This post is a little long, but read to the end and you might think twice about upgrading your radius rod bushings to polyurethane.

A couple years ago, I decided to do some suspension work on my 1991 940 as it had been neglected for quite a few years. Ever since inheriting it from my dad who has since upgraded to an S80, I've been spending a lot more time working on it.

One of my first improvements was to change out the dampers for Boge Turbo Gas shocks, slightly firmer than stock, but still very comfortable. In addition to the four dampers, I also replaced the infamous radius rod cone bushings with polyurethane bushings from IPD. I knew this was a bushing that had a tendency to wear out quickly and everywhere I read, the poly bushings would be the most logical choice. On they went.

Fast forward a couple years, and it suddenly dawned on my that my car seemed to crash over bumps. Mind you, my car felt the same as when I changed my suspension bits. I just figured at the time the extra harshness was because of the new shocks. I then started reading up on suspension comfort and what do you know, the radius rod bushing has a very big impact on ride comfort.

To explain the issue, I'd like to explain the Acura NSX suspension. This car has a front suspension which is designed to pivot when the wheel encounters a bump. Not a regular vertical pivot, but a longitudinal pivot. Essentially, when the car hits a bump, the front wheel will deflect rearwards slightly and convert the harshness you would normally feel from the bump into a horizontal movement of the wheel. The physical movement is not huge, but enough to make a difference.

In our cars (and many other cars), the OEM radius rod bushing will compress over bumps, taking away the shock that gets transferred to the cabin. This compression, like the NSX, will allow the front wheel to move rearwards very slightly. When 'upgrading' to polyurethane bushings, you take away the movement that was designed into the rubber bushing, diminishing the ride quality.

I took out the poly bushings and replaced it with an upgraded-design solid rubber bushing (as opposed to a hydraulic bushing). It was night and day. My car stopped crashing over bumps and took off the edge from expansion joints and ruts in the road that I would otherwise feel. I'm starting to wonder if the original hydraulic bushings would make an even greater difference.

The only disadvantage I can think of is that you'll have to replace the bushing more often, but with the simple removal of two bolts, I don't find that to be an issue. For hardcore purists, you may also feel some sloppiness in the suspension, but you'd have the be running poly bushings all around to notice any difference at all.

Disclaimer: results will vary. If you feel no change at all in ride comfort going from poly bushings to rubber bushings, you may have issues elsewhere.

 

The rubber ones last 150k, even longer, depending on use of course. I'd never put those polyurethane ones on my cars, stock is good enough....

 

there's a lot of real junk poly out there. the Superpro stuff from the land of Oz is good quality and will last for years, the Uro stuff is pure junk. Unclear what IPD puts in their kits.

 

I replaced those rubber cone bushing in my 91 740 with the IPD ones and am very happy with them. With the old ones there was a noticeable delay between me turning the steering wheel and the wheels reacting/turning. This was reduced significantly with the poly pushings. I do get some noticeable feedback through the steering wheel but its not bad. I tend to expect added harshness from performance suspension modifications as a tradeoff from comfort. Im doing a full autocross setup so I expect the comfort to drop significantly more with increased cornering ability.

 

I didn't know original was hydraulic cone bushing. Having a hard time visualizing it. Hydraulic as in it's filled with some fluid?

 

I dunno where that came from, its a solid rubber bushing, 2 cones that sandwich the control arm.

 

When I bought my new rubber bushings, they were the updated design, and in the description it said it was a "solid bushing". I can only assume the original design was not solid and therefore must have been hydraulic. I couldn't actually tell when I pulled out the old rubber ones since they were literally falling apart.