Jaguar IRS question

[Richard85ZXT]

I'm still new to all of this, and since the people on here obviously know what they are talking about I'd rather learn from here, so pardon my simple questions.

Anyway my question is in the early Jaguar IRS they used 2 springs/dampers on each side, so a total of 4 in the rear alone. I haven't found a good explanation as to why this is. I've heard the longer a spring is the more comfortable the ride is, however it can create roll. I'm wondering if this reduces roll, while the 2 combined lengths of spring help with comfort. I can assume all day long, but would really like to understand it properly.

 

[LionelHutz]

Is this to replace the suspension in your Nissan?

Maybe that spring/damper configuration can be shorter (not protruding into the trunk as much) or they don't impart a twisting force into the lower link?

 

[Richard85ZXT]

yep for the Nissan, I'm thinking a double wishbone/multilink rear suspension is the simple option to make a big increase over STA without having to tear as much apart to do so. I'd like to make the upper control arm adjustable to adjust camber should I decide to, and have the lower wishbone connect in 2 locations, as if the point of the wishbone was cut off, and also allowing adjust ability to adjust Toe. I still need to see if this setup will work, but it's still in theory now.

I'm trying to figure out the Jaguar suspension so that I can determine if 2 coilovers will give me the benefit of a smoother ride with less body roll. My thought being I can go with a bit stiffer springs without sacrificing ride quality as much. But that is again if that's why Jaguar used 2. I have read they used 2 short ones rather than 1 long one for trunk space, so I was thinking of utilizing the principal for a different purpose.

 

[NormPeterson]

I imagine that it was done for stability of the mechanism purposes.

A single spring/damper would load the lateral arm in torsion, as would offsetting the lateral arm relative to the axle line. Ultimately, the axle end would wander a bit longitudinally (once you consider bushing compliances at the chassis end of the arm). The half shaft performs a lateral location function only for the top of the upright, so it is of no help.


Norm

 

[gt6racer2]

My suspicion would be purely spring packaging. To get the desired spring rate and travel within the available package space a single spring would be too highly stressed. The two springs does, however, give a nice way to get the spring force around the drive shaft.

 

[Richard85ZXT]

That's my guess as well, the priority was trunk space and so that's how they combated the lack of space.
So if I have room for a longer travel, couldn't I utilize the idea of twin coil overs to retain a smooth ride with a stiffer spring rate, or would twin coil overs not be beneficial in a performance environment? I'd like to retain a smooth ride for street use but also be able to handle properly in a performance environment. I know it's often a trade off, but I'm trying to come up with less trade off and thus more benefit.

 

[evelrod]

You can try a variable rate spring with some adaptation of "intelligent" dampers. A compromise is the best you can hope for. You can never reach the level of road holding of a race car with ANY streetable setup. Lots of cars get close and if that is what your after...

What's the problem with a careful makeover of the Nissan's suspension? Jag rearends have been "done to death" in Socal. I'm not a fan.

Rod

 

[Peter7307]

There were two main reasons for the Jaguar arrangement:

The first as Lionel Hutz suggested being a flat floor for the luggage compartment which was a complaint from some about the previous car.

The second was to allow the rear subframe to be installed and removed as a unit since the coil over dampers were fitted to this and not to the body.

The E type (XKE in the US) used the same arrangement for basically the same reasons.

Peter.

 

[Richard85ZXT]

Rod, the Semi Trailing Arm on the Nissan is really inadequate for real performance use. I would like the car to be comfortable on the street and at least confident on the track. I'm not planning on using the Jaguar IRS, but I'm wondering if the same way the Jag IRS uses twin coil overs, if I utilized twin coil overs, with longer travel, with a double wishbone suspension if I could achieve a better compromise.

I suppose my question is less why they used short ones as much as why 2. Would 2 coil overs give me the benefit of a softer ride with less roll. I'm specifically working on my rear suspension right now as I feel the Semi Trailing Arm is not all that great for even the weekend track racer.

 

[GregLocock]

No, any given rate and travel can be achieved with one spring, if you can package it. There are some notional advantages to using a twin coil-over approach, but I doubt there is any practical advantage.

The fundamental problem you face is that for racing your shocks want to be about 70% or more critical damping, whereas on the road 35 or so is more like it.

If you want a double wishbone IRS then the Mustang Cobra IRS modules are around $1500 or less from a breakers and are not a bad starting point, although personally I think they went a bit mad with the axis of the UCA. I'd use the lower arms and hubs and diff and fab a new upper half.





Cheers

Greg Locock


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[Richard85ZXT]

Thanks Greg. I know compromise will be tough, but new cars and even Miatas can be driven daily and then brought to the track on weekends for fun and simply embarrass people who have spent time and money on their cars. I could go out and buy a miata for track days, but that's not the point. I like my Z, I enjoy driving it, but most important is I love a challenge.

For the rear, wishbone may not be the ideal choice but I think it would be for both street and track use, or at very least a HUGE advantage over Semi Trailing. If there is a better option for mainly street use please let me know.

Greg, just out of curiosity what are the notional advantages to twin coil overs? I had looked into the Mustang Cobra IRS actually. When mentioning it to my cousin who is a Mustang fanatic he said a lot of Mustang guys get the Jag IRS because it's easier to come by, which is what led me that. I do prefer the Mustang IRS over the Jaguar one, though as I recall they chose mounting points over function. They designed it to mount to the existing mounting location on the mustang chassis, even if it was not ideal locations. I believe that is why it looks a bit mad.

Thanks again.

 

[evelrod]

Richard, I fully understand your desire to "have your cake and eat it, too". A lifelong problem with many of us motorheads. ;o)

However, as in your example of a Miata...Yes, I see them every race weekend. Yes, for a stock car, they are fast and handle reasonably well. Still, they are a compromise...
For street they are fun and a bit rough around the edges, depending a lot on your age and ability to accept the ride qualities...rather, the 'lack' of ride quality. I like them, but then I drove a '60 Sprite on a 3000 mile trip and rode a motorcycle to Mexico City and back...I'm not a kid, either.

A stock Miata laps most tracks at about the same times as my '63 Mini Cooper with an all up wt of ~1700 lbs. and an engine of only 1300 cc's (a vintage pushrod 5 port, at that). What I mean...The Miata, while a great compromise, it is still a compromise. What makes racing Miatas so impressive is the fact that there are so many of them racing against other stock Miatas. A fully prepped EP SCCA Miata is probably a good ten seconds plus per lap quicker...but...it would be a lousy/illegal street machine.

Trust me on one thing that I've learned in the last 50+ years (often the hard way), you can have a great race car or a great street car, just not both in the SAME car. The old cliche...
Fast, Reliable, Cheap...Pick any two!

Rod

 

[Warpspeed]

Another fairly effective and easily mounted rear suspension worth a look, might be the Porsche 928.

http://i144.photobucket.com/albums/r166/Warpspeed_photos/928suspension.jpg

 

[patprimmer]

Early 90s Toyota Cressida/Supra rear ends are also upper and lower wishbone on a separate sub frame.

Regards
Pat
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[colinmseries]

Can I join in please, as this particular question covers some issues I've wondered over many times.
I too have heard the reason given as being packaging - the Jags were often criticised for a lumpy boot floor.
Norm also mentions that a possible reason was the more equal distribution of the spring forces about the rear upright's centreline. This is an additional point that I've wondered over myself.
My own car has the delibrate facility built into the chassis for a similar doubling up of the springs about the upright, though it was never originally used.
What in your opinions would be the benifits of doing so from a force perspective? My car is a SLA rear setup and I've puzzled over where the dominate forces are and how symmetry would make any difference.
Thanks, Colin.

 

[BrianGar]

Warspeeds rear axle link is a good one, > Weissach axle, very clever at the time.

BG.

 

[Warpspeed]

As Greg Locock says, one spring can (almost) always be selected to give exactly the same stiffness rating of two springs,

However, that is not true of the dual hydraulic dampers.
On the Jag, these are fitted at a rather extreme angle for packaging reasons, and doubling up on these may be an attempt to reduce the detrimental effect of this fairly radical mounting angle.
Twice the piston area and twice the fluid displacement, is an interesting solution to the rather inefficient motion ratio of the dampers.

 

[evelrod]

Whatever Jags "bottom line" was in engineering the rear suspension layout might have been, it certainly was NOT with any degree of consideration for maintenance.

Rod

 

[Warpspeed]

Rod, that seems to be true of the entire car !!

 

[patprimmer]

We had a saying here about Jags way back when.

It was that you needed two. Every week you went to the dealer to swap them over. Later we changed it to BMW

Regards
Pat
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