Trailing Arm Bushings - Ron’s 70 S2 Crossflow

[RonPNW]

I need a little feedback on the details of the trailing arm bushings. My originals are not in great shape. Likely serviceable but they have cracks and seem dry and stiff. So I ordered new ones from RD. What I got is likely all that is available but they are not the same as the originals.

I’m guessing the originals were 7/16” (.4375). In the 80’s (I was auto-crossing at the time) I noticed a fair amount of slop in the rear end. In an attempt to reduce that I fabricated step diameter bolts for the bushings that more closely fit the holes in the original bushing and the original trailing arm. It was .467 for the bushing and .446 for the trailing arm. I have no idea how the bushing and trailing arm got large enough to need this size bolt (wear? The holes look round). The increased size bolt is needed to get the running play to a reasonable level. (My driving was sloppy enough that I never noticed any improvement in performance, if there was one.)

The new bushing requires a .5” diameter bolt. That is, the central steel tube has a .5” hole. It does not look like there is enough metal in the trailing arm to drill it out to .5”. That’s OK, I can make another step bolt but there are other issues.

In the original installations there is a large thick washer, in the inside of the bushing, that compresses the entire back of the bushing by about .035” when assembled (metal to metal contact). The new bushing has a longer central steel piece that will not allow the large washer to touch / crush against the bushing.  My impression is that this touch / compression is needed to help stiffen the bushing and reduce the amount of movement the is allowed (any movement will alter the rear toe in and negatively impact handling. It is easy to drill out the washer so it fits around the central steel piece to compress the installed bushing although doing that will result in about .065 compression of the bushing when metal to metal contact is achieved. This can be “trialed” in a vice. It does not seem to be excessive but ….?  (I found what appears to be a commercial washer (11/16”ID, 1 ¾” OD, .135 thick) that can do the job. The black bushings are new.

So am I over thinking this? Or do I need to set things up to have compression of the back side of the bushing?

FYI the other dimension of the internal steel tube is also different compared to my original bushing by about .04”. The difference is in the direction to increase toe-out by .04”. So be sure to reset toe-in if you replace your bushings.

Thanx for the feedback.
Ron

[GavinT]

I believe the purpose of the large washer is to 'capture' things should a catastrophic failure of the bush occur.
I reckon the fact that the large washer compresses the rubber on the original bush is more likely a happenstance than a primary design consideration. My foggy recollection is these bushes came from a Rover (?) parts bin or some such.

That said, I guess an additional washer of suitable thickness could be added on the inside to obtain a more precise desired compression but my general understanding of Metalastic bushes is they're not usually intended to be under stress at rest.

Is it worth considering a split tube shim arrangement to take up the space between the original 7/16” bolt and the half inch hole in the bush? I'm thinking that once the assembly is locked up, nothing should move and the bush would flex as intended.

I wonder what RD might have to say on the matter? I expect you aren't the first to notice the bush hole difference.

[BDA]

It’s an odd design. The radius arm takes a 7/16” bolt but the bushing has a 1/2” hole. It is apparently on purpose. Maybe they didn’t want the extra cost of modifying the frame so everything lined up properly?? As far the washer, I think you’re overthinking it. I agree with GavinT that it’s there in the event of a catastrophic failure. I wouldn’t be concerned that it bottoms out on the tube in the bushing. I don’t think any compression (or lack of) of the rubber is part of the design.

Given that the design doesn’t really make sense to me might mean that I’m missing the genius in it or maybe the obvious and someone can correct me, but I think it’s as simple as it looks.

[RonPNW]

Still shooting in the dark but I did find this:    https://www.lotustalk.com/threads/radius-arm-bolt-stuck-on-esprit-turbo-se-and-mods-to-radius-arm-setup.475903/page-4#nested_reply_top_post

Apparently these large washers are called "snubber" washers. This implies they are intended to restrain movement of the bushing in the Turbo Esprit. It has two washers, one on each side of the bushing. It appears that they both touch and sandwich the bushing.

So I am going to go with the concept that similar issues apply to the Europa. I'm going to modify the large washer to apply about .040 of compression to the new bushing when installed and torqued. I think going the the .5" bore is a good thing in that it is likely to provide slightly more control of the suspension components.

Ron

[BDA]

If you want to do it REALLY right, check out this video from our own Serge (https://www.youtube.com/watch?v=0Os40ns8NiI).

[jbcollier]

The arm bolts to the rubber mount and that’s it for location.  The large washer is strictly for safety if the bushing tears out.  The washer just sits there and that’s it.

[RonPNW]

Thanx for all the helpful comments.
I’ve decided  to try and duplicate the original set up as found in my 70 S2. My guess as to the key original features are the 7/16” bolt through the original 7/16” bore in the bushing and the slight compression of the bushing by the large thick washer opposite the trailing arm.

I also found the diagram below, for the Esprit, that shows a large thick washer (#41) on both sides of the bushing. A search for that bushing shows that it is the same are the Europa bushing (actually lists the Europa as a fit) and online pics seem to show that the install is the same as the Europa. I found it at https://www.pnmparts.co.uk/esprit-s4-s4s-s300-gt3-93-99-d/lotus-radius-arm-mounts-6000   (it appears to be an exact match to the original Europa bushing).   It appears that, for the Esprit, the washer is flush to the rubber of the bushing with slight compression when torqued.

The new bushing used a .5” with a steel tube that extends past the rubber. The steel tube in the original bushing ends short of the rubber, allowing compression when fully torqued (metal to metal contact).

I decided to stay with a system that provides some compression of the rubber of the bushing. That will reduce the movement in that pivot providing (hopefully) a more controlled handling experience.

Here is what I did.
•   Fabricate a new bolt with a .5” diameter for the bushing, stepped down to .448” for the trailing arm (the ID dimension of my trailing arm) then stepped to .4375 (7/16) for the nylock nut. Make sure the .5” diameter portion does not extend past the bushing to avoid getting in the way of any spacer washers that may be needed to set toe-in.
•   Drill out the original thick 2” washer to 9/16 to clear the central steel tube of the bushing. This allows the for .050” compression when the assembly is torqued.
•   Add a large .5” washer, between the bolt head and large washer to secure the thick washer and bushing.
•   Assemble as in the original.

I think the .5” bore bushings will do a better job of controlling the suspension compared to the original 7/16” bushing.

Ron

[My S1]

RonPMW, I like the way your mind operates.  Hope you don't mind if I swipe your solution.

[dakazman]

 

Thanx for all the helpful comments.
I’ve decided  to try and duplicate the original set up as found in my 70 S2. My guess as to the key original features are the 7/16” bolt through the original 7/16” bore in the bushing and the slight compression of the bushing by the large thick washer opposite the trailing arm.

I also found the diagram below, for the Esprit, that shows a large thick washer (#41) on both sides of the bushing. A search for that bushing shows that it is the same are the Europa bushing (actually lists the Europa as a fit) and online pics seem to show that the install is the same as the Europa. I found it at https://www.pnmparts.co.uk/esprit-s4-s4s-s300-gt3-93-99-d/lotus-radius-arm-mounts-6000   (it appears to be an exact match to the original Europa bushing).   It appears that, for the Esprit, the washer is flush to the rubber of the bushing with slight compression when torqued.

The new bushing used a .5” with a steel tube that extends past the rubber. The steel tube in the original bushing ends short of the rubber, allowing compression when fully torqued (metal to metal contact).

I decided to stay with a system that provides some compression of the rubber of the bushing. That will reduce the movement in that pivot providing (hopefully) a more controlled handling experience.

Here is what I did.
•   Fabricate a new bolt with a .5” diameter for the bushing, stepped down to .448” for the trailing arm (the ID dimension of my trailing arm) then stepped to .4375 (7/16) for the nylock nut. Make sure the .5” diameter portion does not extend past the bushing to avoid getting in the way of any spacer washers that may be needed to set toe-in.
•   Drill out the original thick 2” washer to 9/16 to clear the central steel tube of the bushing. This allows the for .050” compression when the assembly is torqued.
•   Add a large .5” washer, between the bolt head and large washer to secure the thick washer and bushing.
•   Assemble as in the original.

I think the .5” bore bushings will do a better job of controlling the suspension compared to the original 7/16” bushing.

Ron

  Thanks for all this information. I'm in the stage of working some minor bugs out and I'm sure to get to setup and alignment soon.
 I will pay attention to the handling as well as several other complete resto's will go through.
 Roadability and driving pleasure , I can only imagine.
 
Dakazman