17 replies to this topic

[meb58]

After constructing a couple of crude models I was struck by the relationship between caster angle and camber. It is not a surprise that caster aids camber gain but there appear to be some caster ranges that produce very aggressive changes in camber and as such affect other suspension elements - IC, RC, and CofG as the wheel is turned. I began to wonder about some of these extreme anlges might conspire to adversely affect CofG during transient manuvers...

In addition to this sobering moment, I was struck by how much caster a Mac Strut and an SLA set can actually use; I recently heard an indivdial suggest, "10 degrees of pos caster is common on todays cars..." That feels like a mighty powerful caster angle for an SLA setup but perhaps not for a Mac Strut...and surely, the distance from ground plane to the lower ball joint influences any camber gain? I haven't quite figured out if it is this distance or the angle of the LCA that is important - I understand the rational behind horizontal LCA but the real world is an imperfect place.

[cheapracer]

KPI plays a big part in it too and struts need to clear those large diameter springs needing KPI to do so and caster to keep the camber.

Greg, ever played with KPI that angles inwards from the top (top pivots further out than the lower pivots)?.

Edited by cheapracer, 02 March 2010 - 15:44.

[GrpB]

ever played with KPI that angles inwards from the top (top pivots further out than the lower pivots)?.

Semi-related/similar, I played around with negative castor with large KPI and large mechanical trail for a (gasp) drift car, figuring the rear controlled the slip angle but the front was the real constraint for actual cornering force so negative camber during countersteer would be better than normal geometry as long as there was adequate aligning torque to sense the front tire limit. Subjectively very interesting but as you significantly diverge from conventional geometry it is difficult to know what is a big step and what is a small step (comparitively) as each parameter change affects others.

The Audi SLA double balljoint (upper and lower) system could have played around with these parameters more because of the imaginary KP axis which varies with steering angle, but the variant I looked at long ago was convetional in geometry with the double balljoints more for package than anything else.

[meb58]

Cheapracer,

Am I wrong to assume that camber and SAI move in unison? I think I just relaized that I should be very specific within this group if I am to receive any help. Anyhow, my brain sees camber angle affecting SAI/Kingpin 1:1 for a Mac Strut or SLA???

It was somewhat interesting to study the front strut in my 2005 mini...there was barely enough room to get a finger between the strut tower unibody area adjacent to the spring. And still, the strut body incorporated an indent to clear the tire. Very tight stuff front drivers...

I also see caster as an anti...sort of...at least over the outside front wheel and as such has the potential to do funny things to shocks and springs during a turn...I cannot imagine driving a car with lots of caster and lots of anti dive - front driver. If my logic about anit over the outside front wheel is correct then the exact opposite is tru for the inside wheel...perhaps this is a wonderfull setup for a highway truck?

[Greg Locock]

re the Audi - I suspect they use DBJ UCA to stop the steering axis wandering around, which it does if you just have a DBJ LCA. Th main disadvantage of that is that the steering tends to stay on full lock even if you release the wheel, and some subtle effects to do with phasing(SWT/SWA) on higher speed turns. These can be resolved by increasing the trail but that has its own downsides.

[Fat Boy]

re the Audi - I suspect they use DBJ UCA to stop the steering axis wandering around, which it does if you just have a DBJ LCA. Th main disadvantage of that is that the steering tends to stay on full lock even if you release the wheel, and some subtle effects to do with phasing(SWT/SWA) on higher speed turns. These can be resolved by increasing the trail but that has its own downsides.

When I first saw that setup, I thought it was the bees-knees. Then I tried laying it out to give me some sort of advantage, and never could even remotely justify actually using it.

[Greg Locock]

When I first saw that setup, I thought it was the bees-knees. Then I tried laying it out to give me some sort of advantage, and never could even remotely justify actually using it.

Even the DBJ in just the lower arm is frankly more of a packaging advantage than any great shakes geometrically, so far as I can tell. For a driven axle it makes sense, but for a non driven axle I'm not a big fan. Of course, if you design your suspension with it in then going 4wd later, say for a hybrid, is a doddle. Oh, it is pretty light as well, you have two tension compression links instead of an L arm, but you could do that the old MacPherson way.

[GrpB]

re the Audi - I suspect they use DBJ UCA to stop the steering axis wandering around, which it does if you just have a DBJ LCA. Th main disadvantage of that is that the steering tends to stay on full lock even if you release the wheel, and some subtle effects to do with phasing(SWT/SWA) on higher speed turns. These can be resolved by increasing the trail but that has its own downsides.

A little more trail couldn't have made things worse, the DBJ Audis I've driven had terrible steering feel, numb on center with very strange torque buildup at customer representative speeds and almost no feedback approaching and exceeding the limit. Maybe the PS pump/gear tuning or friction, but still with all that hardware...

[Greg Locock]

A little more trail couldn't have made things worse, the DBJ Audis I've driven had terrible steering feel, numb on center with very strange torque buildup at customer representative speeds and almost no feedback approaching and exceeding the limit. Maybe the PS pump/gear tuning or friction, but still with all that hardware...

Yup, the additional ball joint friction is a serious consideration, and is very hard to calibrate around. A good EPAS lets you add friction compensation, but that is piling fix on top of fix.

[meb58]

The lack of steering feel you both note is quite disconcerting; I have been driving a late model A4 for a few months and despite the four wheel drive, this is numb beast especially when driven in the snow.

Greg, in a different thread from a while back you indicated that "stiction" is a serious concern although the conctext there was about dampers and low profile tires. I imagine that if marketing latched onto a design concept worth exploiting, from a pure marketing and sales perspective, the company might be willing to give up some purity...?

[Greg Locock]

Yup, marketing often force things into programs that cynics would prefer to be left out.

Sometimes development want to get rid of a feature but marketing want it left in. Then marketing get to pay for it, not us.

[meb58]

Well thankfully I don't have that problem...I am working on a funky front end design that I hope generates some atention. I have found in my creative past that when a novice attempts to make sense of an environment he or she is not fluent with, the results create some laughter, joking, but on occasion, something unexpected. Obviously I am not constrained by too much engeering knowledge so I fulling expect the unexpected...

[cheapracer]

when a novice attempts to make sense of an environment he or she is not fluent with, the results create .. something unexpected.

If amateurs didn't 'have a go' we wouldn't advance in some areas, there is too many qualified experts around who are willing to immediately say "it won't work".

Countdown is on for my double beam axle, getting closer by the day even though "it won't work"

Not sure about your KPI/camber question.

[GrpB]

If amateurs didn't 'have a go' we wouldn't advance in some areas, there is too many qualified experts around who are willing to immediately say "it won't work".

Countdown is on for my double beam axle, getting closer by the day even though "it won't work"

Yes, absolutely! The conservatism of convention could also be called the fatalism of the unimaginative. And lots of experts don't actually do much themselves anyway, they just have access to better literature than someone who isn't part of 'the club'. By all means, TRY IT!

[meb58]

Cheapracer,

Forgive me, sometimes I know so little I don't even know how to ask a question...and I am having a wee bit of a brain fart just now...If we maintina a given caster angle but change SAI does dynamic caster change? I see caster angle increase with steering wheel angle if only SAI is increased, written another way.


GrpB,

I have something funky...I just want ot make sure different is for the sake of different...and I am pretending I do not have to worry about money, manufacturing or marketing type folk. Then where's the fun?

Edited by meb58, 08 March 2010 - 23:29.

[Greg Locock]

One of the reasons I don't like antidive on double wisbones is that it generates changes in castor. I think I've always ended up with the upper wishbone hinge axis parallel to the lower wishbone hinge axis, in both plan and side view.

The exception is on 2 production cars where we found that mechanical antidive does seem to increase impact harshness, so we stuff around a bit in side view.

However, in the USA production people seem to keen dial in non parallelness in both views at both ends of the car. eg Mustang Cobra IRS. I don't know what they think they are gaining.

[meb58]

Greg,

Regarding your last sentence...aren't they then relying upon bushing material to allow for more compliance? Else the ball joints get a workout...?

[Greg Locock]

No, a double wishbone is determinate, it doesn't need compliance to articulate unless you do something insane