Steering axis inclination, how much is too much?

Billy_the_Kid

Well-Known Member
Posts
166
Reaction
0
Recently there was a discussion on here about bumpsteer with radical arms. http://www.race-dezert.com/ubbthreads/showflat.php?Cat=&Board=shop&Number=64924&page=2&view=collapsed&sb=5&o=93&fpart=1

My question is how much steering axis inclination is too much. Seems like a couple of those pictures show a ton of it. Don't you always want that projected point to be centered on the tires contact patch at ride height, correct? What happens when the projected point is inside (negative?) of the center of contact patch? And what happens when it's outside (positive?)? The amount of inclination must change a bunch as the suspension cycles too, right?
 

geoff

Well-Known Member
Posts
356
Reaction
1
i was trying to figure this out, too, then i thought to check out RC cars/trucks. Those things have ridiculous kingpin angles and they work well... i think as long as the steering is on the right axis it will work. seems to make sense that way
 

ntsqd

Well-Known Member
Posts
2,429
Reaction
27
Re: Steering axis inclination, how much is too muc

SAI goes back to Scrub Radius and how much or how little of that, that you want. Also, the more laid over the angle, the more it will give an on-center effect similar an increase in castor. Castor aids high speed stability, something SAI may not. Or may not w/o other undesirable traits.

It does change as the suspension cycles. Where it is at ride height and how fast it changes from your ideal are important, not trying to make it the same thru the whole travel range.

The trend on modern cars is to use nearly, but not quite verticle SAI's. Look at where Honda has been putting the front upper BJ in recent years. It's either that, or they go to larger rim sizes so that the designer came stuff both BJ's inside the rim.
 

JrSyko

Jerry Maguire
Posts
7,888
Reaction
1,600
I have a question for you guys. Could you run a 4-link setup in the front with a solid axle and run a hydraulic steering set up? This would eliminate all the steering components that limit travel and allow you to run longer hydraulic lines to make up for the increased travel. Basically, like some of the rock crawler guys do but for a long travel desert application.

Just a thought?
 

tedmales

Well-Known Member
Posts
541
Reaction
10
i thought of that for my excurion, but felt it might limit feedback and the road "feel" that lets you know when things are going to give out. i have driven tractors like that and it works for them, but do not think it would be a good idea for an street driven truck, plus its probably not legal because if a line was to fail there would be no control and no warning signs.
 

geoff

Well-Known Member
Posts
356
Reaction
1
Re: Steering axis inclination, how much is too muc

on the other hand, anyone who road races, drag races or really goes fast in any honda, swaps the L to R upper control arms to give them some caster... it makes a world of difference
 

FABRICATOR

Well-Known Member
Posts
5,147
Reaction
107
There have been a few discussions on this topic. As always, application is an important factor.

RE: “Don't you always want that projected point to be centered on the tires contact patch at ride height, correct?”

Not necessarily. It depends on what type of axle/suspension arrangement you have. Some require it offset, some centered. Ride height is only applicable to a vehicle cruising down a smooth road. There is a substantial difference between static and dynamic ride height on a vehicle with long travel and good power. There basically is no ride height at speed over rough terrain.

Re: “What happens when the projected point is inside (negative?) of the center of contact patch?”
This depends on other factors such as castor, wheel off set, and whether the wheel is driven or not. There are too many variables to address this all here. Generally, it will reduce the steering’s return-to-center characteristic, and increase stress on nearly all components, especially the steering.

Re: “And what happens when it's outside (positive?)?”
Again there are too many variables, but generally it will increase the front end’s return-to-center characteristic and make the front end difficult to turn.

Re: “The amount of inclination must change a bunch as the suspension cycles too, right?”
It may change very little. But it should change enough to avoid positive camber.

There is a big difference between having these things designed in, and modifying an existing design. Several OEM designs have some pretty radical numbers. Nearly all FWD (front wheel drive) geometry has virtually zero, or even negative scrub. Different types of axles and suspensions require different types of spindles and king pin angles.
 
Top