Suspension Design

[DPpatrol]

Currently i'm reading a couple of books on chassis and suspension design. However these books were written for road cars and I
was wondering how applicable some of the information is to offroad suspension. I basically a few specific questions.
1. How much camber change is reccomended per degree off bump travel, or is the amount of camber change needed calculated
another way?
2. How much anti-dive do most off road trucks have built into the suspension?
3. It seems most race trucks have a huge amount of squat under acceleration. Does anybody try to incorporate any anti-squat
into the trucks rear suspension to try to prevent this and get more traction?
4. How do people control body roll on long travel front suspensions without using a stabalizer bar?

jason

<P ID="edit"><FONT SIZE=-1>Edited by DPpatrol on 11/01/01 09:13 PM (server time).</FONT></P>

 

[ntsqd]

In a road race car you want the camber to change enough so that the tire is always flat or, to account for carcass squirm, just slightly off the ground at the outside edge of the corner in compression and the inside edge of the corner in extension. My inclination is to use the same approach to the dirt as that makes the most sense to me. Ford TIBs and TTBs prove me wrong. That front suspension design is an abomination, but it happens to work well in the dirt and success is hard to argue with.
So instead of looking at it from a compression/extension vs camber delta stand point, look at it from a chassis roll angle vs tire normalcy (sp ?) to the ground stand point. What you'll find is that the easiest way to keep the tire flat on the ground is a live axle. Don't go there with the front unless you're a masochist. I've yet to see a deDion front axle tried in an off road truck, wonder who'll be first.

After watching beam front buggies this weekend I can see why A-arm cars are gaining. Their anti-dive doesn't exist.

B4 you put a lot of anti-squat in your design, think about the instance of the "OH S**T !!!" gulley. If the rear squats under accel then you can shift weight off the front so that the hit from that gulley you forgot about isn't so extreme.

Most of the trucks I've looked at have a rear sway bar. This makes the rear end 'loose'. Increasing the roll stiffness of an axle reduces it's cornering power, it takes away traction. By having the rear 'loose' the driver can flat track it or Rally Car it around corners if need be.

Which books have you got ?

TS

"Teach you all I know and you're still stupid"
-- Howdy Lee

 

[DPpatrol]

In a past post you and another guy reccomeded Herb Adam's "Chasiss Engineering" and Carroll Smith's "Tune to Win." I just about
finished reading Chasiss Engineering and I'm starting on Tune to Win. Adam's book gave a nice, clear, broad overview on suspension
but I was a little disapointed that in didn't get into much specifics. I hope Smith's book will dig a little deeper into suspension design.

jason

 

[ntsqd]

Try "Race Car Engineering and Dynamics" by Van Valkenburg as well. It's a HP publication. Might offer some of what you're looking for. Staniforth's "Competition Car Suspension" also has some good info, although sorting thru his British use of the language takes some doing. He talks a lot about suspension natural frequencies and his spring rate calcs stem from that.
If you can afford it ($100+), the book by the Millekin's (sp ?) is supposed to be nearly a textbook. I've never seen it. It's a SAE publication and you can find it in SAE's bookstore on their page.

TS

"Teach you all I know and you're still stupid"
-- Howdy Lee

 

[Quick_Prerunner]

Both MacPherson's TT and the last Lil Mac both used anti squat and anti dive geometry. They would also move the pivots to make more or less squat or dive. Depending on what type of terrain they were on. Pretty cool stuff.

Mike

 

[Dylan]

I have millikens book, it’s got tons of info but I only use about 20% of it, a quarter of it is aerodynamics(desert trucks should start dealing with that soon though).

Anti squat is an interesting balance. You can manipulate the AS effects from ride height to bump fairly easily with your 4 link geometry but into droop it almost always gets out of hand and goes way over 100%, for that reason I think starting low(30-40%) @ RH is a good idea so it doesn’t get nasty in an area to close to RH.

If you look at a stadium truck they have tons of dive in the front end as do beam cars??. They also have wheel recession in bump which is nice if you are hitting large obstacles. It is possible to get wheel recession and some anti dive with SLA front suspension but you often get a lot of castor change with it, but that’s not necessarily a bad thing, again it’s a compromise of the different characteristics that your looking for. Dive is only really a problem when you hit the breaks, so you could build a high dive car w/ lots of wheel recession and just stay on the gas or use mostly rear break with the weight in the back…oh wait that’s a beam car.

I agree TTB really is an abomination isn’t it, they can have really good camber vs. roll characteristics for good tire contact under extreme roll though, which is a good thing since they are sooo bad with jacking a body roll.

Where all these parameters should be is a good question??? What I’m trying to do is determine which characteristics are making which vehicles work and slowly close in on an envelope of successful parameters(But it’s also important to think outside the envelope), maybe in 5-10 years if I’m lucky I’ll have a better guess of the perfect vehicle dynamics

 

[FABRICATOR]

Application??? It depends on how much travel is available and how fast you want to go (not how swell you want to stop). If your interest is in speed over the rough:

1. Avoid positive camber if you want to go fast. Negative is good, there are no set ratios.

2. Forget the anti-dive. This would translate into poor stopping and anti-smooth ride.

3. Forget the anti-squat. This would translate into poor traction and anti-smooth ride.

4. Most of the body roll is taken care of in the rear. Addressing it in the front greatly affects the ride.

You must decide what you what out of the vehicle. All 4 of these items have been helped a lot by lowering the ride height over that of just a few years ago.

Basically the vehicle has to have a ton of high diving qualities and be very sloppy in comparison to any pavement machine. To fight any of these qualities to any significant degree, will hurt the ride and your speed. The only exception to this would be on-the-spot system modifiers that are interactive with the various controls, or what some automakers call "active suspension." We aren't there yet, are we?

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>

 

[BradM]

Milliken's text is written for the engineer and may go way over the heads of most. It is extremely complete in the area of vehicle dynamics. My suggestion would be to read everything that you can get your hands on, if you have the time. Then assimilate all of the information and make up your own mind. Thom is right that most of those texts are written with the road racing application in mind. The dynamic effects are transferable but will have much different results with 20+" of travel than they do with 2-3" of travel.

My personal opinion on anti-dive and squat is that they are not practical in an offroad application. The instant centers change continuously with suspension movement and the only effect that could be built in is anti dive. When you have a good working knowledge of what anti dive/squat is and how to determine it you will see that perhaps some can be designed in but it will likely be a very small percentage. 100% Anti squat on a straight axle rear end with 20+ inches of travel can not be achieved without some serious compromises. The same can be said for camber change. When you have 2-3" of suspension travel it is relatively easy to design a short/long a-arm suspension system that will match body roll to the camber change of the outside tires in order to keep the contact patch of the laden tires flat on the ground. However, if you tried to do that with lots of travel, the total camber change would be huge and it can only be considered from ride height up. The effects in full droop will likely lead to interference problems as well as increased loads as the vehicle contacts the ground from mid air. I believe that you are better off to make the suspension work in all situations and adapt your driving style to the dynamics of the vehicle.

I am not saying that these issues are irrelevant or that they are not worth your consideration but rather that they will lead to complex issues that result in drastic compromises of your design goals. Again, learn all that you can and make your own determination.



"The only source of knowledge is experience." - Albert Einstein

 

[geoff]

The milliken is the best book i have used to date. It is complete and very accurate. I really dont think its all that hard to understand either. Just read it over and over... every time i read it i learn something new.

Staniforth's Competition Car Suspension is very good albeit difficult to read. You really have to read each paragraph over and over =) My boss has it sitting on his desk.. its the most used book on the shelf.

Most suspension books just scratch the surface and arent the least bit comprehensive. One thing to note, is that all of these books are meant for race cars. Ackermann is the number one objective, something which seems to have little importance on the dirt. I think for the most part a standard unequal length A arm with a decent camber gain should prove to be more than adequate IMO. Its interesting that the guys who are building suspensions are just trying to duplicate the factory arcs.. nothing new/innovative.

Mathmatics is the language of nature.

 

[FABRICATOR]

Ackermann all depends on whether the vehicle usually slides around corners or not. Ackermann can help a bit on a rear engined dirt car as they do not slide as much. A front engined car for the dirt is always sliding during cornering.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>

 

[DPpatrol]

geoff, where are you studying mechanical engineering? That's what I plan majoring in next when when I go off to college and it's that time off year to start applying to schools.

 

[geoff]

Im at Arizona State. Its not a GREAT school, but the desert is here for you guys (i havent been into anythign but rock crawling until now), i had a job at a local turbo and chassis fab shop, and the school is really affordable. Also the girls are insane.... =)

All in all tho, engineering is an extremely time consuming major. If youre an engineer you really need to study HARD...

Mathmatics is the language of nature.