suspension design

[Cement Legs]

Well everyone thanks for all of your help so far, but Im back! I'm looking for a little advice in carrying on with our suspension design.

OK so far

1. wheel base 69"
2. front track 48"
3. rear track 44-46" (not quite finalized)
4. IC's below ground
5. roll center approx. -5" (front)
6. camber gain through 1.5" jounce ~4 degrees
7. approx wheel freq 2.5 hz

Some of these numbers we have taken as good starting points for our first car and others to achieve a certain purpose. Since we could not get in to FSAE for this summer we will only be testing this year and hopefully be competing in 2006. Are there any people experienced with this phase of the design who could perhaps share with me some foreseeable problems, or what to get into next. Thanks for any help.... http://fsae.com/groupee_common/emoticons/icon_eek.gif

[Denny Trimble]

The only thing that stands out to me is the RC at -5". That means your anti roll bars are going to have to resist a fairly large moment. Not a problem, but I don't see the advantage of going that far below ground.

Also, negative jacking forces will start to be significant with that RC height.

[fade]

with that camber gain how much RC migration do you have?

[Denny Trimble]

Yeah, I should rephrase that "only thing that stands out" to include the camber gain. 2.6 degrees per inch of bump travel? That's pretty high.

[Cement Legs]

Yeah the -5" number was a very rough guess I didnt have the CAD model in front of me so I will double check that and see about lengthening our top a arm to raise that a little. That would also change our camber rate. I thought those numbers were a little high so thanks for the reply guys.

How do i go about calculating the front and rear roll stiffness ratios? I've been looking through the Milliken book but I'm a little bogged down in the wealth of info in here and good use a nudge in the right direction.

[TA]

You might want to take this advice with a grain of salt (since my FSAE experience is limited to stealing stuff from work to support my local Uni and cheering from the sidelines) but I might pack a softer set of springs as well.

A 2.5 Hz wheel frequency might see the wheels in the air for most of the drive at the Silverdome, which more closely resembles the surface of Mars than a proper racecourse.

Cheers, Ted

[Angry Joe]

Miliken has a set of 'quick' calcuations for wheel frequency and roll stiffness that are not as complicated. I forget the page but could probably find it...

[Denny Trimble]

We run a sprung mass natural frequency of around 3, and I think we're on the softer side of the FSAE spectrum. That, or the other cars were running out of travel. We used all 2.7 inches of ours. But Cornell and UMR did well with less, and with noticeable airtime / chassis dragging.

[Cement Legs]

Well I took the replies here to heart and after a few minor mods have brought the front roll center up to -.75" and the camber gain now sits at 1.6 degrees through 1.5" of jounce. Thanks for the heads up guys.

Anyway I've also been working on designing an anti-roll bar with a variable length lever attached to the bar that will be subjected to the torsion. Has anyone played around with this type of adjustability in their ARB or is there another more common way to design in some form of tunability?

[fsae racer]

Cement, Allan Staniforth has a very simple and through calculation of roll stiffness in, I think, Competition Car Suspension? As far arb adjustment, yeah, we use the exact type of adjustment in both front and rear. check the pick @ www.gatormotorsports.com (http://www.gatormotorsports.com) if you need a reference. If you would like more, pics just let me know.

[Denny Trimble]

Nick,
Is there something you'd like to tell us about your car's relationship with curbs, dirt, etc?

You'll have plenty of time to fix it, though. http://fsae.com/groupee_common/emoticons/icon_smile.gif

http://www.gatormotorsports.com/gms/formula/2005/images/IMG_4051.jpg

[mysticv6]

Denny,

http://plaza.ufl.edu/wheelerc/nick.jpg

[fsae racer]

Hey Denny,
Just kiddin, but seriously though, that happened in early-mid January. We actually had the whole thing repaired in the same afternoon. Made 2 more contol arms, a tie rod for the steering and the pull rod, and arm for the steering rack that got ripped out, and we would have been on our way. We ended up missing the Jackinsonville auto-X the next day, but only bc one of our guys sucked air into the masters around 12:30 that night. The saddest part of the whole thing is that after the car was fixed, it only drove once more before a slightly worse accident occured. The front end of that frame was pretzelized by one of our younger guys about a week later. Since then we have hacked the front off it and are almost finished rebuilding it. However, we are not exactly happy with how many portions of the car seem plasticly strained, so we are going to try and build an entire new one by the end of our spring break just 2 weekend from now. If they would let us, we would try to race both of the frames this year. Has any school ever had a 2 car effort in one year?

[Denny Trimble]

Nick,
No worries, I thought it was pretty funny http://fsae.com/groupee_common/emoticons/icon_smile.gif

I know RIT and Cornell have come back from mangled frames in the weeks before competition. And I think in the early '90's a few schools entered two cars, an old one and a new one. But I don't think it's feasible any more.

Were both crashes driver error, or was a suspension failure involved to get things rolling? Just curious if there's something for us all to learn from this http://fsae.com/groupee_common/emoticons/icon_smile.gif

Good luck building that 2nd car...

[Cement Legs]

Denny is right. Only one car per school permitted. Even if you had two separate teams, the first to register would get to compete. Allthough that may change if they ever get the west coast competition organized. hmmmmm west coast hmmmmm... how did I end up 4000 miles from where I belong hmmmmm

[D J Yates]

I'm having some problems deciding wether to include anti-roll bars or not. Although they're a useful tool for distributing the lateral load transfer between the front and rear, i could do without the extra weight and the undamped osscilations.

Are there any major problems in raising the roll centres to reduce the roll moment and compensate for a lower roll stiffness, were i to ditch the ARBs?

I'm liking the idea of using small/mild ARBs for fine tuning only with the spring dampers providing most of the roll stiffness. Will the dampers alone be sufficient to damp the ARBs? I'm entertaining the idea of using 3 or 4 way dampers for this purpose.

I've only done the front geometry so far and am leaning towards using the small/mild ARBs:
# 60mm RC height,
# 3 deg KPI and static camber. (will reduce to 2 if the wheel and steering allow it)
# camber gains are -0.056 deg/mm in bump, 0.403 in roll.
# Negligable lateral RC movement.
# 1:1 vertical RC movement.

[Nihal]

The biggest negative I can see from raising the RC's is an increase in jacking and scrub (not scrub radius). Also if you RC is below ground you might want to see if raising it will ever allow it to cross the ground plane.

[Frank]

keep the roll centre near the ground

too high = jacking = bad camber anyway

make the virtual swing arms fairly short .. about the same as the track

[D J Yates]

Of course, Jacking. It's hard to quantify. But at 1.4g, the jacking component on the front outside tire is about 180N. Lowering the RC to 30mm brought this down to half.

The virtual swing arms are just inside the track. The actual swing arms are quite long though (narrow chassis) so camber changes and scrub are quite low anyhow.

Thanks guys.