Roll steer and pitch

[Cinerous]

How could you have roll understeer without introducing changes in toe during pitch in a double wishbone suspension? Can that be avoided or do you have to deal with it?

 

[GregLocock]

You can. I don't like it, but you can use the sta bar to steer the knuckle, by angling the droplink and terminating it on the lower arm (I particularly dislike this) or spindle so the line of action, viewed with the steer axis in end view, has a moment arm around the steer axis.

However small amounts of bumpsteer (as toe in pitch is more usually called) don't cause problems and usually give enough roll understeer. Most cars these days also rely on a fair bit of compliance understeer, which can lead to subjective problems.

Here's the understeer budget for a large RWD car with reasonable steering/handling. Notice how little roll steer they use (problem) and how much compliance steer they use (problem), that's all the stuff in italics, as most of steer/mz is compliance as well. So >80% of the understeer is compliance related

************ T.I.N.ADAMS: This Is NOT ADAMS***********

Greg Locock v1.0 24 Jan 2020

TROLLpg = 5.6580
SROLLpg = 4.4717

Understeer Budget Front Rear

Weight/Tires 3.2 -3.097
Lateral Steer Compliance 0.8357 0.1696
Lateral Camber Compliance 0.04074 -0.03829
Steer/MZ 1.265 -0.1472
Camber/MZ 0.001151 0.0009279
Roll Steer 0.004472 0.2459
Roll Camber 0.1129 -0.09972
RBAT 6.765e-005 7.192e-005
Cornering Compliance 5.46 -2.966

Understeer 2.494

Cibachrome should recognise this! here's the original thread

https://www.eng-tips.com/viewthread.cfm?qid=463747

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Cinerous]

Ok I understand the sway bar idea though I have never seen that being done in practice. Sounds like a nightmare to me

Sorry for being a noob but I am just not sure what those numbers mean, e.g. Lateral Steer Compliance 0.8357 0.1696. Is 0.8357 the variation +/- of steering angle in degrees at the front due to bushing deflection? In Roll Steer 0.004472 0.2459, is 0.2459 the variation +/- in toe for a rear wheel over the range of motion of the suspension?

Thank you so much

 

[GregLocock]

That table is called a Bundorf cornering compliance budget. Here's an article on it.

https://en.wikipedia.org/wiki/Bundorf_analysis

. The first of each pair is the understeer contribution from the front axle, the second is the rear. They add. Then at the end we add the whole lot up to give the vehicle understeer.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[cibachrome]

How about using a "true" tierod connecting both steer arms cross-car. Then use a single tierod from the gear to push and pull on the knuckle/upright. In pitch/jounce there is very little toe change because of the LONG tiebar, in roll, an offset steering gear will create the roll steer, with it's sign determined by the front view angle. My lifted golf cart has the 'feature'. Otherwise you wear out front tires on pavement in a few seasons with it's very soft springs. I don't play golf, just use it around the farm and neighborhood roads.

 

[TMAPV]

Greg, why do you say using little roll steer and a lot of compliance steer is bad? Aren't there inherent issues from roll steer due to the delay between steer input and roll (and therefore roll steer)? And how does this compare to compliance steer? In the end both require lateral force build up to produce steering effects

 

[GregLocock]

Compliance steer is a subjective rubbery feel in the steering, caused by excessive compliance steer(!). Unfortunately I have never been able to define an objective metric that correlates to Golden Bum's subjective comments.

Roll steer is admittedly a two edged sword. If the roll and yaw responses get out of phase then you end up with 'threepenny bit', or two stage cornering, where you have to re-adjust SWA once the roll angle settles down.

https://en.wikipedia.org/wiki/Three...edia/File:British_threepence_1967_reverse.png

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[cibachrome]

Compliance steer due to tierod forces (hence MZ steer + caster induced compliance from FY) is the source of rubbery feeling. Also contributed by a soggy steering column isolator and large angles on the intermediate shaft. I'm no fan of front roll steer because of the convolution issue, but rear roll steer has use for vehicles with large delta payload capacity (trucks, lots of passengers, etc. This because it will be used to add understeer to the vehicle hence a higher natural frequency and resulting quicker response times. Easily defeated by means of Level Control systems, either automatic or manual specification. For example: a compact car on fuel economy (low cornering stiffness) tires is fairly decent with 5% rear roll steer which increases to 15% at 4 passenger + luggage in the boot, full gas tank. Needless to say, on a 60 psi high pressure compact spare tire on the rear that's 3 years old and is now at 30 psi, it can mean the difference between getting home OK or not getting home at all.