Poly, Delrin vs Rubber bushing fitment

[67-LS1]

I have a full Hotchkis suspension in my 66 Chevelle and would like to swap out all of the Delrin bushings for rubber. I want a smoother ride and am willing to sacrifice a smidge in the corners. This is daily driver on normal streets. I never track it.
I’m trying to understand about the interference fit of bushings in the various parts, ie, control arms, frame mounts, etc. I’ve found a listing of bushing by dimensions but I’m not 100% sure how much interference I need to get them to stay put and operate correctly.
Any help would be appreciated.

[dontlifttoshift]

Factory rubber bushings are bonded to the inner and outer sleeves. The rubber then twists with suspension travel. I don't know how you can replicate that using existing sleeves if that is what you are proposing.

If you are looking for an existing rubber bushing, with a sleeve already attached to press into your tubular(?) control arms I think you would be looking for .005" press fit.....maybe tighter.

I think the problem you will run into is that most OE type bushings have a lot more rubber than an aftermarket bushing has Delrin. Consequently, rubber bushings will be larger by a significant amount.

[Jonathonar89]

Interference should vary by the bushing/material used. Isolation and articulation bushings have different requirements depending on use. In the pro-touring world, we’ve moved away from rubber body mounts to solid/poly/delrin. In leaf spring cars, for example, a lot of suppliers sell delrin/solid upper shackle bushings to keep the rear end location but use rubber on the spring eye to allow for more spring articulation.

As you can see, this subject can be answered in various ways. Is there a specific part or bushing you’re looking/referring to?

[67-LS1]

I would really like to take some of the NVH out my car. I drive it everyday and it’s just plain harsh. My thought was that if I changed the front and rear control arm bushings from Delrin to rubber it would make the ride a little more compliant.
The body mounts are stock replacement rubber. I don’t know the durometer.
I have 380 lb springs in the front, 160-190 lb in the back. Rears have 1/2 coil cut so may be slightly stiffer. 2” dropped spindles, 0.9” taller upper ball joints and front/rear sway bars. I had 515 lb front springs and they were brutal.
I like how it handles now but since I don’t track it I don’t think I will be giving up as much in handling as I would gain in comfort.
I didn’t think about the rubber bushing needing steel out sleeves. Except the rear uppers which have always been steel sleeved and it’s the one bushing I can buy off the shelf.
My front lower control arms take two bushing per “leg” of the arm, pushed in from the ends. I don’t remember if there was a steel sleeve in the center of the Delrin bushing but I would assume so. I thought if I could get a rubber bushing with the same dimensions and a steel center sleeve that it would be an easy swap.
For the front uppers the bushing presses into the arm and onto the shaft. I’m not sure what the Hotchkis version looks like so that may take some homework.
The front of the rear uppers and both ends of the rear lowers take 2-piece bushings installed on each side. Probably also have steel sleeves.
I’m going to drop one of each piece off so I can take good dimensions of the bushings and the sleeves they fit in.
I was hoping to just press rubber ones back in with an appropriate interference fit.

[68Formula]

Delrin move more smoothly and allow the control arm to follow it's rotational path more accurately than rubber bushings (rubber has stiction - more resistance, in addition to deflection). This transfers the loads to the springs and shocks better which actually improves the ride feel. So if you want a better ride than you have now, keep your delrin bushings and buy double-adjustable shocks.

The added benefit, is if you were to ever track it, you could dial up the shock settings temporarily.

[67-LS1]

Delrin move more smoothly and allow the control arm to follow it's rotational path more accurately than rubber bushings (rubber has stiction - more resistance, in addition to deflection). This transfers the loads to the springs and shocks better which actually improves the ride feel. So if you want a better ride than you have now, keep your delrin bushings and buy double-adjustable shocks.

The added benefit, is if you were to ever track it, you could dial up the shock settings temporarily.

The suspension as it is now with Delrin bushings is jarring. Hitting a large crack in the road at 90 degrees jars the entire car. The ride is great, ie, smooth and flat through the corners, until I hit a defect in the road and my teeth loosen. My 67 with stock control arms with new rubber bushing did not exhibit this trait at all.
I don’t think my tires are super low profile and are the same as my 67 had. I just can’t think of another way to try to minimize the jarring other than a softer durometer material in the suspension.

[V8240Z]

The Delrin should provide a smoother movement through the range than a rubber bushing. I have Delrin in my car and the jarring feeling you describe is due to shock settings. If I firm up the compression and rebound the car can become harsh and non-compliant. Backing down those settings provides the smoother feel for daily driving. I have Viking Triples on my car with a UMI Performance Cornermax kit and could not be happier.

[67-LS1]

The shocks I’m running now are from Hotchkis as part of the complete kit. They are made for Hotchkis by Fox. They are not adjustable that I can tell.

So talk me through shock selection....

I can’t overstress how much more important NVH is to me than the ultimate corner carver. I drive this everyday, rain or shine, to the grocery store or movies or ?? and am trying to make it smooth and quiet.
The car is pretty light (3260 with full 20 gallon tank) with an aluminum V6 and I’m going to change the tires/wheels to save 6 lbs of unsprung weight per corner.
I’m not opposed to adjustable shocks but once I find my comfort spot I would never adjust them. Probably worth it for initial tuning though.

[stab6902]

Rubber bushings do not have stiction - that would be impossible since they do not move inside their sleeves. As Donny said, the inner and other bushing sleeves are bonded to the rubber. The rubber portion of the bushing twists (think of it as a low rate torsion spring, with a little damping from the rubber) to allow the control arm to move up and down. Poly bushings, on the other hand, do have stiction, since the poly does move inside of its sleeves; that's why they sometimes squeak.

The additional spring rate from the twisting of the rubber bushings is pretty negligible in my experience, especially if you tighten down the control arm bolts at ride height like you're supposed to.

I'm confident that rubber bushings would help the OP's impact harshness issues, but it might not be worth it to make rubber bushings work in control arms designed for Delrin. Do you have an extra set of stock control arms you could rebuild with new rubber bushings? That would probably be the most cost effective solution.

What others have said about shocks has merit as well. What are you running? KYB Gas-a-Justs can make a car feel like you're describing - they have a ton of compression damping and not much rebound, which is a bad match for stiffer than stock springs. Your rear spring rate sounds a little high too, especially for a lightweight car. The rear of your car has a bigger impact on ride quality since you sit closer to the back.

All that to say, I think rubber bushings would help, but they may not be the silver bullet you're hoping for.

[stab6902]

If you're concerned about NVH and do only street driving, rubber bushings are the best option for sure. Delrin has no "give" to it to take the edge off of sharp impacts.

Those Hotchkis/Fox shocks should be right in the ballpark. I'm guessing they were tuned for a slightly heavier car, but they shouldn't be that far off. What tire pressure do you run?

[67-LS1]

If you're concerned about NVH and do only street driving, rubber bushings are the best option for sure. Delrin has no "give" to it to take the edge off of sharp impacts.

Those Hotchkis/Fox shocks should be right in the ballpark. I'm guessing they were tuned for a slightly heavier car, but they shouldn't be that far off. What tire pressure do you run?



I’m running 36 psi cold all around. Fronts tires are 225/45-17 and rears are 275/40-18.
The jarring is definitely from the front. Or at least WAY worse in the front. I even avoid Botts Dots.
It’s not bottoming out as I have plenty of clearance to the bump stops.

[stab6902]

36 psi is high for the street in my opinion. I usually run 30 psi on the street - that was the standard for years until cars/tires were designed to optimize fuel economy (at the expense of everything else). I'd try 30 psi as a first step and see how you like it - it'll ride better for sure. With a light street driven car, you don't have to worry about the tires rolling onto the sidewalls.

[67-LS1]

36 psi is high for the street in my opinion. I usually run 30 psi on the street - that was the standard for years until cars/tires were designed to optimize fuel economy (at the expense of everything else). I'd try 30 psi as a first step and see how you like it - it'll ride better for sure. With a light street driven car, you don't have to worry about the tires rolling onto the sidewalls.

.

I’ll try 30 psi and see what it’s like. The tires say something like 50 psi max so I though 36 was on the low side already.
And I know the exact road to check it on today. I normally avoid it.

[dhutton]

Lower tire pressure should make a big difference. Don’t forget about unsprung weight. Are you running heavy cast wheels?

The phenomena you are describing is commonly found in cars with lowered suspensions etc. I also attribute it to delrin bushings, and worse yet heim joints which I see on some setups. Shocks can make a difference imho.

I also suspect that high spring preload combined with inadequate rebound damping can cause the harshness you described.

If you take a look at DSE suspensions you will see they are running rubber bushings. So does Art Morrison. So do the OEM Corvette suspensions. Maybe they are onto something...

Don

[67-LS1]

Lower tire pressure should make a big difference. Don’t forget about unsprung weight. Are you running heavy cast wheels?

The phenomena you are describing is commonly found in cars with lowered suspensions etc. I also attribute it to delrin bushings, and worse yet heim joints which I see on some setups. Shocks can make a difference imho.

I also suspect that high spring preload combined with inadequate rebound damping can cause the harshness you described.

If you take a look at DSE suspensions you will see they are running rubber bushings. So does Art Morrison. So do the OEM Corvette suspensions. Maybe they are onto something...

Don

My wheels/tires are pretty heavy at 47 lbs front/55 lbs rear. I mentioned earlier that I’m looking at new wheels which would drop these weights to 41/49 lbs per corner.

The rear springs are 2” lowering with another 1/2 coiled removed but as I mentioned I feel the jarring is from the front.

The front has stock height springs with 2” dropped spindles. I did reduce the front spring rate from 515 to 380 lbs/in. This is still up from the stock 290 lbs/in

I’m also going to have to do more homework regarding shocks.

[dontlifttoshift]

DSE uses delrin in all their front control arms and only uses rubber in the rear on their swivel links but Don's point stands and I agree.

However, changing those bushings to rubber is not what I would call low hanging fruit nor do I think it will have a significant impact on your harshness issue.

Tire pressure is a big deal and is free. I'd give it a shot. After that, I would revalve the shocks. Does Hotchkis send dyno sheets with their shocks?

[67-LS1]

DSE uses delrin in all their front control arms and only uses rubber in the rear on their swivel links but Don's point stands and I agree.

However, changing those bushings to rubber is not what I would call low hanging fruit nor do I think it will have a significant impact on your harshness issue.

Tire pressure is a big deal and is free. I'd give it a shot. After that, I would revalve the shocks. Does Hotchkis send dyno sheets with their shocks?”



I didn’t get “dunk sheets” for the shocks. It’s actually the first time I’ve heard of one.
I’ll call and ask them.
wait...this isn’t a trick to make ask Hotchkis a dumb question is it???

[dontlifttoshift]

Dyno. I didn't edit my post, it says dyno sheet. It will look like this.

186462

[CSG]

Since the subject of shocks is up I'll add my .02. One of the first things I would want to do is to determine how much of my shock travel I am using in my typical driving. An easy way to do that is to smear some grease on the shock shaft and then drive around & see what got wiped away. If you have shocks apart I like to put tight o-rings on the shafts for markers. Then investigate where your bump stops come into play in normal driving. You need to verify that you are using your compression travel without slamming the bumps too hard in normal driving. Personally I feel that most of the harshness that people encounter is due to the rapid change of direction between compression to rebound. Slamming the bumpstops can cause this along with weak rebound. For my "soft" setups (on properly matched shock/springs) I run soft compression and a lot of rebound. It sounds like you may need softer high speed compression as I am assuming the the "cracks" in the concrete are not compressing your suspension much. I also find that if the car has very much bump steer that I perceive it as being rougher when I hit rough areas. Have you attempted to determine the frequencies front & rear?

[67-LS1]

I adjusted the tires down from 36 psi to 30 and it made a big difference! Not gone but WAY,WAY BETTER.

[dhutton]

DSE uses delrin in all their front control arms and only uses rubber in the rear on their swivel links but Don's point stands and I agree.

However, changing those bushings to rubber is not what I would call low hanging fruit nor do I think it will have a significant impact on your harshness issue.

Tire pressure is a big deal and is free. I'd give it a shot. After that, I would revalve the shocks. Does Hotchkis send dyno sheets with their shocks?

I stand corrected! Thanks Donny.

Don

[dhutton]

I adjusted the tires down from 36 psi to 30 and it made a big difference! Not gone but WAY,WAY BETTER.
Awesome!

Don

[67-LS1]

Since the subject of shocks is up I'll add my .02. One of the first things I would want to do is to determine how much of my shock travel I am using in my typical driving. An easy way to do that is to smear some grease on the shock shaft and then drive around & see what got wiped away. If you have shocks apart I like to put tight o-rings on the shafts for markers. Then investigate where your bump stops come into play in normal driving. You need to verify that you are using your compression travel without slamming the bumps too hard in normal driving. Personally I feel that most of the harshness that people encounter is due to the rapid change of direction between compression to rebound. Slamming the bumpstops can cause this along with weak rebound. For my "soft" setups (on properly matched shock/springs) I run soft compression and a lot of rebound. It sounds like you may need softer high speed compression as I am assuming the the "cracks" in the concrete are not compressing your suspension much. I also find that if the car has very much bump steer that I perceive it as being rougher when I hit rough areas. Have you attempted to determine the frequencies front & rear?

My suspension bump stops have tons of clearance. And there is not a mark on the frame/arm where they would contact
If there are bump stops within the shocks I wouldn’t know. The shocks were part of the Hotchkis kit so I assume they were matched with the intended use which was a small block. After a very short while I did change out the Hotchkis springs for lighter rated springs. I could barely budge the front end they were so stiff. I can bounce it a little now and it really was an improvement. I did not change the shocks at that time so the shocks in the front of the car may be designed to work only with the much heavier spring??
I don’t know what you mean by the “frequencies” front and rear.

[CSG]

You can google "car spring frequency" and get a better description than I could give. I had a discussion with some OEM suspension engineers a long time ago and they were pretty big on having the right front to rear frequency ratio for comfort. From my little bit of experience, with the limitations most of us are not going to put the fab work into overcoming, the frequency ratio is not as important on a race car because it is not always "balanced". For a 100% street car I believe it would be worth investigating. Of course after that then the shocks have to match the springs...

[dhutton]

You can google "car spring frequency" and get a better description than I could give. I had a discussion with some OEM suspension engineers a long time ago and they were pretty big on having the right front to rear frequency ratio for comfort. From my little bit of experience, with the limitations most of us are not going to put the fab work into overcoming, the frequency ratio is not as important on a race car because it is not always "balanced". For a 100% street car I believe it would be worth investigating. Of course after that then the shocks have to match the springs...
Interesting. I think maybe this refers to the natural frequency of a second order control system. The natural frequency combined with the damping ratio of the shocks will determine the response of the system to a step input. At least this is my SWAG...

Don

[stab6902]

Ride frequencies are a big topic, but here's a nice intro: http://downloads.optimumg.com/Technical_Papers/Springs%26Dampers_Tech_Tip_1.pdf

Modern performance oriented production cars tend to have ride frequencies in the 1.5 Hz range (+/- a few tenths), with the rear ride frequency being 10-20% higher than the front. The article above does a nice job explaining why.

Spring manufacturers are well aware of this stuff, and as long as you have a 'typical' car with a matched set of springs, you should be in the ballpark. If your vehicle weight and/or weight distribution is way different from stock or you're mixing and matching parts, it'd be worth looking into.

You can roughly determine your vehicle's ride frequency by doing a simple "bounce test" (with the vehicle parked) and counting the number of oscillations in a 30 second increment for example (Hz = cycles per second). So if the rear of your car bounces 40 times in 30 seconds, your ride frequency would be 40/30= 1.33 Hz. Each end of your car will have a frequency it naturally tends to oscillate at (that's why it's called a natural frequency) with minimal effort; like pushing a swing. Damping has an effect on frequencies as well, but not as much as you might think. I prefer to determine the natural frequencies with the shocks removed, and once I'm happy with them, reinstall and adjust shocks to application/preference. I'm a nerd so I use the accelerometer in my phone and Matlab to measure and analyze ride frequencies and damping, but at the end of the day it's all about how you want it to feel going down the road.

[dhutton]

Ride frequencies are a big topic, but here's a nice intro: http://downloads.optimumg.com/Technical_Papers/Springs%26Dampers_Tech_Tip_1.pdf

Modern performance oriented production cars tend to have ride frequencies in the 1.5 Hz range (+/- a few tenths), with the rear ride frequency being 10-20% higher than the front. The article above does a nice job explaining why.

Spring manufacturers are well aware of this stuff, and as long as you have a 'typical' car with a matched set of springs, you should be in the ballpark. If your vehicle weight and/or weight distribution is way different from stock or you're mixing and matching parts, it'd be worth looking into.

You can roughly determine your vehicle's ride frequency by doing a simple "bounce test" (with the vehicle parked) and counting the number of oscillations in a 30 second increment for example (Hz = cycles per second). So if the rear of your car bounces 40 times in 30 seconds, your ride frequency would be 40/30= 1.33 Hz. Each end of your car will have a frequency it naturally tends to oscillate at (that's why it's called a natural frequency) with minimal effort; like pushing a swing. Damping has an effect on frequencies as well, but not as much as you might think. I prefer to determine the natural frequencies with the shocks removed, and once I'm happy with them, reinstall and adjust shocks to application/preference. I'm a nerd so I use the accelerometer in my phone and Matlab to measure and analyze ride frequencies and damping, but at the end of the day it's all about how you want it to feel going down the road.
Good info. Thanks!

Don