MG Sports Cars

The Roadmaster is now safely stored in Indianapolis at Dave Kirkman's house.

Post-mortem of the failure will have to wait a bit. Still cannot see the upper portion of the rear member/coil over assembly.

In addition to this failure and the steering wheel failure....the barrel to the driver's door lock fell out of the assemby (and is safe inside the console) and the cigar lighter works sporadically at best. (My GPS would power down quite frequently.

Obla-di, obla-da.

Received a new sending unit for the fuel tank of the Roadmaster today, courtesy of Curtis Jacobsen. THANK YOU!!! Maybe this will correct the reading of the fuel gauge and eliminate the close calls/near misses/actual incidents of running out of fuel.

I'll take it over to indianapolis to add to the Roadmaster parts-to-be-installed/repaired.

I wonder if we should make the effort to get the Roadmaster back down to Jim's to evaluate/repair the rear suspension as it was he who made the original design and is most familiar with it?!

I also learned that the Roadmaster has a fly-away hand brake....it's very frustrating to attempt to release the hand brake in a conventional manner when one of these prawls are installed. Please advise all drivers/caretakers of the Roadmaster of this installation when handing over the keys. (I had never driven a car with a fly-away hand brake...it was very frustrating until I learned what was going on......)

Sorry about the brake Rick, I thought by now everyone knew about it. Could indeed be frustrating, but could be handy for some on uphill starts I suppose. Not that you'd have that issue...

Re: rear suspension, maybe first we need to know what we are dealing with. If it's just a broken coil-over that should be easily replaced and the car is back in service. I am no authority on coil-over units so if it is thought that something needs changed there such as longer springs as Bill G has advised, I would recommend conferring with Ted Lathrop as he knows more about them than anyone else I am familiar with and if we need longer springs he can get them from his regular supplier. If he agrees with Bill then I'd say that would be something we should do.

There was no binding in the suspension as built and installed and aside from the guys shortening the adjustment of the forward strut to bring in the alignment there still should be none. But I suppose we could look at that. I don't see how that could break the shock though. I think replacing the shock first so the car can be driven ought to be the first step. Then if you or Dave wants to drive it over we can take a look.

Hey guys, a thought just occurred to me. The eye of that shock is threaded and screws on to the shock rod, right? So what happens if the rod screws back out of the eye? If that happens doesn't it do just what this one did? I wonder if maybe they forgot to put the locktite in the threads before it went together. If that happened then it could be that all we need to do is thread the pieces back together (with a generous application of locktite of course) and reinstall the shock.

I'm not saying it will be just that easy, but certainly it seems like a possibility. Just think. you could be the hero that finds the easy fix! ;-)

Jim

To be honest, Jim, that was my first thought.

But I could not see under the car enough to see what was going on with the top of the unit.

It's going to be a couple of weeks before I can get over to Indianapolis to see what's going on.

Great minds think alike? Just that some of us are a little slower ;-)

Jim

Quote:

thread the pieces back together (with a generous application of locktite of course)

Jam nut, perhaps?

Guys:

That IS a very short spring, there is lots of adjustment to bring the base down. Maybe lighter weight 12" springs would be best?

I don't think the rod screwed out of the eyelet-- wouldn't the spring have shot out? The fact that it's all there but "bent" makes me think the shock rod bent rather than disassembled.

$.02

M

Max, if memory serves I believe those are 350 lb springs and the rate is about right. Could be a little stiffer maybe but lighter would probably not be a good thing. Longer springs would have less of a progressive rate I would expect, in essence almost the same thing as a lower rate. So going to a 10 or 12" spring (depending on how much adjustment there is now) I'd be inclined to go up another 25 lbs on the rate. I don't think it'd hurt the ride noticeably.

But again, I have very little experience with coil-overs and I'm anything but an authority on them. The good thing is that the springs are not particularly expensive.

Jim

Jim B, 350lb is 350lb rate tested, regardless of length. If you have the same diameter wire and cut 4" off a 12" spring you would be correct. Less coils for the same diameter wire is stiffer. My guess is the 12" spring rated at 350 will have either thicker wire or stiffer spring to develop the same rating. I agree that will be about right for the rear. 450-500lb would be about right for front coilovers depending on engine weight or preferred stiffer handling(and distance of shock to the wheel). I have a theory that the spring being too short when the wheel is in droop and when it came down hard broke the shaft. I recall you said the Roadmaster had 6" of travel, you need approximately a 12" spring. If 5" travel a 10"spring. The Mustang II Front suspension uses a real short shock and at best has 3 1/2" total travel so a 7"spring would be ideal. MGB common rear tube shock conversion has 5" travel so with same mounting distances would use a 10" spring. An inch one way or other probably won't matter.

This is very helpful information. Thank you!

Thanks Jim. The shocks are at a 2:1 ratio with wheel travel, which is 8" so shock travel is 4" and spring length is 8". I think that falls inside those guidelines, so I'm not sure we are any closer to figuring out why it failed. Could we be at coil bind? I have no idea.

What I meant above is that the progressive rate on a shorter spring will be higher than it is with a longer version of the same spring. In other words, with any spring pressure will increase as the spring compresses. You can take two springs that start off at the same pressure, one short and one long, and as you compress them the shorter one will get stiffer faster. So the same spring only made longer would need to be made with thicker wire in order to avoid bottoming out the shock. Maybe that means the rate is the same. I guess if you are going from tested compression and pressure figures it would, and if that is how coil-over springs are rated the 350 lb/in springs in a longer one should be fine. But 25 lbs stiffer wouldn't do any harm either. If coil bind is the issue than longer springs should cure it. I have moved my car back over to the lift bay so I'll raise it back up and take some measurements to see if coil bind could be a problem.

Jim

Jim B and Rick, If you jacked up the spring too much to increase road height you could be at coil bind. I have experience with Pro street parallel 4 bar set ups and long travel off road buggies.Another problem may be initial shock setup length. Normally I have setup my shock length to be slightly higher than mid point travel at road height desired. For example if you had 6" travel I would set the shock length at the 3 1/2" level at desired road height. Hope the info helps.

Most of the wheel travel is in the downward direction from ride height so the shock would definitely be compressed beyond the midpoint normally. I'd say the travel is probably somewhere between the center and around 3" up and 5" down so at the shock, between 2 : 2 and 1.5 : 2.5 depending on the ride height. That should be OK, right?

I took some measurements on mine and got .7" between the coils, 8 spaces, .444" wire. That should give 5.6" of travel but again, depending on the ride height adjustment. My car sits kinda high right now but the RMs could have been adjusted up further. Over an inch and a half further I sort of doubt but I can't say for sure. It's had a lot more miles on it.

It also has rubber bump stops fitted inside the top of the springs which means there should have been no sudden banging at the end of travel.

Jim

Could just be a faulty shock or top bracket broke. I've used Chassis Engineering or Morrison rear set ups that used a standard shock(usually Gabriel adjustable) and QA1 coil overs. What brand are those?



Edited 1 time(s). Last edit at 07/21/2013 06:22PM by mgb260.

These are QA1.

The top bracket is a 3/4" tube welded through a 2 x 3 x 1/4" rectangular tube, sticking out a couple inches on each side. A long 1/2" stainless bolt runs through the tube and the shock's spherical rod end, mounting the shock in single shear across the full 1/2" shank diameter with the threads and the nut out on the opposite end of the bolt I believe. I know it isn't ideal and double shear would have been stronger but so far we don't know where the failure happened.

Jim

IF you look at the photos, the spring is adjusted VERY high up the shock.

IF there is coil bind, or even a slight chance of such, then all it would have taken is ONE instance of bind (Hard bump/ curb? etc) to have bound the spring and started a Bend in the shock!

And then it's just a matter of time before it bends more and more until... THAT.


I was thinking (perhaps incorrectly?) that the 12" spring *lighter* would then be cranked in more (to 11"? there is about 3" on the shock we could lower the spring to) to set the actual ride height, thus getting it up to the working rate of our (currently) shorter spring. whilst still having longer travel, less coil bind, less force on the shock rod, etc.

Did the Roadmaster get Corner Weighted at Omaha? And what spring rate would those suggest?


M

The coil-overs have 10" springs on them. At most the springs on the RM are adjusted up less than 1" above mine, which are adjusted up about 3/8" to 1/2" above free length. So with 5.7" of free space between the coils (from actual measurement of my springs) and a maximum of 4" of shock travel the coils would never come close to touching.

I think I can claim categorically now that coil bind was not the cause of failure. But even so, perhaps a 12" spring wouldn't be a bad idea. There is room for it after all, so if you guys want to go to 12's I have no objection. I'm not sure what the advantages would be.

Dave is looking at what it'll take for him to remove the shock. The spring could be preloaded by as much as an inch so he has to proceed with caution.

Jim

Looks to me as thought there ***might*** be enough room to decompress the spring on the lower end to relieve the pressure ***if*** the coil wrench will fit in the tight space available.

Watcha think?