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Roll Centres, Front to Rear, my learnings and Jaguar E type calcs

Roll Centres, Front to Rear, my learnings and Jaguar E type calcs

Roll Centres, Front to Rear, my learnings and Jaguar E type calcs

I have been digging deep into vehicle dynamics in general and Roll centres inparticular.
My OEM background has had me deliver various vehicle projects where Ive had a good overview but Im now digging in technically deeper. I intend to learn and also apply these learning to various project cars for different purposes. I'll cover what my key take aways have been and then share the roll centres I have (mostly from this very informative thread: https://www.eng-tips.com/viewthread.cfm?qid=431615) and share my E type data. I picked an E type because, it generally has a simpler set up than modern multi link cars- so is easier for a green person like me to get my head aorund and because I own one ad can drive it , compare and get it on a lift and double check stuff.

These are some of my take aways- please feel free to add and comment as Im eager to improve my knowledge:

•The Steering axle benefits from low RC , long equivalent control arm lengths result in steering predictability due to minimal track change, minimal RC height change , minimal camber change etc

• High RC at the rear controls roll/yaw coupling and having a higher RC at the rear stabilizes the vehicles by causing yaw in the opposite direction to the direction of turn as the car rolls

• TLLTD at the front builds progressively with body roll. Higher rear RC means lots of nearly instantaneous geometric weight transfer from the rear end, causing transient overtsteer that compensates for other perceived sources of steering sluggishness (improves turn-in)

• Raising front roll centre improves on-centre steering feel

- A School of thought- as shared by Gordon Murray and practiced on the Maclaren F1- was to have the distance between the C of G and Rc the same front to rear, and keeping the Rcs both low.
By having Rcs higher rather than lower (or zero) a larger part of the TLLTD will be geometric . With lower Rcs more load will be transferred via the suspension system itself.With zero Rcs the load transfer will transfer immediately via the tyre contact patch while the rest of the Wt will pass through the springs and shocks. In addition zero Rc allows the lionshare of the vehicle dynamics tuning to be done by fine tuning the suspension. Im not sure how I feel about this one- I have no direct experience of a zero Rc (frt and rear) vehicle and suspension tuning it.

And finally

The E types roll centres seem rather high- 280 mm at the front and 146 mm at the rear. This seems weird to me-with high Rcs in general and the rear being lower than the front

Can anyone corroborate this?

Ive never pushed my E type to the limit, I know it has close to equal weight distribution, and have never heard of evil handling characteristics. The steering is very 'talky'.


Sideways To Victory!

RE: Roll Centres, Front to Rear, my learnings and Jaguar E type calcs

280mm front roll center height does not even seem plausible. That's swing-axle roll center height. Double-check your geometry.

I know only as much about Jaguar E-types as I can look up on google, and from what it looks like, it appears to be a pretty conventional upper-and-lower-A-arm arrangement with the arms more or less parallel and horizontal at nominal ride height, and that's a recipe for having a roll center somewhere near ground level ... which would be pretty conventional on a vehicle like that.

Also keep in mind that high roll center + independent suspension design = jacking. Jacking + soft spring rates = badness.

RE: Roll Centres, Front to Rear, my learnings and Jaguar E type calcs

Data I have on your Jag indicates front roll center at 60 mm and rear at 150 mm (Force based, measured on K&C rig).

A few things to clear the air.

Setting the roll axis requires knowledge about the tires, wheelbase, weight distribution and sprung mass inertia (dressed with powertrain, battery, fuel, passenger(s), glass, etc.). Then, figuring out the cornering compliances (tires, weight distribution, kinematic and compliance parameters factored into net front and rear axle sideslip gains (deg/g), you compute the yaw velocity (yawrate) natural frequency and damping metrics. These are speed dependent, the faster you go, the less damping and the higher the peak frequency becomes.

Next, you figure out or measure the sprung roll inertia. Knowing this and the parallel axis theorem, figure out what the total sprung roll inertia needs to be, given the roll gradient you desire, so that the roll natural frequency is lower than the yawrate frequency. This prevents the yawrate frequency from hooking up with the roll frequency ('Q') to animate any roll yaw coupling that you have in the body or chassis. Things like roll steer and roll camber and load transfer are the culprits here.

Now that you have a roll axis height at the sprung mass cg location, you figure out the load transfer you need to produce a TLLTD you need or can live with. This establishes the load transfer delivered by your suspension and springs and bars and track and tire Fy, and Mx, etc. The kinematic roll centers deliver the load transfer first (suspension path, followed by the elastic components delivered by springs, bars and roll inertia. So, low roll center at the front with matches with a reasonable roll bar library given the instant centers intended to produce camber changes during steer and cornering force application. This gets you easy track deltas because bars are cheap and often incrementally adjustable. Not so with a spring library. Spring increments are usually pretty big and cost a lot. For production engineering, you might want a Level 1 handling vehicle with no rear bar for cost and mass savings. So, your rear roll center is set to deliver this 'feature'. Yes there is the danger of jacking force contributing to rollover. Yes the low roll center drives a lot more lateral force into your control arms, (and that's what you signed up for), so changing horses in the middle of the stream may break some parts if you lower it a bunch after the fact.

Low is fast load transfer, high is slower load transfer. A vehicle can feel 'lazy' if you lower it to much, as the distance between the sprung cg and the roll axis height gets too large.

Of course, changing the roll centers and roll axis can be quite a project. You need new attachment points in the chassis where structure may not exist to support the loads. Breakage and compliance and wreckage often result.

My first response to this thread died somewhere or was removed by a Propeller Head. Maybe this one will survive !

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