Chassis Setup for a Jensen - Its Welded

Ron Earp

Ron Earp

Administrator
Looking for some hard and fast recommendations as a baseline for the Jensen chassis setup. Bear with me, info will not be complete and I am sure I will miss something, but work with me here - especially the guys who have good experience.

The car weighs in at around 2015 lbs near as I can estimate with careful attention. Race weight is 2240lbs so we'll be over it a little with me in at 175lbs.

The wheelbase is 92", front track is 53", and the rear is 52". Overall car length is 165", sort of long.

Weight distribution is unknown, but was published about 51.5 front and 49.5 rear. Cage has probably centralized it some more.

Front suspension has upper A arm with a lower arm, shock and spring inside there. Travel is limited with the car lowered, maybe 1.5" to 2" til it hits the upper stop. Lower arm is located with a 5/8" diameter threaded rod that goes back at a 45 degree angle to the frame. Camber is not easily adjustable, but I think we'll be able to rig to get whatever people come up with for suggestions.

Rear is a weld axle, drum brakes with trailing arm four link suspension. The upper arms now have air bushings and a pan hard rod has been done on the car.

Bushings are mostly delrin assemblies with some stock new rubber pieces on a couple of places.

There are sway bars, steel, about 5/8" inch thick (cannot rememebr right now), front and rear. No adjustment in them right now, but we can obviously disconnect them.

So, in a way, it is sort of like a little bit lighter Z car. Power at this point is unknown, but stock the 2L voodoo motor was rated at 140hp. Diff ratio is about a 3.75 with few other choices. Close ratio rare Getrag 5 speed. Tires, at the moment, are some 225-45-13s crammed on some 6" Panasports.

I'm looking for some ideas on spring rates and setup. I have ordered some 425 lb springs for the front and have some 250lb springs in the rear. But, I think the car is going to want to push with the welded rear and need some setup advice to minimize that as much as possible. As a rookie brought up driving a fairly good driving Z and Miatas I've a steep learning curve I realize, but with some advanced help from you folks I will hopefully get off to a decent baseline setup.

Thanks,
Ron
 
castrolhansen06.jpg


Hasn't everyone built one? :)
 
Well, you won't get anything super useful from me, but some comments.

1-2015 + 175 = 2190, so you will be a little UNDER, not over. Lucky dog.

2- Get yourself that $1000 scale set up...with all the cars you guys have it will get plenty of use.

3 Put the ballast where the scale tells you to, wink wink.

4- I think you're missing a link. In the rear you say the upper bars of the 4 bar are "air bearings". Have you added an adjustable traction bar for upper axle location?

5 Is the panhard stock? or fabbed? What is the geometry on it?

6 Your front suspension travel is tight, you'll need stiff springs and good damping to keep from hitting the "infinite spring" bump stops. AKA "instant understeer"

7 I would do some real measuring of pickup points and try to get a handle on the geometry and roll centers and such. That will help you choose the best solution for the system.
 
Some comments.

1. Damn. I am actually good at math - calculus, EM field equations, and what not, but I'm bad at arithmetic since I cannot add. So I can eat a little more now.

2. RTP money could be well spent here Jeffy....

3. Hmmm...

4 and 5. Four links. Two trailing arms on the axle down low, two upper links on the pumpkin to locate it. Those two have some air bearings (air bushings, brain damage). Panhard rod is a fab piece and connects very low on the pumpkin.

6. No joke, it is hard to work in there as is. I have some 650lb springs too.

7. Will be done here soon with some Carol Smith chassis arithmetic done.

I called JensenWorld, but nobody picked up the phone.

Andy, where is that picture from and what is it? Is that the fabled ITS JH in the South that never ran much and was put away? Looks like it needs some serious chassis development with that mean lean going on.
 
I think you are going to need high front spring rates (but maybe not 625, which was my call in the first place right) to keep it off the stops and then tune out the understeer. No front bar? Soften up the shocks?
 
OK, by air bearings I take it you mean that you have rendered the stock bushings useless as the geometry of the new set up and ride height created interferance?

If I understand the setup, you have some issues with location. What will stop the axle from twisting about it's longitudinal (that is, hub to hub) axis? The upper links used to do that, AND they were doing lateral location as well. (I bet they were angled outward, so your axle may have had some toe change characteristics under roll as well, but ...)

But now your lower links hold the bottom, and there is nothing holding the top from moving fore and aft. I would go to Gforceengineering's website and read about the tri link. Jim Susko is a ex GM chassis engineer, free lance engineer and racer, who has designed and sold parts for race cars based on real live math and calculated chassis dynamics. The RX-7 and the Jensen rear end are somewhat similar. He doesn't have anything for your car, but might be able to save a ton of your time by sending you in the right direction. heck if I were you I might want to pay him a fee just to do some math, LOL. (I own an RX-7, so his math cost me $75...cheap in the long run, LOL)

I'm thinking you need to add an upper link of the proper length, ala the Trilink, but that length is critical.
 
Oh, at 625 on the front springs you will need some serious dampers, and the rear will need some stout springs too.
 
I don't believe you can accurately determine spring rates unless you know the proper motion rate for the suspension. IE.. How much of that spring's rate is actually being used to control the suspension? A 900 lbs front spring on my 68 mustang is going to act far differently than a 900 lbs spring on my ITA mazda.
(I'm a finance techy guy, not an engineer or suspension guru, so my terminology is likely not copletely correct)

marcus
 
I'll be checking into the upper arm thing here. It is no problem for me to add another bar, or, to use existing bars with bushings.

One thing I am concerned about is not only the spring rates, but, setting the chassis up to deal with the effects of the welded rear.

Specifically - what on a Z car changes to deal with a welded diff? I'd like to know a baseline 240Z setup with spring rates and roll bars, and then a setup for a Z car that would account for a welded diff. Having this sort of comparison would be useful to me which is why I was asking trying to get some of the Z gurus on the thread.

Ron
 
I'll be checking into the upper arm thing here. It is no problem for me to add another bar, or, to use existing bars with bushings.

One thing I am concerned about is not only the spring rates, but, setting the chassis up to deal with the effects of the welded rear.

Specifically - what on a Z car changes to deal with a welded diff? I'd like to know a baseline 240Z setup with spring rates and roll bars, and then a setup for a Z car that would account for a welded diff. Having this sort of comparison would be useful to me which is why I was asking trying to get some of the Z gurus on the thread.

Ron
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Ron you can't just compare weights of this car. This is a 2 a-arm front where is the spring mounted? On the rear were are the springs mounted how far inboard are the shocks and do they have any angle. The best way to get to where you want to be is likely just to call me next week in the shop. i am out of town at a national right now but I will be checking in daily.

If the spring is in the control arm I need the length of the control arm from the pivot to the ball jont and the the distance from the ball joint centline to the center of the spring.
 
Have you tried the other JH (Joe Huffaker) to see if they have any setup info from when they campaigned one in the 70s? I'm guessing they did not not do a prep manual like they did for the MGB
 
I did, about a year ago, but it was sort of vague info and some didn't apply due to changes they could make and I cannot.
 
Because of lever arm considorations and such if you do not have the math to compute the spring rates does it make sence to measure the stock springs and at least start with the same ratio of spring rate front to rear.

i am not too smart but I would take my best guess and then buy springs on either side in the rear, if those are the easiest to change, and test the car.
 
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