Suspension Settings

[rjwoods77]

Hey guys,
Me and a teammate were working out a suspension setup and we got some shocking numbers. We ran it through a suspension analyzer and checked it by using dynamic sketching in solidworks to compare. How does this sound:

.1" of lateral R.C. travel per every 1 degree of body roll.

.8 degrees of positive camber gain per every 1 degree of body roll.

1:1 ratio of dive to R.C. height

Static roll center height of .85"

We are designing adjustable anti-dive into the chassis. Neeed to test to see if it is necessary but since our car is way different we don't think it will be necessary.


How does this sound? http://fsae.com/groupee_common/emoticons/icon_cool.gif

[rjwoods77]

Hey guys,
Me and a teammate were working out a suspension setup and we got some shocking numbers. We ran it through a suspension analyzer and checked it by using dynamic sketching in solidworks to compare. How does this sound:

.1" of lateral R.C. travel per every 1 degree of body roll.

.8 degrees of positive camber gain per every 1 degree of body roll.

1:1 ratio of dive to R.C. height

Static roll center height of .85"

We are designing adjustable anti-dive into the chassis. Neeed to test to see if it is necessary but since our car is way different we don't think it will be necessary.


How does this sound? http://fsae.com/groupee_common/emoticons/icon_cool.gif

[rjwoods77]

Also the roll center height doesnt chnge with body roll. Only when you dive does it move up and down. The entire front roll center movement can be contained in a 2" dimeter circle from static r.c. location and it really only uses a little bit of that circle.

[Denny Trimble]

That's lovely, wish my suspension behaved that well...
http://fsae.com/groupee_common/emoticons/icon_wink.gif

RC height following CG height is a good thing, by the way. Keeps roll moments constant. So if you draw your circle around a point that follows the CG and lines up wiht the static RC at ride height, the dynamic RC should stay within a very small circle.

About anti-dive, it depends on your wheel rates, wheelbase, CG height, expected braking G's (1.3 to 1.5), etc. Find out how much forward weight transfer you get, what this means in terms of suspension compression (available travel and camber change), etc.

And about the .8deg/deg camber change in roll, why don't you look at what happens when you steer a wheel 20 degrees with 7-10 degrees of caster. The car will handle differently on large radius vs. small radius corners because of this.

Don't forget about scrub radius and weight jacking, either. With infinitely stiff rear roll resistance, the effect of scrub radius * caster * steered angle will change your cross weight dramatically.

[Paul V.]

I noticed you are looking at large caster. What kind of Pnue. to Mech. trail ratio do you find the most advantageous.


Paul Vaughan
UAH FSAE

[rjwoods77]

Hey Denny,
The previous settings were using 13" rims. We improved on those settings just a tad using 10" rims which we are jazzed about. Smaller everything which helps with our superlight goal. As far as castor, why in the world would you run so much castor. Baja cars run that much. We found with our settings that in dive and roll our suspension actual registors a smaller camber gain. We did some testing with the old car. It has zero kingin inclination, zero castor, zero trail, zero anti dive and a considerable scrub radius. The thing has more steering stability than we can describe. The new setup is gunna run no anti-dive, zero castor, zero kingpin and zero scrub radius. We will have the obvious toe adjustment, scrub adjustment and we have a trail adjustment designed into the upright. Initial settings are 2 degrees of camber, some toe out and zero trail. If we find stability lacking or the steering feel too "floppy" then we will dial in some trail. The point being is that we are getting rid of all geometry in the front suspension that cause odd arc travels and odd r.c movement. By isolating the front r.c. to such a small range of motion we will be able to tune the handling by adjusting the rear roll center that practically doesnt move side to side. Less that what the front does. The rear suspension design allows for a huge range of adjustability in weight transfer along with tuning in roll understeer and roll oversteer and nuetral if that is so desired.

[Denny Trimble]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Rob Woods:
why in the world would you run so much castor. <HR></BLOCKQUOTE>

Camber change with steering and weight jacking. You're not locked into high mechanical trail just by running large amounts of caster, either.

FSAE cars run on some of the tightest corner radii of all racecars, so steering angle-induced effects such as the camber change due to caster are significant. Any time you can do something that helps camber in mid-corner without excessive negative camber under brake dive, do it.

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>The point being is that we are getting rid of all geometry in the front suspension that cause odd arc travels and odd r.c movement. <HR></BLOCKQUOTE>
I don't think the judges will buy this as a coverup for "we didn't fully understand the effects (benefits/problems) of these parameters, so we set them all to zero".

You can build whatever you like and be proud of it, but be careful of chasing one particular aspect of the system, at the neglect of others.

I'm also wary of your claims of zero scrub radius and zero KPI in a 10" wheel, unless you're running your brakes inboard of the uprights on a live axle. Watch out for suspension structural stiffness in braking and cornering loads (it's easy to lose as much camber through compliance as through kinematics).

Since we finished 2nd in Endurance in '04 with this "baja" amount of caster, I think we'll look into some 12" travel Fox air shocks for next year. That should be baja enough to catch Cornell...

[rjwoods77]

Denny,
I am runninn inboard front brakes. Not a problem with stiffness. Design for upright is good. We are good getting everything packaged. As far as the comment about "we didnt fully understand" we get the broader picture of things. Once you see our rear suspension you will see what I am saying. The weight jacking dynamics that you are talking about are set and compensated for by the rear. You'll see what I mean when we show up. As far as the judges are concerned all we have to do is show them the sim data versus the actual function of the suspension/driver feedback. If the driver is fast with it that is all that matters. If your car is built around those settings and works then you are good. We are just building off different settings and we will see if it really works. We ran it by a vehicle dynamics guru and he seems to think we are on the right track. Those fox air shocks are cool. I used 5.5" travel ones on my baja car. Neat shocks. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[jack]

with so many zero's and ten inch wheels, i cant help but ask what your max steering angle is?

[Kevin Hayward]

Rob,

Just a quick question. Given that we have established the number of zeros in the 10" wheel and that the brakes are inboard I have to ask how big are your front brake rotors?

We have a 13" setup and the packaging of our inboard brake rotors prevented us from being able to have so many zeros without serious comprimises.

I do have to say that I know nothing about Baja but around 5-10 degrees of caster appears to be pretty common in FSAE. But maybe your car is different enough to warrant going outside of this range.

Kev

[rjwoods77]

Kevin,
We can go as big as 7.25 without any collision problems but that is right on the edge. I was going to use a 7.1" MCP rotor. They are cheap as hell if you ask for a SAE discout(20/piece), they are mild steel and take a hell of a beating. I may turn them down a little but right about there should be good. My "balljoint" spacing is 8". We concentrated on getting the front roll center to stay super isolated. The rear suspension will then be adjusted to have the corresponding rear roll center. The rear can also be tuned to roll understeer, oversteer or just neutral. Anti dive can be adjusted from around 40 percent to 300 percent. In testing we are gunna adjust clutch shims and center to center spacing to get the clutch to grab super hard on the accel event. With good grip(which we will have), big anti squat and a hard hitting clutch we hope to pop the first wheelie in competition. As far as the suspension was concerned we have analyzed the front in roll,dive steer conditions and the camber gain actuallyt reduces when we do all three. It is a little more in other cases but only in the order of about .1 degree. Thats about the same castor people use in baja but it is for different purposes. The rear suspension controls weight jacking/transfer so we dont have to engineer it into the front. We are going in a completely different direction so we hope it works good by competition. No reason it shouldnt because its already in use in certain race cars. But we know how that goes.

[rjwoods77]

Kevin,
Forgot to add. The whole setup of our car is to be user freindly. We are making it for the average guy to be able to make front and rear suspension changes on the fly and know what he is doing. We are making charts for "do this for this" kind of thing. We will have ajustable camber, mechanical trail and castor in the front. I havent completely dismissed these things. Just leaving them out to see if we can get it to work nice without them. Again we are build a "drivers" car. Something an average joe can screw with quickly and repeatably.

[Denny Trimble]

Didn't your team win an MSC award a year or two ago for your ADAMS/Car model? Have you modeled the new car in ADAMS?

I hope you're not serious about the wheelie thing, unless you jack the car up 8" or find an extra 1.0 to add to your tire coefficient of friction.

[rjwoods77]

Don't really know.
The team did really good for 2 years, then did pretty good, then it went to shit for 4 years.
We won best exhaust for a turbo manifold on the pink car i think. We finished 9,11,27,3# the first 4 years. Then like 2 guys were working on the project for 2 years. Then a bunch of guys that are now on the team worked on it and got it to comp. Tested at 127db and they couldnt get it down in time for the enduro. We have good guys and we just threw out the old design and started fresh. There isnt a single transfer part on the new car. We still haver the old one but it doesnt handle that well.

As far as the wheelie its a pipe dream but the car car be changed between events to suit the individual events. We are gunna be down on power because we are started development on an engine we never messed with before. I suspect the power this year wont be too great but we are started a engine development program now so by next the year after we should be up to snuff. It would be cool to do it though but i realize the limitations. We also are making just about everything adjustable because we are using it to develop the chassis concepts. We have a goal to score 600 points this year. Then the year after we want to get 800. Kind of a 2 year program. W e have an awful lot of catching up to do put the car is dirt simple.

[jack]

wow, now that you say your ball joints have 8" between them, i am even more curious how you get enough steer angle with zero scrub and tens. my current suspension iteration (tens too) has 2.25" scrub with 6.75" between the balljoints, and i can only hack a little more than 25 deg of steer angle (50 total). just wondering how you are pulling this off?

[rjwoods77]

Working on it. You only really need that amount of steering anyway. My tire sit centerline of the front bulkhead and front hoop. Although an in plane a arm would be perfect, many passenger cars are not because they need to attach points. Maybe that is something to consider if you are having problems. I just getting done with the upright so getting the a arms is next. I recognized that problem early. If I cant get it to work then I need to change some stuff around. As a side note my lower arm at ride height is parallel to the ground and the upper is inclined downward from the upright to the frame. If my a arms have to be a little ugly or a little heavier than I would like them to be then so be it. Its worth it for the setup that i currently have.

[fade]

how narrow are the 10" tires?

[BryanH]

Rob, I have found that the most difficult thing to do is get your racecar suspension into its operating window. When I'm outside the window changes make no change, or effects reverse to expected. When you get into the window very small changes can have large effects...... Very Frustrating/character building.
Too much adjustability is the last thing weekend racers that I know need or want.
Think this is all bullshit Rob?
RMIT 03 has run with virtually no suspension changes from it's 1st shakedown run, top 7 of 15 fastest laps at FSAE-A, until the end of winning F-Student!
IMHO RMIT 03 is a commercially viable racecar, possibly the basis of a one make series.

I'm guessing your not going to like the High temp/high pressure alloy honeycomb carbon tub RMIT 04 but everyone else will love it!
Bryan H

[rjwoods77]

Halfast,
I understand this all too well. Thing is the first time around we will have to make it more adjustable than it needs to be to find its dial in points. Why would I think it is bullshit? I didnt say I didnt think it was cool I just said impractical for normal people. Guess your too busy holding you hard on for me but thats ok. I'm a big boy.

[BryanH]

Sorry Rob, didn't mean to offend your tastes.
The bullshit will stop when the flag drops.
Bryan H

[rjwoods77]

cool