Thraxbert
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I'm forking this thread into something new, with an aim to help any random Google passerbys who are experiencing rear suspension clunking on their MkV/A90 Supra after installing lowering springs. I first want to credit @jvang85 for spurring this conversation in the direction of a fix. I also wanted to share my own story.
A Primer on Progressive Rate Springs
Like everyone else in the other thread, I have lowering springs that are progressive rate. Let's start there: what is a progressive spring? You can identify progressive rate springs by a section of equally-spaced coils and a section of tightly-wound coils (illustrated). I have RS-R Super Down springs. "Progressive rate" means that it might take 1X unit of force to compress the spring the first 1" (hypothetically speaking), but it might take 1.5X unit of force to compress the next 1". In other words: The force require to compress the spring increases disproportionately to the amount of compression. The alternative is a linear spring, where each unit of compression always requires the same amount of force: 1" compression = 1X unit of force, the next 1" also = 1x unit of force, and so on until the spring bottoms out.
Lowering springs are almost always progressive because they handle small bumps in the road better than linear springs. When you're taking out spring height (or making the spring rate softer) to lower the car, a spring that can readily resist full compression and rapidly return to "relaxed" position is very advantageous. It absorbs bumps well, doesn't easily reach full compression, and maybe you could say a progressive spring has a very "dynamic character."
Progressive Descent Into Madness
Buuuut that dynamic character is also the drawback. The coils in the tightly-wound section of the spring, under high compression or rapid oscillation, can touch. This is called coil slap. Here's a video of a Toyota 4Runner exhibiting coil slap when the operator manipulates the suspension on jackstands. This is a good visualization of the phenomenon, and you can hear the coils touch in time with the suspension compression. Now, when this happens to you on the road: clunk. Yes, the clunk everyone has complained about, and the clunk that maybe brought you here from Google. The clunking you are feeling from the rear of your car is the force of coil slap being transmitted through the rear subframe.
Can I Avoid This?
Trick question. Eibach, H&R, RS-R, HKS, Swift, Whiteline, Tein... Everybody for the A90 is using progressive springs for the Supra because they have good road manners. You will get rear clunking with every one of these produc-- wait a minute. What's this? Tein SKTJ4-S1B00 for the A90 has COIL ISOLATORS PRE-INSTALLED.
You see, coil isolators are the solution for rear-end clunking with progressive lowering springs on the Supra A90 and, well, any other car. Tein is the only gigabrain company that figured this out and solved the problem from the factory. Kudos to them. So happens they also sell coil isolators to the rest of us under model TEISPR02-G1497. You may also find them listed as Tein Rubber Silencers 90-130mm. These are perfect for the A90 Supra and the in-market spring options.
Make Your Springs Very Lonely With Isolators
To use coil isolators effectively, you should clean your springs up real nice first. Hit 'em with brake cleaner (protect your plastics), soapy wooder, isopropyl, whatever makes you happy. We're doing this because the isolators have a very sticky adhesive inside, and you want that adhesive to work right so the silencers don't come loose and get smashed. You don't need to go nuts with these things, because two sets per side (four isolators per spring) is enough. Like, you're just stopping coils from touching not making a mummy, ok?
Start towards the top of the spring near the upper stop and put isolators on every other coil of the tightly-wound section. Going back to our earlier-in-the-post progressive rate spring illustration, I've flipped and cropped it to match the Supra rear orientation of most springs. After the isolators are installed, your coil should look something like our new reference image at the end of this post. Just in case it isn't clear: red is raw spring, turdy Tein brown are the isolators. (Why Tein did not make these things in the nice Tein green I will never understand.)
I've also attached an image of my RS-R Super Down springs at full extension on a lift to show you just how close these progressive rate coils can be, leaving no question about why they can sometimes touch. The more lowering your springs provide, the worse this rear clunking issue will likely become and, dear reader, let me tell you my car clunked running over a dead leaf. So, you can see that isolators on every other coil isn't just the appropriately miserly thing to do... it may also be a technical requirement of the job.
Exceptions and Disclaimers
I am not your mechanic or your shop. I cannot guarantee all clunking will be solved, because there are a number of ways a lowering spring install can be botched. But, presuming you've installed everything else correctly: your clunk is donezo.
As a series of general tips...
A Primer on Progressive Rate Springs
Like everyone else in the other thread, I have lowering springs that are progressive rate. Let's start there: what is a progressive spring? You can identify progressive rate springs by a section of equally-spaced coils and a section of tightly-wound coils (illustrated). I have RS-R Super Down springs. "Progressive rate" means that it might take 1X unit of force to compress the spring the first 1" (hypothetically speaking), but it might take 1.5X unit of force to compress the next 1". In other words: The force require to compress the spring increases disproportionately to the amount of compression. The alternative is a linear spring, where each unit of compression always requires the same amount of force: 1" compression = 1X unit of force, the next 1" also = 1x unit of force, and so on until the spring bottoms out.
Lowering springs are almost always progressive because they handle small bumps in the road better than linear springs. When you're taking out spring height (or making the spring rate softer) to lower the car, a spring that can readily resist full compression and rapidly return to "relaxed" position is very advantageous. It absorbs bumps well, doesn't easily reach full compression, and maybe you could say a progressive spring has a very "dynamic character."
Progressive Descent Into Madness
Buuuut that dynamic character is also the drawback. The coils in the tightly-wound section of the spring, under high compression or rapid oscillation, can touch. This is called coil slap. Here's a video of a Toyota 4Runner exhibiting coil slap when the operator manipulates the suspension on jackstands. This is a good visualization of the phenomenon, and you can hear the coils touch in time with the suspension compression. Now, when this happens to you on the road: clunk. Yes, the clunk everyone has complained about, and the clunk that maybe brought you here from Google. The clunking you are feeling from the rear of your car is the force of coil slap being transmitted through the rear subframe.
Can I Avoid This?
Trick question. Eibach, H&R, RS-R, HKS, Swift, Whiteline, Tein... Everybody for the A90 is using progressive springs for the Supra because they have good road manners. You will get rear clunking with every one of these produc-- wait a minute. What's this? Tein SKTJ4-S1B00 for the A90 has COIL ISOLATORS PRE-INSTALLED.
You see, coil isolators are the solution for rear-end clunking with progressive lowering springs on the Supra A90 and, well, any other car. Tein is the only gigabrain company that figured this out and solved the problem from the factory. Kudos to them. So happens they also sell coil isolators to the rest of us under model TEISPR02-G1497. You may also find them listed as Tein Rubber Silencers 90-130mm. These are perfect for the A90 Supra and the in-market spring options.
Make Your Springs Very Lonely With Isolators
To use coil isolators effectively, you should clean your springs up real nice first. Hit 'em with brake cleaner (protect your plastics), soapy wooder, isopropyl, whatever makes you happy. We're doing this because the isolators have a very sticky adhesive inside, and you want that adhesive to work right so the silencers don't come loose and get smashed. You don't need to go nuts with these things, because two sets per side (four isolators per spring) is enough. Like, you're just stopping coils from touching not making a mummy, ok?
Start towards the top of the spring near the upper stop and put isolators on every other coil of the tightly-wound section. Going back to our earlier-in-the-post progressive rate spring illustration, I've flipped and cropped it to match the Supra rear orientation of most springs. After the isolators are installed, your coil should look something like our new reference image at the end of this post. Just in case it isn't clear: red is raw spring, turdy Tein brown are the isolators. (Why Tein did not make these things in the nice Tein green I will never understand.)
I've also attached an image of my RS-R Super Down springs at full extension on a lift to show you just how close these progressive rate coils can be, leaving no question about why they can sometimes touch. The more lowering your springs provide, the worse this rear clunking issue will likely become and, dear reader, let me tell you my car clunked running over a dead leaf. So, you can see that isolators on every other coil isn't just the appropriately miserly thing to do... it may also be a technical requirement of the job.
Exceptions and Disclaimers
I am not your mechanic or your shop. I cannot guarantee all clunking will be solved, because there are a number of ways a lowering spring install can be botched. But, presuming you've installed everything else correctly: your clunk is donezo.
As a series of general tips...
- Hopefully your shipping spacers were removed by your friends at Toyota.
- Hopefully you oriented your top hats, dust boots, and spring perches correctly -- line up those notches with the ends of the spring!
- Hopefully you didn't do anything untowards like remove the front suspension support sub-assembly. You only have to free that one central nut!
- Hopefully you fished your old bumpstop out of the dustboot and replaced it with the new one if your vendor gives you one
- Hopefully you put everything back to the right torque:
- 52 ft-lbs for that central nut on the front struts ("Front Support to Front Shock Absorber Nut")
- 21 ft-lbs +90 degrees for the four torx bolts on the top strut mount (topside of engine bay)
- 41 ft-lbs for the endlinks on all corners
- 122 ft-lbs + 90deg for the rear lower control arm to the knuckle
- 74 ft-lbs + 90deg for the rear strut to the rear lower control arm
- Best to tighten endlinks to the strut + swaybars when the car is on its weight, but not everyone has this luxury
- Do yourself a favor and buy a set of Torx + E-Torx for this car.
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