Objectively measuring and defining subjective steering "feel"
Objectively measuring and defining subjective steering "feel"
(OP)
I recently read an article on the new C6 Corvette and noticed GM got high marks for many things, but steering "feel" was not one of them. Many of the BMW's, especially the previous (mid 90's?) model M3, are widely touted as having nearly perfect steering.
What is it about steering system design parameters that make that tiny difference and push a great car into the realm of the nearly perfect? Are there books out there on automotive steering system design? Are there experts who make fortunes at BMW, know enough to write the books, and are impossible for GM to lure away? How does one objectively measure and design in a certain steering feel, when it seems to be a subjective criteria?
What is it about steering system design parameters that make that tiny difference and push a great car into the realm of the nearly perfect? Are there books out there on automotive steering system design? Are there experts who make fortunes at BMW, know enough to write the books, and are impossible for GM to lure away? How does one objectively measure and design in a certain steering feel, when it seems to be a subjective criteria?





RE: Objectively measuring and defining subjective steering "feel"
An aside...the seriously overboosted brake servos of yesteryear seem to be dramatically improved! ??? Maybe I'm just getting old and lazy? Even my race car now has a servo!
Rod
RE: Objectively measuring and defining subjective steering "feel"
- feedback
- discernible play
- accuracy
- path accuracy
Others?
RE: Objectively measuring and defining subjective steering "feel"
Anyone else have additional specifics or analysis they care to contribute?
RE: Objectively measuring and defining subjective steering "feel"
BMW's new EPAS is getting bagged for poor steering feel, so I guess even BMW don't always get it right. They also made some slightly odd trade-offs on the X5 (apply full lock when parking. release the wheel, for example).
There are two sides to it, there's eliminating all the wrong things (friction and lash and other non linearities for example), and then there's doing the right things, which can be split into things we know about, and all the rest. Rumsford's philosophy applies, and there's an awful lot of unknowable unknowns around.
Important objective measures include frequency response of the yaw velocity/ SWA curve, and linearity of that curve against amplitude and speed, and the shape of the loop you get when you when you plot SWT vs SWA for a gentle weave.
You should also be aware that different parts of the handling spectrum have different requirements. On centre feel needs completely different evaluation techniques, and hardware mods, to limit handling.
And as one seasoned campaigner pointed out to me, 70% of the model on model improvement is down to the tires.
For what it is worth, in my whole car dynamics model, 50% of the code is tires, and 25% is the model of the steering system. That leaves just 25% for everything else. If you ask me, the worst (least representative) part of the model is the tires, and the next worst is a tie between the shock absorbers and the rack/PAS system.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Objectively measuring and defining subjective steering "feel"
-Joest
RE: Objectively measuring and defining subjective steering "feel"
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Objectively measuring and defining subjective steering "feel"
BMW seems to be able to balance ride/handling better than other companies. Maybe because they set the priorities from the beginning to have "The ultimate driving experience" not "Beat Viper". Design of the frame and subframes, mounting attachment stiffness and polar inertia are things that are hard to tune.
As for measurement, steering feel is just that. Feel. Different demographics have different priorities. Tuning is done by the seat of the pants and maybe some instrumentation to create arguments later. Vehicle dynamics engineers have to put their mind in "customer mode" and try to figure out what a typical corvette buyer wants from the steering. As soon as a magazine writer gets in one, they want razor sharp response and "heft". They typically drive tracks or short term, which may justify racier steering. More conservative daily driver types, may want lighter effort to park without hitting a curb and scratching those nice rims. Again, it's balance which takes alot of tuning time to sort out interactions and find the right combination of stiffness and comfort. BMW seems to have the edge.
RE: Objectively measuring and defining subjective steering "feel"
Regards
Sandy Cormack
RE: Objectively measuring and defining subjective steering "feel"
My feeling is that before the mid 60s the only tools for good "handling" were light weight, rearward weight bias, rack and pinion steering, reasonably balanced front and rear roll stiffness, and radial tires. There were way more clumsy, ponderous RWD cars than good "handlers."
It took the trans-am series to get Ford and Chevy to understand thoughtful camber curves, and to re-design the Mustang and Camaro suspension to incorporate them.
Nowadays every vocal owner of a RWD car is pretty snooty about how bad FWD "feels." When the Renault Fuego (FWD with the longitudinal engine hung out the front) first came out with nice feeling fast power steering, exotic Michelin tires, etc, Road and Track reported it had a real impressive slalom speed.
A Mustang with 54% of the its weight on the front end with nicely balanced roll stiffness may delight its owner/driver in dry weather. That's fine. Body Piercing, Cosmetic surgery and "reality" shows populated with young folks with snippy attitudes are pretty popular these days too.
Around 1983 an acquaintance who campaigned a Lotus Elan in SCCA for years was VERY enthusiastic about the VW GTI's street entertainment value. I asked if it was as fun as his Elan, and he said no, but.......
I bought a new 1984 GTI. Hey, with a responsive engine and steering, pretty decent tires, it was very entertaining, and very well balanced when cornering hard enough to set off the oil pressure light and buzzer. A friend described it like a frisky pony.
Empty, In the snow, my mom's Accord and similar vintage Camrys are almost precariously balanced between trailing throttle oversteer and understeer.
From time to time FWDrivers have ruled some and rallying and SCCA stock classes (Integra and Saab).
Ice racing classes often have separate FWD classes. I don't think that is to make the FWDrivers feel better by not having to compete with the superior RWD.
RE: Objectively measuring and defining subjective steering "feel"
which one is the best performing EPAS currently?
RE: Objectively measuring and defining subjective steering "feel"
Recollection of my first experience driving a FWD car included "stiffening up" of the steering under power such that one had to physically steer it back straight. Fortunately, things have improved considerably since, though there are still noticeable differences between different FWD cars that are probably the result of philosophical differences among mfrs. For example, turn-in with my Mazda 626 is better than that of the Maxima by more than can be explained by differences in weight and tire size.
Norm
RE: Objectively measuring and defining subjective steering "feel"
And of course the Lotus Elan II did manage exceptional handling and reasonable steering feel.
The GTi has always had fairly bad steering feel, in my book. Quite why anyone would use it in this thread is beyond me, unless it was to define what lifeless, heavy, innaccurate feel is all about. They might as well have stuck 100% power assist on it and used a spring to tell you where the middle was.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Objectively measuring and defining subjective steering "feel"
Take one moderate to high hp FWD, add limited slip differential, mix in about 5 degrees of positive caster, select highest reduction gear, remove hands from steering wheel and apply power...voila!...instant center.
Rod
RE: Objectively measuring and defining subjective steering "feel"
Are you talking about the original 1984 VW Rabbit GTI? If so, I'm surprised to say the least- my subjecitve opinion (is there any other kind?) is that they had excellent steering feel. I wonder if they came spec'd with differnet tires where you are.
RE: Objectively measuring and defining subjective steering "feel"
To be honest, this is an area where RWD snobbery does have a place, a RWD car can have all sorts of nastiness in its front suspension, and it will have good steering feel, whereas an AWD/FWD will almsot always compromise its steering feel.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Objectively measuring and defining subjective steering "feel"
I repectfully request opinions on the steering "feel" of these examples - any GM or FORD, or even MOPAR full sized or intermediate vehicle with power steering made between 1955 and 1978. I think there are a few RWDs in there.
RE: Objectively measuring and defining subjective steering "feel"
Rod
RE: Objectively measuring and defining subjective steering "feel"
Bad examples prove little. The market apparently wanted effortless steering, so that's what they got.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Objectively measuring and defining subjective steering "feel"
I'm sorry.
RE: Objectively measuring and defining subjective steering "feel"
It may even be a deliberate choice.
Typically I find AWD cars have lousy steering feel, but are quicker, thereby demonstrating that the ability to feel what the tyre is up to is not necessarily the most important thing.
One of the more reliable indicators that you are approaching the limit of the tyre is that the SAT gain drops off, or in other words, the pneumatic trail reduces. But you should bear in mind that through the wheel you are feeling the total force due to both tyres, so the inner tyre can mask what the outer one is up to.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.