October 23, 2021 at 9:24 am #219827
Hello everyone I have a desire to have my (1998-2002) X308 XJR ride and handle like an (2000-2005) E39 BMW M5. Let me preface this by saying that I’m NOT interested in buying an E39 M5: The Jag stays, I love the looks and style of the Jag and it represents a special time for me- from when I worked for Jaguar. The BMW rides very well over the terrible broken surface mid western roads over here but remains very composed and taut. I never cared for how the E60 M5 rode and IMO I thought BMW hit a highpoint in terms of R and H compromise on the E39 M5. I drove one again recently to see if my memory of driving one (during the development of the Jag XJR at Whitley) had drifted but alas I found it as incredible as I found it then. The Jag is very soft, -which is fine, but it seems to lose composure over bad roads and at 9/10ths, in addition, if I boot it hard to kick the tail out and then catch it, the ‘wallowness’ makes it degenerate into a drift in the other direction. Very annoying. The only other car that I remember that used to do this was my 1970s BMW E21 323i.
X308 XJR steering is delightfully quick but lacks ‘feel’. The BMW has GREAT steering, its slower ( about 2.7 turns LTL vs 3) and it also still utilizes a steering box! Someone suggested a slower rack may transmit more steering feel- but going to a slower rack is not an option for me!
I managed to change the servotronic ZF relay in my Jag to one that lowers the assistance level more at higher speeds. The difference is barely perceptible
I had the E39 M5 spring rates sent away to be measured. The values were supringingly soft. When I factored in the vehicle corner weights, unsprung mass, the motion ratios- I’m getting 1.34 Hz ride freq at the front and only 0.95 Hz at the rear. This is contrary to flat ride theory so it has me miffed.
Torsional rigidity comparison: 13,000 Nm/deg vs 15,000 (BMW claimed 22,000 at the time but we tested this at Whitley and found different results)
Jag less than 52% front weight, BMW close to 50%
Unsprung masses surprisingly similar! (100-120 lbs)
Dynamic index: This is a concept a colleague and vehicle dynamics/R and H expert taught me about. He said that all the vehicles well tuned for great dynamics on the street and track have a dynamic index of 0.9-1. Ive attached a diagram of what this is. Suprisingly both the Jag and the BMW are inline with each other.
Roll centres – I will be looking at that next and comparing
Toe settings- I’ve heard that increasing tow out can increase on centre steering feel
Castor, traditionally BMWs have used a lot of castor- between 5-8 degrees, while Porsches have used up to 11 degrees. The Jag is at about 4.5 (between 3-6 degrees). I could target a value of, say 9 degrees . I don’t know how adjustable this is- and may have to resort to custom parts.
Raise the front roll centre- this is probably unwise as it has other handling implications….
I am unable to get the OEM damper rates- but I was able to get hold of SOME for the previous Jaguar X306 inline 6 XJR shock data (bump and rebound at various velocities). I know for a fact that the previous inline six XJR was set up stiffer than the X308 V8 XJR I have.
I have found a place that will make fully adjustable dampers (for low speed and high speed bump and rebound) for my Jag to my baseline curve.
In light of the almost unbelievable spring rate measurement data I got about the M5, I may look to get the X308 XJR springs measured also.
I tried estimating the rate using various methods- using the properties of steel, number of coils- (assuming number of active coils etc) looking at the front to rear motion ratios, looking the amount of spring travel while knowing the corner weight. I worked out the unsprung mass component by component. None of these methods correlated to one another and for the BMW none correlated to the measured data.
Next steps will be trying to assess the roll centres front to rear- comparing the Jag to the BMW (although with my terrible results in trying to calculate the E type roll centres (see my other recent thread) I’m a bit worried, and then get the stock Jaguar XJR spring rates measured.
How do I measure the roll stiffness of my Jaguar XJR?
I’m open to any other ideas.
October 23, 2021 at 9:44 am #219828
Couple of things (I actually prefer the E46, BTW):
By messing with the ZF to reduce the boost, you added to the net steering compliance because you lowered the gear stiffness. So, you lowered the steering gain because you increased the understeer contributed by the steering system. This adds load/torque (and thus even more compliance) because other parts are worked harder. Since steering parts are always nonlinear (soft then hard), I don’t ‘feel’ you accomplished what you want. In fact, gone the other way.
The Bimmers use caster offset as well as caster to accomplish their steering feel goals. I’ll let you guess what their knuckles/uprights look like. This can make front driveshaft action quite a mess if you also desire 4wd.
BMW uses tires with VERY SPECIAL FY and MZ traits. Their real OEM tires have a circled star on the sidewall. Replacement tires installed by an official BWM dealer may not even have the star tires if your experiments require several sets of replacements. The car(s) will be different even though you changed nothing in the suspension, steering, chassis, pressures or rims. BMW’s are best on the Contis, BTW. You will need to guess why.
You need to know that Jags are lame because they need to have large (i.e. stiff) tires for their total vehicle weight because of all the other sponges in the suspension. This gives them poor transient response because they even have to mount the steering gear in Jello to give it some undertseer for GVW load, replacement tire issues and spare tire use, you know, SAFETY reasons.
Get a VBOX or a yawrate gyro and a speed signal and run a fixed steer test over a speed sweep and you can measure your car’s steady state properties in as many conditions as you can think of. Heck, I’ve even done it with my BOAT !
BTW: you may need a good ‘haptic’ driver to make sense out of all this. Greek word for FEEL. If you don’t have a driver who is “better than the car”, move on to the next project.
October 23, 2021 at 10:11 am #219830
Maybe you better think about a new car that will be more powerful and faster? I now devote a lot of time to sports betting, so I expect to buy a new car by the end of the year. If you want to try it, then I advise you to visit https://www.dimers.com/best-books to find a good bookmaker, which is very important because there are a lot of scams now.
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