Advanced Suspension Geometry and Design

Quote
Iowa999
Raising the nose of the car relative to the tail, in effect, lowers the rear pivot of the front lower LCA relative to the front pivot. That's the opposite of what you want to do in order to add more anti-dive. So, yes, it will affect it and unless you want more ride comfort and less anti-dive, it will have the wrong effect.

This is what the sketches I made were telling me.

Although I disagree that it is the opposite effect to what you'd want in a rally car. From what I've seen most use a neutral to slightly pro-dive geometry. Since most road-cars come with a fair amount of anti-dive to allow them to run soft springs getting rid of it has some benefits, especially when dealing with loose surfaces. Under acceleration anti-dive geometry attempts to keep the front down but does it at the expense of actually unloading the front wheels.

Now whether changing ride-height has much effect on it who knows, I'd have to do something a lot more detailed then just drawing a rough diagram then rotating it.

-Grant

P.S. Alex: is that a 2nd gen front subframe I see? I did a 1st gen up like that maybe 7-8 years ago, as I recall it had a pile of caster and was super fun to get VERY sideways. Unfortunately youthful enthusiasm played a much larger role in it's creation then actual engineering or fabrication skills.

I was thinking about bumpsteer recently. I added pins and converted my tie rods to hiems to adjust bump. I havent set it yet wasnt sure where I should set it.

I was thinking of going from ride hieght to full compression because thats where the tire sees the most weight? I am planning on max .20 out all positive If I can get it that close. Rally cars have a lot more travel then what ive worked with in the past.

Okay lets talk slip ratios lol. My car is ass happy when I lift and roll into corners at higher speeds. What should I change and where would you start? Was thinking of stiffer rear springs and more rebound in the shocks will this help?

I totaly re vamped the rear susp. From stock.08 sti. I took pics of Higgins car and stabbed at it as close as I could. The car is huge improvment as far as hitting the big bumps but the ass happy didnt change 1 bit. I dont have the big wing but niether does Osullovin and they seem to have it figured out. Ive called Vermont sports car. They will fix me right up with there shock package for 10k plus used. Wouldnt say at all what springs they were running. They jusy said there CG is differnt so they wont work any how.

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First rally 2013
Rally car type AWD subaru
Total rallies as driver 6
Total rally cars built 2
Total rally cars caged 3
Total rally cars repaired from offs 4
Total years racing exp other then rally 19 yrs
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Sounds more like you're describing lift-off oversteer than a suspension issue. If you lift right as you brake into a corner, you take all the weight off the rear and then throw it sideways. Always going to get ass happy if you take all the weight off the rear.

Steer less and then get back on the throttle one you're turned in and get the weight back on them.

lamb



Edited 1 time(s). Last edit at 02/05/2014 08:20AM by Iowa999.

Okay that sounds correct. Now how come this car has way more lift off over steer then the gc,gd subaru chassis?

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First rally 2013
Rally car type AWD subaru
Total rallies as driver 6
Total rally cars built 2
Total rally cars caged 3
Total rally cars repaired from offs 4
Total years racing exp other then rally 19 yrs
Like 31motorsports on FB!
Check out 31motor sales on ebay for used Subaru parts

Quote
MeCalledEvan

That's an interesting point actually. There is a ton of info out there on slip angles and slip ratios within the traction circle, but traveling beyond the traction circle is not examined nearly as in depth. I know that lateral load being generated by the tire drops off gradually as slip angle increases beyond max FY, but comparing it to the loss of road surface is something that I haven't found much on.

That depends GREATLY on the tire construction, too. Look at the slip angle curve of bias-plies versus radials for an extreme example - radials having a much sharper peak and drop-off after peak grip.

That's one thing I rather like about Grassroots Motorsports tire tests. They may not be able to offer actual charts and graphs but they do tell you things like, okay this tire has the highest capability but you really have to drive it perfectly because it falls aways if you overdrive it, whereas this other tire has slightly less grip but it is much more forgiving of overcooking so it is easier for someone who isn't a robot to make good times...

Quote

By the way, I'm recently relocated to the state of Ohio. Working in Columbus. Are you going to be rallycrossing your RX7 with OVR this year? I'm working on getting mine up and running and hope to be pitting it against you. We should meet up sometime and talk about dirt, beer, and silly spinning triangle things sometime.

OVR and WOR and occasionally Detroit and probably the big one in Omaha and possibly a couple in Indy. RX-7 needs essentially everything in the drivetrain rebuilt, and if I have time I want to cut the strut towers out and relocate them up and in so I can get some damn negative camber already, and I'm thinking of going back to a Panhard too, but that car is second in line behind getting my new VW put together so I can drive something that isn't the automotive equivalent of a zombie, and the VW is steadily undergoing project creep enough that I am now porting the THIRD head I got for the thing, this one a big valve head with some fancy fabricated intake manifold that a friend half-built before he bought a bunch of 20-valve Audis so he has no more point for 10v stuff, so more than likely I'll be bumming co-drives off of people like I did for most events last year.

But definitely going to be hitting all/most of them... I might try to squirm my way into driving some Subarus or maybe a certain 944 if he's serious about rallycrossing it, and we're a beer-friendly bunch down here...

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Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.



Edited 1 time(s). Last edit at 01/31/2014 08:41PM by Pete.

Quote
Grantmac

Although I disagree that it is the opposite effect to what you'd want in a rally car. From what I've seen most use a neutral to slightly pro-dive geometry. Since most road-cars come with a fair amount of anti-dive to allow them to run soft springs getting rid of it has some benefits, especially when dealing with loose surfaces. Under acceleration anti-dive geometry attempts to keep the front down but does it at the expense of actually unloading the front wheels.

From looking at what the people who spend money on lots of R&D are doing, they are running a lot of pro-dive geometry in the control arms and anti-dive in the struts. The key is that the brakes put a torque on the strut assembly while acceleration only puts a forward thrust on it, so accelerating drives the tires into the ground and the anti-dive in the struts negates that somewhat (100%? more? less?) under braking. Also, the wheel motion is not purely up and down anymore, it's fore/aft, which has got to also pay big dividends when it comes to being able to follow the road.

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Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

Quote
Reamer
Okay that sounds correct. Now how come this car has way more lift off over steer then the gc,gd subaru chassis?

That's a very complicated question, but I don't think it has to do with the chassis. Are you running swaybars?

but



Edited 1 time(s). Last edit at 02/05/2014 08:21AM by Iowa999.

If I under stand anti dive. Isnt it the binding of the susp under braking. If the top is going back and the lower is going forward it binds up only while the brakes are applied. I thought that in slick conditions like loose gravel roads anti dive would be your enemy because the tires would lock up and slide easier?

Anti dive in aspahlt good softer springs without bottom out. Dirt bad becasue you bind up the susp and loose travel and grip under braking.

In the oval car when my car is loose on entry I will lower the panhard bar to tighten it up. How do you move the rear roll center with out a panhard bar? Control arm angles?

Yes I am running sways. Bigger in rear or smaller?

---

First rally 2013
Rally car type AWD subaru
Total rallies as driver 6
Total rally cars built 2
Total rally cars caged 3
Total rally cars repaired from offs 4
Total years racing exp other then rally 19 yrs
Like 31motorsports on FB!
Check out 31motor sales on ebay for used Subaru parts



Edited 1 time(s). Last edit at 01/31/2014 08:46PM by Reamer.

What I am looking at is that the angle of the strut is NOT parallel to the steering axis - that same kick-forward that allows them to package 18 inches of travel without having the strut tops at roof height also means that the upright is going to rotate more with suspension travel than merely the caster measurement would allow for... especially close to maximum compression, which is where you'll be when you're braking the hardest, yeah? You're not at full droop when you're hard on the brakes.

---

Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

Armchair guy over here. Yeah, I'm an engineer too. Also I'm late into the party here. Oh well. Grains of salt, possibly stating the obvious, etc, whatever.

I think when trying to transfer anything from any of the roadrace books, you have to keep in mind that the basic objectives are different. Roadrace geometry is focused on having the outside tire as square to the ground as possible, letting both rear tires work at the limit of grip on corner exit and making everything respond as fast as possible. I'd argue that rally is more focused on having as many of the tires on the ground as possible with as much force as possible, and having the car not do anything so quickly that you can't catch it.

Everything in modern WRC seems geared around having softer springs. Longer suspension travel allows softer springs before you bottom out, hydraulic bumpstops let you not smash anything when you get close to bottoming out, valving that opens up when the wheels aren't on the ground (I found patents from Bos and their brochures talk the same language as Reiger and EXE-TCs on that kind of shit) lets you run softer springs and still have things extend out. So it seems likely that you want to use geometry to correct for some of the other issues with softer springs - raise the instant centers maybe to keep body roll down, play with pro or anti dive and squat to get things to move how you want under acceleration and so on (no, I'm not really sure what you'd *want* with the pro/anti geometry but it's there as a knob) Raising the instant center may do weirdish things about jacking up the body during cornering, but I suspect that's one of those things where you balance the pros with the cons and decide whether or not the cons are relevant to dirt. Like camber. If camber mattered, there wouldn't be every top level car migrating to struts. Ergo, it probably doesn't matter much.

Anyhoo. Make sense to anyone?

her



Edited 1 time(s). Last edit at 02/05/2014 08:21AM by Iowa999.

Quote
Reamer
Yes I am running sways. Bigger in rear or smaller?

Well, here's my take on it, and hopefully someone smarter/more experienced than me can correct me and give you a better explanation.

Your rear sway is probably too big, you're probably going into the turn too hot, and you're probably not getting back on the throttle early enough. Quite possibly also giving it too much steering input.

You brake in, tip all the weight forward, and you steer a bit too much, and the car rotates - what you want it to do, but perhaps a bit more than you wanted. Now the weight is sideways and the sidewalls are helping you brake. All good there, except the force that used to be forward is now on the side, so the outer rear suspension is getting a lot of compression. Your heavy-ish rear swaybar is now also lifting in the inside rear which is already unloaded/low on grip. Now since you're using the sidewalls of the tires as part of the braking, you're now only using 1 of the 2 in the rear, so it's going to keep going more than you wanted.

Now, on top of that, you've unloaded a rear tire, so when you go to get on the gas because the car is pointed the right way and you're nearing the apex, one of your rears is unloaded and spins away power, so it doubles down on the problem because the diff is only giving you so much to the one wheel that is actually still loaded in back.

I'm guessing the same thing is going on up front too since we're talking AWD, you've got 2 wheels on the outside where you can put power down, but if you're using the weak Subaru stock differentials (unless we're talking about an STI), you probably can't actually put much power down at all with those 2 unloaded inside wheels.

So, rear goes around too much, and you blow the corner wide in general because you couldn't start the momentum forward.

Disclaimer: I'm not an engineer, nor a particularly great driver, this is just my probably very flawed understanding of how to get around corners and how the various bits of suspension that people normally play with can harm them.

Quote
mekilljoydammit
Armchair guy over here. Yeah, I'm an engineer too. Also I'm late into the party here. Oh well. Grains of salt, possibly stating the obvious, etc, whatever.

I think when trying to transfer anything from any of the roadrace books, you have to keep in mind that the basic objectives are different. Roadrace geometry is focused on having the outside tire as square to the ground as possible, letting both rear tires work at the limit of grip on corner exit and making everything respond as fast as possible. I'd argue that rally is more focused on having as many of the tires on the ground as possible with as much force as possible, and having the car not do anything so quickly that you can't catch it.

Everything in modern WRC seems geared around having softer springs. Longer suspension travel allows softer springs before you bottom out, hydraulic bumpstops let you not smash anything when you get close to bottoming out, valving that opens up when the wheels aren't on the ground (I found patents from Bos and their brochures talk the same language as Reiger and EXE-TCs on that kind of shit) lets you run softer springs and still have things extend out. So it seems likely that you want to use geometry to correct for some of the other issues with softer springs - raise the instant centers maybe to keep body roll down, play with pro or anti dive and squat to get things to move how you want under acceleration and so on (no, I'm not really sure what you'd *want* with the pro/anti geometry but it's there as a knob) Raising the instant center may do weirdish things about jacking up the body during cornering, but I suspect that's one of those things where you balance the pros with the cons and decide whether or not the cons are relevant to dirt. Like camber. If camber mattered, there wouldn't be every top level car migrating to struts. Ergo, it probably doesn't matter much.

Anyhoo. Make sense to anyone?

Famous and been around forever rally engineer Christian Larioux


said in a long LONG interview in Racecar Engineering (which showed an amazing amount of unintentional arrogance) put the problem very simply and clearly "on asphalt were are searching for handling, on gravel we are searching for grip".

he said on asphalt in testing it was very easy to put sensors everywhere and measure everything you desire and can imagine. On gravel it was very difficult to get reliable information not the least of which because as he put it "a good driver will "drive around' all kinds of problems" and the drivers---he said again in so many words---are monkeys that don't have a clue how anything in any area of the cars work "because they are so complicated" and so their feedback is suspect and again said several times in "so many words" and their observations must be ignored...

Despite his shocking arrogance it did zero in on the essence of the dichotomy: asphalt its engineers and equipment that is primary because it is so consistent and simplistic, gravel is difficult, and the driver's perception and skill is central.

That is nice words for what I've been saying for 40 years ----the difficulty and unpredictable nature of the surface---different often left to right, different turn by turn, different lap by lap, different during the day...------makes doing the job far more challenging---and the challenge to one's flexibility and adaptability is the main attraction on loose surfaces.
It's hard to do it well. And the more powerful the vehicle , the more challenging finding grip is. (hence why I personally look at 2wd cars as the more desirable because its more challenging) and thus more satisfying.

Remove the difficulty in finding grip and any schmuck and drive fast as long as they can afford the equipment and tires.

That said it doesn't mean we can't think of ways----within reason----to optimise the vehicle which I believe was Paulinho's original intention rather than a fap fest of theoretical posturing.

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.



Edited 1 time(s). Last edit at 02/01/2014 01:26PM by john vanlandingham.