help with Amod suspension design (sorry its long)

[Sean in CT]

I am building an Amod autocross car and am looking for some suspension design input.
The car is intended to be used locally only, no national events. The objective for this car is to be easy to drive fast, especially for my novice friends.
Here in New England, the lots tend to be pretty rough, not just pot holes but dips, crests and off camber turns. The car will include: a zx11 engine (about 125 hp, good torque curve) and a torsen diff. The wheel base will be about 90"�, 62"� front width, 60"� rear, 9"� available suspension travel, pushrod/bell cranks to coilovers and ARBs. Wheels will be 13x7 front with 5.5"� BS, 13x9 rear with 7.5 BS. My target weight is 800lbs, everything but driver
The front suspension has 25"� LCAs, 23"� UCAs, zero KPI (actually KPI=camber) I offset the spindle 1"�, so 10* castor gives about .75"� mechanical trail. Camber gain is .25* per inch of travel Front RC is about .25"� above ground. I did the suspension analysis with performance trends software...the RC migration was reasonable.

For the rear suspension, I already fabricated the uprights ( typical ˜Y' style), but I am unsure of what to use for camber gain (?) and that is my question. I compensated for the low camber gain in the front with a high caster. Since the rears don't turn, I think I need a higher camber gain, but due to my suspension travel reqirements, I don't want to go too high.

BTW I am a doctor, not an engineer, so be nice �

[Sean in CT]

I am building an Amod autocross car and am looking for some suspension design input.
The car is intended to be used locally only, no national events. The objective for this car is to be easy to drive fast, especially for my novice friends.
Here in New England, the lots tend to be pretty rough, not just pot holes but dips, crests and off camber turns. The car will include: a zx11 engine (about 125 hp, good torque curve) and a torsen diff. The wheel base will be about 90"�, 62"� front width, 60"� rear, 9"� available suspension travel, pushrod/bell cranks to coilovers and ARBs. Wheels will be 13x7 front with 5.5"� BS, 13x9 rear with 7.5 BS. My target weight is 800lbs, everything but driver
The front suspension has 25"� LCAs, 23"� UCAs, zero KPI (actually KPI=camber) I offset the spindle 1"�, so 10* castor gives about .75"� mechanical trail. Camber gain is .25* per inch of travel Front RC is about .25"� above ground. I did the suspension analysis with performance trends software...the RC migration was reasonable.

For the rear suspension, I already fabricated the uprights ( typical ˜Y' style), but I am unsure of what to use for camber gain (?) and that is my question. I compensated for the low camber gain in the front with a high caster. Since the rears don't turn, I think I need a higher camber gain, but due to my suspension travel reqirements, I don't want to go too high.

BTW I am a doctor, not an engineer, so be nice �

[KU_Racing]

Sounds to me like you have things pretty well sorted. 9" of suspension travel? you should be able to handle the whoops sections and the tabletops without any problems.

When you read these replies, dont forget that a lot of suspension design is compromise, educated guesswork, and opinion. There are definately people out there who will read this and disagree with me.

All jokes aside, your rear camber curve should be a lot sharper than your front curve. I would shoot for a curve on the order of .5-.7 deg/in assuming that you are starting with a relatively low camber setting. The more camber you intend to run at ride height, the less camber gain you will need. Tires are also a factor- the typical hoosier slicks that formula cars use are not terribly camber sensitive, but moving up that far in width means you will have to pay more attention to how your tires will react to the camber changes you feed them, as your wider rear tires will provide more grip variation as camber changes.

[Sean in CT]

Thanks for the quick reply KU. My thinking was to use more static camber with less camber gain as you eluded to. I dont expect to use all 9" of travel too often but it would be nice to have it available. At our lot, the SM cars do very well, a Z06 and a 350Z in particular which i think is due in part to the longer wheelbase and large suspensaion travel (relative to typical formula/SR type cars). I also think that since most of the braking is on the front tires, the rears can tolerate more static camber.

[Steve Yao]

I'll echo KU's comments that you certainly have your design pretty well sorted.

I would caution against compensating for low rear camber by having a great deal of static camber. While the front tires are dominant in braking, the opposite is true in acceleration. In addition, large static camber will lead to significant amounts of camber thrust, cutting down on your grip available for longitudinal acceleration even further and inducing additonal rolling resistance. This could be good for building heat in the tire, but it is definately bad for wear. You've indicated that you are a doctor so maybe re-occuring investments in tires is not a big deal to you, but for most autocrosses I talk to, its a big concern.

To determine your ideal camber gain rate, I would be looking at your chassis roll stiffness and thus roll rates. Is your chassis going to be rolling 1deg/G or 1.5deg/G? In addition, how much does the car roll due to lateral load transfer compressing the tires? Add these two roll rates together and you have a starting point for how much camber your suspension needs to generate in response to maintain your static camber setting in cornering. however, this does not taking into account compliance in your suspension components...

[KU_Racing]

Steve is definately right on all the points he made. Just for clarification, my opinion is that the initial camber setting should be designed to help put a little heat into the tires, as steve explained. I think that the grip gained from the additional heat in the tires overcomes the small amount of grip lost from the extra degree or two of camber. If you were talking about a road racing application, I would feel differently. In an autocross situation, however, my philosophy is that getting as much heat into the tires as possible is of primary concern.

I also agree with steve that your total roll rates should be taken into consideration when you sculpt your camber curve. Just as more camber gain in bump can reduce the amount of static camber that is needed, a car with a higher roll stiffness will need less camber gain as well. The whole goal, again as steve said, is to determine what camber angle is ideal for your tires and chassis, and then design the suspension so that the tire stays as close to that amount of camber as you can manage in roll.

One more thing, and I will step off of my soapbox. Dont forget that the way your suspension is designed and idealized may not acutally be the way it turns out when everything is said and done. What I mean is that when most people design a suspension system, they get to the point where the dimensions and locations or their pickup points and links are set down to thousandths of an inch. While its pretty easy to build links that are very close to designed dimensions, it can be very difficult to fabricate a pickup point with that sort of accuracy, not to mention what happens to a space frame when it gets welded. The point is, try to make sure that your design wont go to hell if your pickup points are off by, say, 1/16th of an inch or something. Usually it wont be a problem, but if your design relies on something being exactly 90 degrees or somthing not rotating over center, then when things move a little you could have some trouble.

Also, sorry about any bad jokes that may pertain to your unusually high amount of suspension travel. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Sean in CT]

Even though i am a doc, this is a low budget car. I anly run about 5-6 events a year, usually take a friend. One set of used tires gets the job done...

I am not equipped (time, knowledge, software) to calculate roll angles/roll rates with any accuracy.
Id prefer to spend the time testing/tuning/getting seat time etc...no judges to justify anything to.
Plus static camber is easily adjustable as well as ARBs and springs. I was thinking (as mentioned) 0.5*/inch gain with 2.5 static would be a good starting point.
As far as fabircation goes, it should be pretty accurate. I am building uprights and control arms on jigs i fabricated. I have a 5'x10' acorn welding table for the chassis (5000lb cast iron welding table) and i plan on fabricating the cross members then drilling the pickup points in the bridgeport. I dont think extreme accuracy is needed, as long as the alignment is good. I am building teh frame rails out of 1x2x0.090, the main hoop out of 1.5x0.083 and the rest on the tubes and suspension out of 1x0.063, so it will be pretty rugged compared to an FSAE
All my friends laugh at my suspension travel too, but the way I see it, turning, braking and hitting at bump at the same time will use quite a bit of it. Softer springs will keep the tires happier. plus I can limit travel easier than i can add more.

[JHarshbarger]

One more thing about the wheel travel: in order to get that much travel out of a pushrod/bellcrank suspension, you are going to need some pretty large bellcranks and coilovers. Packaging a full-sized car coilover inboard is quite a feat. You could run a very small motion ratio with some very stiff dampers, but you'll get a lot of stiction that will make transients very unpredictable and sloppy. Not criticizing on the large amount of wheel travel, just questioning the feasability.

[Sean in CT]

bilstein make a small body coil over with 9" travel. With my 90" WB i should have enough room (hopefully) I would like to stick to a 1:1 ratio if possible

[Marshall Grice]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">All my friends laugh at my suspension travel too, but the way I see it, turning, braking and hitting at bump at the same time will use quite a bit of it. Softer springs will keep the tires happier. plus I can limit travel easier than i can add more. </div></BLOCKQUOTE>

Nothing wrong with hitting the bump stops (or scraping the ground) every once and a while. I am certain that a z06 does not have 9" of suspension travel and they weigh over 3x what you're planning on, and you say that they do pretty well on your lots. I hope you don't plan to run 4.5" of ground clearance or run springs soft enough to use all 9" of travel.

[Brian Evans]

This project sounds like a lot of fun. I first thought that by "autocross" and 9" of suspension travel you meant off road grass-lot racing like they have in England, but it sounds more like parking lot autocross.

Probably the biggest reason people are looking for 1:1 motion ratios is to accomodate extremely stiff suspensions and very little shock/wheel travel. To use 9" of travel you will have to be very soft in the springs, you may find it advantageous to consider a 0.5 MR - that will make it a lot easier to buy springs, for one thing. If you aren't going to use the travel, then you probably shouldn't worry about building it in. It forces other compromises that you might not need.

Brian

[Sean in CT]

thanks for all the input.
In my legends car, i found using stiff springs would cause severe stability problems over bumps and dips but was very quick through a fast, smooth slalom, and the softer springs would cause it to bottom out more, but was easier to drive. My goal is (obviously) to keep the tires on the ground as much as possible without severe/sudden loading or unloading. I like the idea of the 0.5:1 motion ratio. Since my control arms are relatively long, I can get about 12" travel without binding the ball joints. The reason I was looking at 9" total travel was because bilstein make the 9" shocks, and seemed better than 7"http://fsae.com/groupee_common/emoticons/icon_smile.gif
BTW If anyone live in/around eastern connecticut, you are welcome to stop by my shop.

I will be building the rear control arms this weekend if time permits

Sean

[Chuckster]

Sean,
Have you looked at the latest SCCA rules? One can also run a MC or snowmobile powered car in BM instead of AM.
BM does not allow as much wing as AM (In BM: FA rules for open wheel cars and 8 feet square total for wings for sports racers-both can have gnd effect tunnels).

The min weight is not much higher then AM (AM is 950 w. driver) and you might be more competitive in BM if you do not buld an extreme big winged wonder.
With a 1000cc MC engine or 900cc 2-stroke, you could run to 1020# with driver. With a 1300cc MC or snowmobile engine, you would have to weigh 1075# with driver.


I am originally from CT (Mansfield) and used to run in Solo there from about 1972 to 1988. Do you know Soter Slomski (P&WA engineer-former DSR road racer) or Bill Gendron (Small Fortune Racing)? They can provide local advice.

[TG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Sean in CT:
thanks for all the input.
In my legends car, i found using stiff springs would cause severe stability problems over bumps and dips but was very quick through a fast, smooth slalom, and the softer springs would cause it to bottom out more, but was easier to drive. My goal is (obviously) to keep the tires on the ground as much as possible without severe/sudden loading or unloading. I like the idea of the 0.5:1 motion ratio. Since my control arms are relatively long, I can get about 12" travel without binding the ball joints. The reason I was looking at 9" total travel was because bilstein make the 9" shocks, and seemed better than 7"http://fsae.com/groupee_common/emoticons/icon_smile.gif
BTW If anyone live in/around eastern connecticut, you are welcome to stop by my shop.

I will be building the rear control arms this weekend if time permits

Sean </div></BLOCKQUOTE>

I would never compare an A-mod with a Legends car. Excuse me if I sound a little harsh right here, but I think Legends are pieces of shit. The reason why you were experiencing what you described is that Legends front clips are very much undersupported and with a stiff spring, you were just making the chassis act as the suspension. That will cause a lot of non-linearities in the springing force experience at the tire due to twisting and bending of the frame rails.

http://store.600racing.com/images/site_images/legends_diagram-framecage.gif

Also, from what I remember, anti-roll bars aren't allowed. That's another issue entirely, and makes it all the more different from A-mod.

Are you using Bilstein monotube gas shocks? If you are, I'd suggest upgrading to an adjustable unit (get as much adjustability as you can afford). If you able to dial in damping values for your shocks, that will go much further than having a ton of suspension travel tied with really soft springs. Stiffer springs will also allow you to lower your car substantially, dramatically reducing roll and pitch propensity. If you are running aero, this will help keep the attitude of the wings more level and closer to a constant ride height, too.