good to know that drilling holes in it won't hurt the material.

One other thing. If you still have the metal SLP spacers on your rear bumpstops, it's safe to remove them. They seem to be utterly useless as well as rob you of usefull suspension travel.

 

Bump to the top

 

Nice write-up Jason!!! Courtesy Bump......

 

Just thought I'd add this pic of the front Koni SA limits.

 

Very cool jason! Great info.

 

Nice info. I’m running the 55mm Koni MCU style on the Koni’s on both my 3rd and 4th gen TA’s, and the really are much better than the stock ones if you’re going to use the bump stops for tuning or just prevent bottoming when you cut off the stock pads.

For anyone doubting that the rear bump stops get used, take a look at the pads that are welded on the top of the axle for them to contact, I’ve never seen a car that they’re not shiny on.

That being said, does anyone know of any MCU style bump stops that are single stud mounted? I'm not even sure that I care what shape, since it's easy enough to carve them to the shape (and rate) that you need.

 

The main picture that this thread started with is for FRONT shocks right??? So you can change them out? I have Koni SA's in the front and I was told the car would ride better but I noticed no change over stock. I have them on the softest setting with the same tires and tire pressure. I think these are from '05.

My other question is about the rears. Do the Koni SA shocks NOT come with the 55mm bumpstops? Did this guy buy them and add them? And so the 55mm stops on the shock body would allow me to ditch the factory style ones that hit the rear axles? I have an SS with the spacer and the hard rubber. I was going to get new Z28 bumpstops but not if I can buy Koni SA's with them or add them to my new Koni shocks. I have the fronts already. I've been meaning to get the rears anyway.

 

Yes, the rears look totally different.
It depends on the shock, but usually yes.

That seems hard to believe. You did not mention the springs. What do you have?

Of course not. Why would they?
Yes, I did. I was running 315's and they would rub, so I cut the bumpstops off completely, then set up the new bumpstops on the rear shocks so as to gain a little more travel.
For me, yes. For you, I don't know. There are a lot of other factors and measuring that needs to be done. Regardless of where the rear bumpstops are located, the important thing is to know how far the shocks can compress and make sure your new bumpstops do not allow the shock to compress beyond that. If they do, you will damage the shock. This is also true for any shock on any vehicle.
You can swap the bumpstops to the shocks like I did, but remember what I said earlier about not compressing them beyond their limits.

If your not running 315's and can keep the stock bumpstop location, then why not do that? Are you trying to increase compression travel?

 

No, I just want a better ride. I guess I want to have my cake and eat it too. My car is noticeably more stiff than my girlfriends TA. She has all stock shocks, springs and bumpstops. I have Koni SA's in the front, stock rear shocks, and factory cut springs. I've lowered the car about 1.5 all round. What I'm really noticing is that the back has all kinds of racket going on back there. I have L.G. Motorsports've race/adjustable LCA's and PHR where one side has a Poly bushing and the other side utilizes a tie-rod end for binding. Yes, I have the LCA's in the right direction with the poly bushing in the front to the body and the rod towards the rear. I'm not sure if there is a correct side for the poly and tie-rod on the PHR. Anyway, I just want a smoother ride. I don't mind a firm ride but this is noisy, beat you up rigid. I'm going to remove the aluminum spacers first and see if that helps. Then I am going to get the Koni SA's for the rear. After that, I want to try either Z28 bumpstops or the Koni 40mm shock mounted bumpstops since those sound to be more progressive and not so harsh. I just need it top absorb the bumps better. Will the softest setting on Koni SA's give you the smoothest ride or is there a point where it just gets more sloppy. Of the eight settings, what is the best setting for someone who is 80% worried about ride quality and only 20% worried about handling?

 

I think the reason your riding so rough is because your hitting the bumpstops so hard and so often.

When you cut a spring, it's rate goes up. The rate increase on the rear is actually good (approximately 20lb. per coil increase), but the fronts are still going to be too soft.

Remove those rear spacers, then get the rear shocks. That should solve most of your ride quality issues. Set the rear shocks rebound to full soft. It won't be sloppy. (There are not 8 settings. I show this in my Koni adjusting thread)

Your current bumpstops are a rubber like material. When they compress they want to push the axle back quickly. This leads to a bouncy ride. The regular bumpstops are made from cellular polyurethane. They compress easily, but don't spring back super fast. This helps get rid of the bouncing.

If you have room for more rear suspension travel without over compressing the shocks, you can cut down the new bumpstops some (maybe 3/4"). Just make sure you keep a tapered shape.

There are a lot of other things that effect ride quality as well. What rear tires do you have and at what air pressure do you keep them?