What are the negative effects of increasing positive caster 6

[TMcRally]

I have just bought a Porsche cayenne as a project to modify to improve its off-road capabilities.

I'll give you a moment to stop laughing.

Objective
Ok, that's enough laughing. When I started to consider the car, I never really thought it would be a fun car on the road, I just thought it was a soccer mums car but I was surprised when it is actually quite good on road so now I want a transformer. One that goes from good on road to reasonable off road with a wheel change.

I was going to fit a 3in spacer type lift kit, now I'd rather lift it with big tyres so I can switch back and forth to have both capabilities in one car with a set of street 29in and off-road 35in. It has adjustable air suspension now so it has a max height of 275mm with 29in tyres, if I can fit 35in tyres I'll have the 3in lift plus all the other advantages, the problem is 33in is as big as I can go because the tyre contacts the metalwork at the back of the wheel arch, not bad, but I was thinking.... There is room at the front because I am able to clear away the bits and pieces in front of the wheel.

Finally the question.
What down sides are there to moving forward the lower arm. It is double wishbone independent front suspension. Currently it has too much understeer and its feels too light with poor feedback.

 

[BrianPetersen]

What I'm seeing online suggests that it already has 8.5 degrees caster, which is quite a bit. The German manufacturers typically like using a lot of it, so that's not surprising.

Understeer at higher speeds should mean it is happening with low steer angles, which means the caster won't have much effect. There's about a thousand other things that affect this. Is it just the "feel" of understeer, or is it actually the front end losing grip?

The numb steering is the calibration of the power steering. It's hydraulic on that vehicle, so this has to do with the characteristics of the servo valve that controls it - basically, the gain is really high, so that a featherweight driver can steer this 3-tonne vehicle with fingertips. I doubt if there is anything that could practically be done to change this. It's already got quite a bit of caster, which means it has quite a bit of mechanical trail, which already translates lateral force into moment around the steer axis. The power steering just isn't letting much of anything through to the steering wheel.

 

[cibachrome]

The reason it has so much caster is because the Gigantic 255/55-R18 meats found on this sled don't stomp out very much self aligning torque. With average boost flow and a slumber/yawn steering valve profile, tierod loads on the gear are low and steering torque is light. You need to install/make/beg/borrow or steal a different (thicker) torsion bar in the steering gear valve body. This defines the manual/unboosted component of your steering wheel torque. Bigger t-bar, higher manual effort contribution. Even with all this light headedness, the initial Mz compliance steer at .81 deg/100Nm makes a pickup truck look like a winner. That's a pretty big number for a porch pickup (That's a Craigslist and Marketplace joke. Don't take it literally).

Just for your info, these Cayenne Powders have about 3% front roll steer with 5% in the rear. A bunch of lateral force steer (from the caster). If you raise the chassis relative to the tierod outers, you are going to loose all of that front roll understeer.

But, here's what I'd be concerned about if I were you: When you drop off those giant sized Bridgestoners with high cornering stiffness and replace them with some aftermarket baloney skins and raise the c.g. you may be looking at worse that GVW loaded pickup truck response parameters. Me thinks you could be headed for a rollover event because of the longer phase delay, the higher steering gain, and the higher c.g.

So, be careful. "A word to the wise is sufficient"

 

[gruntguru]

On the spectrum extending from flat-track dirt to rock crawling, where does your definition of "off road" lie? (Is there a hint in your handle?)


je suis charlie

 

[GregLocock]

To be honest the limitations when taking modern SUVs off road are (1) desire not to bend metal and (2) the tires. Our management took our proto AWD SUVs off for a week of fun evaluation. My manager came back and said he couldn't see why anybody needed a high ground clearance 4WD, ours was fine. My mechanic, ever keen to help, pointed out $5000 of damage.

Cheers

Greg Locock


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

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

 

[gruntguru]

Helps with spare parts turnover.
Tick.

je suis charlie

 

[TMcRally]

Hi Brian, I call it push not sure if that is right or wrong but I don't mind a bit of understeer/slip angle into a corner but back on throttle the front pushes wide. Dropping the front helped a little but the inside front squeals now. Maybe that means I am a bit quicker ?

I suspected the steering was not something easily fixed. It has an option which gives it more assistance. This is fused and they talk about removing the fuse to give great feel but it didn't improve feed back it was just heavier.

Hi Cibachrome, it's worse than that it has 295/35/21 tyres, very small slip angles which is new to me. Point taken on the roll. I will swap tyres for off-road and back to street tyres/wheels for street. It has adjustable air suspension which will allow me to have the best of both worlds, kinda.

Hi Gruntguru, most of my off-road is sand but I want to do a trip to the Cape and a few glass house mountain trips. I enjoy difficult but don't like walking, so as far as how difficult. Pretty close to the capability of the car and myself. I'm not sure what that is yet but if I'm going to be "that guy in the Porsche" I don't want to be that guy stuck in a Porsche, I know it's the wrong choice for a 4wd but different is good. Tarmac Rally definitely is/was my passion but this just a fun project, I'm not looking for a race car but it is quite good on the road and I would like to keep that side of it.

Hi Greg, haha, a very high %of stuck cars are stuck because they bottomed out. I like that these don't have the diffs hanging down but it has huge limitations with the lack of suspension travel.

Thanks all for your answers. I'm hoping the extra caster will allow me to fit the larger diameter tyres without the loss of its on road ability when I put the road tyres back on.

How much more caster is too much, can I add 4° ?

 

[BrianPetersen]

You're already at or near the high limit of the normal range of caster in production cars. There's probably a reason for that. I can't see how an excessive amount is going to be helpful.

 

[cibachrome]

Wheelhouse intrusion is probably the main reason for a limit. Of course tierod loads mean steering effort mitigation (pressure, flowrate, cooler(s), and wheel lip strikes. Since most vehicle manufacturers use the same wheelhouse design front and rear, extra room for tire movement means tight rear passenger spacing, reduced trunk volume, smaller fuel tank maybe and, of course, appearance. Front drive hardware will have a hard time with it too, from driveshaft plunge. There's even some load transfer distribution changes that accompany it also.

 

[RRiver]

Camber is a driver specific demand. The same car with two different drivers will expect different adjustments so I'm not sure there's a real answer for you. The Cayenne with all the options and a full tank weights 3 tons and then add passengers? Isn't that your real issue in sand? You're right about bottoming out in sand but that is 99% of the time from sinking or digging themselves in. I'd invest in a good winch and good sand\land anchor.

I remember as a kid driving circles around big lift, big tired pickups and jeeps in the sand at Sleeping Bear Dunes in a skinny tired VW bug. They looked impressive though.

 

[TMcRally]

RRiver,
When rules were guidelines we used to cut up VWs and anything else we could find and race each other around the bush tracks and on the beaches. Good fun .

I definitely agree sand is mostly about floatation. Tyre size/weight ratio. Tread pattern also makes a difference, I know that's against popular opinion but from my experience I believe it does, something between a mud terrain and an agressive all terrain.

We're not quite that heavy, we're about 2,500kgs, about the same as a heavyweight Japanese 4x4.