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This is by far one of the best explanations of FWD suspension physics, charicturistics and overall theory I have ever read. I thought others might enjoy it. If you want to read more from the same site, go to the Far North Racing website. There is an abundence of worth while reading there....

Well enjoy :cool:

"On FWD

Nothing says "triumph of enthusiasm over physics" like a FWD race car.

A FWD has all of physics working against it. You are trying to put power down through the front wheels, but power transfers weight off the front wheels to the rear wheels - so the harder you accelerate, the more weight goes rearward, the less grip you have on the driven wheels. Eventually, this reaches equilibrium at a grip/acceleration level far less than should be possible for the power/weight of the vehicle.

That being said though, there are a couple of advantages to FWD:

They can be smaller and lighter than the average production car. FWD packages the drivetrain very compactly (think about the original Mini) and that lets you build a car to much smaller and lighter physical dimensions. This means that FWDs, sized appropriately, can fit into smaller gaps and can generally avoid slowing down very much. A car like the Jim Harnish CSP CRX may not make much power or be able to put it down (even if it had it) but it can negotiate slaloms and similar course elements at much higher speeds than larger, heavier cars.
FWD cars tend to compete against other FWD cars (meaning a level playing field) and when FWDs are mixed with other driveline configurations, the rules tend to give the FWD cars weight and size breaks compared to RWD/AWD.
I think that FWDs are physically easier to drive. No matter what, an FWD car is never going to experience power-on oversteer. This gives the FWD driver a powerful tool, in that he can manage longitudinal weight transfer with his throttle foot - more throttle means more rear grip, without ever crossing the threshold into power-on oversteer that occurs with a RWD car. (Try watching a novice/hack C5 Corvette driver; see how often they spin on corner exit). It is possible to set up a FWD car with a lot more corner-entry oversteer than with other driveline types, because catching the car is a simple matter of mashing the throttle. Done correctly, you get a car that rotates on lift throttle, but plants the rear firmly once throttle is applied. This can make for slaloms/offsets that follow a steer-lift-rotate-plant driving rhythm that, especially when coupled to small and light physical dimensions, can be easy to drive stupid fast.
So then, the keys to setting up a FWD are going to revolve around the facts that you can't put power down, and when you do, it is going to understeer. I'm making the assumption here that your class rules allow you to make reasonable power levels. If your rules limit power production, the same advice applies, but to a much lower degree.

The very first step (outside of proper tire selection) is thus going to be putting a GOOD limited-slip front diff in the car. If you're going to be forced to drive the car with the wrong end, you might as well get both of the wheels on that end working for you. Otherwise, you're going to be plagued with inside-front wheelspin which is not going to help you put down power and will magnify your power-on understeer problem. Furthermore, there are some neat things that happen when you can change the thrust vector of the car with the steering wheel; meaning that the LSD is going to help you with throttle-on understeer by channeling some of the thrust from the driven wheels into cornering force. This is commonly reported by drivers as the car "sucking the nose in" under power, and it's a good thing.

The next is going to be balancing the car. You want the car biased much more heavily towards entry oversteer than you would with a RWD/AWD; partially because entry is the only chance you get to rotate the car, and partially because a lot of rear lateral weight transfer is going to (thanks to diagonal weight transfer) put more weight on the inside front, and get more grip out of that inside front tire. You're trading rear grip for front grip, but given that you're trying to do everything with the fronts anyway, that's also a good thing.

It's probably worth mentioning that you want as much front weight bias as you can get away with. This will hurt braking, but you aren't going to be braking all that much and you need every ounce of weight on those front wheels. Note, however, that there is an upper limit to how much front weight you can stand; the FWD drag racers were up in the high 80% area at one point, and some very odd things started to happen when the CG got forward of the front contact patch..... So lots of forward weight bias, but not too much.

Now things get interesting.

Given that we want corner-entry oversteer, and given that we want a lot of rear lateral weight transfer (to help plant the inside front) that means it's pretty much a given that we're going to lift the inside rear off the ground on corner entry. That's not a bad thing, in of itself, but it does mean that once we've got that wheel up in the air, the rear has given all it can in the way of total roll resistance, and if our total roll angle is still too high to keep the fronts in their Happy Place, we're going to have to add front spring.

Things get really interesting when we have a front McStrut suspension to deal with. McStruts have really crappy camber compensation in roll; the suspension does not gain negative camber when compressed like a double A arm does. This means that as the car rolls, it is trying to pry the outside front increasingly positive in camber, to the detriment of front grip.

The usual hot ticket for setting up a front McStrut is to admit that one happy tire usually makes more grip than a pair of pissed off tires, so you slap a lot of front bar on the car, get 100% front lateral weight transfer (lifting the inside front off the ground in the process) and then set the static camber such that the outside front tire is in its Happy Place at full roll - look at any fast M3 or CamaroStang.

But with the fronts being the driven wheels, we want LESS front lateral weight transfer, not more. And that means that there is going to be a tradeoff somewhere, especially when it comes to static camber angles. More will help out the outside front at full lateral G, but it will also hurt power-down - and we've been battling to get all the power-down we can get.

And to add further complexity, if our front diff is a Torsen, like a Quaife, it absolutely cannot tolerate having one of the wheels it connects hoisted off the ground. As soon as that happens, it goes open and you lose front drive. So if you have both front McStruts and a Torsen front diff, there is a limit to how much front weight transfer you can get away with before the diff stops working. That shouldn't be an issue because we're trying really hard to keep the front bar soft - but a high CG and too much roll angle without moving to stupid springs might not give us much choice.

This means that there is going to be a compromise in here, and it is probably course-dependent. On a course where you can maximize momentum-maintenance, and especially if your competition is mostly wide, high-power AWD/RWD cars that cannot fit into the same spots you can, you want maximum lateral grip and you crank up front camber to get it. On a pointy-shooty course with a lot of tight corners that you cannot maintain momentum through, you back off the camber in order to get more power down.

Probably the best solution is to find a small, light car with a double-A-arm front suspension; I think there are some Civics that might fit this mold....

Anyway, in practical terms I think we're looking at a front natural frequency of around 2Hz with a rear frequency around 2.5Hz - at least as a start point. Were I setting up a FWD, I'd start there, and then try front frequencies of 1.8Hz and 2.2Hz and see what happened. Based around what I learned there, I'd set the rear bar and/or spring until I had the inside rear off the ground at entry, tune front spring to either get the roll angle I wanted (or found the point where power-down started to suffer) and then start playing with front static camber. This means a lot of iteration around front natural frequency and front camber angle trying to find the happy medium between peak cornering force and peak acceleration - on several surfaces, and on several course layouts, such that there's a "concrete open setup" and a "asphalt tight setup" etc etc.

I'd also seriously consider a traction control system like the RaceLogic system, especially on a high-HP, non-turbocharged car." - Source
 

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yeah really good read here. puts a lot of things into words that I've never really tried to verbalize, but feel on the track. cool to see how mashing the throttle to straighten out your car while exiting a corner actually works.
 

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understand all of this except the whole "frequency" thing what is it,how do we figure it out and how do we change it to meet our needs?
 

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Discussion Starter #6
You should check out his website that I linked, he literally has his whole book posted there, and it is free to read, I am half way through and my view on tires and suspension has been warped and twisted from its original form into a much more complex understanding. I feel like I just went through a static/dynamic model college course. Seriously though his information is great.
 

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Good stuff. If you can put these words into practice at the track. You will be faster at the end of the day. I set up my car to transfer as much weight to the front under braking & off the throttle to maximize rotation & kill understeer. the car easily lifts the inner rear wheel & using throttle modulation I can balance the car while turning. which makes the car faster & easier to turn. But never get on the brakes while the rotation is initiated because it will want to spin out. so just like he says on the article for you to be faster somethings gotta give.

Now the hz I don't know what that means or. how you apply frequency to a suspenssion!? Maybe he is referring to the stiffness for a given end since to make a fwd rotate easier you make the front softer than the rear by at least 100lbs between front/rear spring rates.
So my guess he is referring to a shock & spring dyno in which case we don't have access to. If that is the case then we can easily play around that, with stiffer rear springs & sway bars vs the front ones. If you are getting to know your car in the auto-x or road course start neutral or factory as far as the front vs rear spring rate balance & go from there once you find a set-up that suits your style then focus on mastering it.
 

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def a great read and derseves a sticky im gona read on to fully understand how to setup my bubble bump
 

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man I keep reading his guy's site. a wealth of information and tire, suspension, alignment setup and a lot more. well worth reading through, I've already learned a lot, thanks for posting :up:
 

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I'm bored @ work since gotta pull an overnighter I'm gonna read as much as possible from his site
 

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Discussion Starter #12
There is another site that I found that does a great job of explaining all the options and calculations associated with the suspension, brakes, tires, and auto plumbing...it is intentioned for 4x4s, but def. not limited to them and I dare say goes into the information Dennis G. covers in his book and breaks it down even further, math being a big focus. Links below:

Coilover Bible Part 1
Coilover Bible Part 2
The Brake Bible
Tire Tech
The Plumbing Bible

All the above guides (linked) were written by Bill "BillaVista" Ansell. The org. article I posted is by Dennis Grant.
 

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So here is some more information that ties into this thread, I sourced the below from DG's book, here.
 

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^^^ I read all that and its fascinating. I guess the $1600 Teins I bought 4 years ago are crap then. I've still had good experience with them though...

but as an amateur racer/hobbyist, how can I (or anyone else) dyno all their shocks and spend that kind of money? I do like the idea of trying some koni yellows and doing a group buy and dyno matching them... that may be the only way with my budget to get better balanced shocks. But for the time being I still kill a lot of high end cars on the track with my teins, toyos, and the K20 so what I have will have to do til the budget allows more experiementation.
 

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Discussion Starter #15
^^^ I read all that and its fascinating. I guess the $1600 Teins I bought 4 years ago are crap then. I've still had good experience with them though...

but as an amateur racer/hobbyist, how can I (or anyone else) dyno all their shocks and spend that kind of money? I do like the idea of trying some koni yellows and doing a group buy and dyno matching them... that may be the only way with my budget to get better balanced shocks. But for the time being I still kill a lot of high end cars on the track with my teins, toyos, and the K20 so what I have will have to do til the budget allows more experiementation.
Great question, how to invest more $$$...well I think that the best thing that any of us could do would be find a shop with an interest in people like us or one of us that is willing to spend $$ to make money. WE need to find someone we can pay to dyno a shock for us. As for suspension travel, I am sure we can still use the calculator on his site given that you take the time to figure out all the numbers needed. Although keep in mind we are not running $3k data loggers so all of ours will be theoretical ranges of motion and trial/error testing. The fundamentals are there in his book and I will adapt how I must to make use of them, period. It seems to be the best possible plan. Also AMR told me they will be willing to provide football, aka force vs. velocity dyno charts for the shocks they sell me...which could be invaluable info.
 

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Great question, how to invest more $$$...well I think that the best thing that any of us could do would be find a shop with an interest in people like us or one of us that is willing to spend $$ to make money. WE need to find someone we can pay to dyno a shock for us. As for suspension travel, I am sure we can still use the calculator on his site given that you take the time to figure out all the numbers needed. Although keep in mind we are not running $3k data loggers so all of ours will be theoretical ranges of motion and trial/error testing. The fundamentals are there in his book and I will adapt how I must to make use of them, period. It seems to be the best possible plan. Also AMR told me they will be willing to provide football, aka force vs. velocity dyno charts for the shocks they sell me...which could be invaluable info.
interesting,because when I inquired about shock dynos I was told flat out that they will not provide them.
 

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Discussion Starter #17
I am not sure why they would make that claim to myself and not you...I hope my posting that was not a problem for them...but at the same time I feel like each shock should come with a three plot dyno that is a form of certification. That goes for all companies...after hearing many former pro tuners write/talk about how "off" shocks and springs can be from their claimed rates it makes me want proof with every purchase. I think it is stupid for them to believe that people will continue buying their product once they find out the difference between claims and reality.
 

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something interesting on his full article is he complains that the problem with more basic suspensions (tein in my case) is the inability to adjust bound and rebound independently.

My Monoflexes adjust them both the same, and when I purchased, I chose them because what little experience I had at the time, I wouldn't have a clue how to make fine adjustments (bound and rebound) to increase performance. With the current setup, I can confirm that the car handles as though they are more or less accurate based on the way they car responds when I make changes. Maybe I got lucky and have a decently balanced set.

I also know tein does make coilovers that offer the exterior gas reservoirs and independent bound and rebound adjustments. who knows, maybe those are no good.
 

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Discussion Starter #19
I am going to look around, but I might drop some serious change and get Penskes. They are currently only competing with AMRs. Granted my car will be a DD/Road race...and (occasionally) local track racing vehicle, but I care about precision, and I plan to own the car a very long time so there is no other choice than to buy what I want/need for the "long haul".
 

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I am going to look around, but I might drop some serious change and get Penskes. They are currently only competing with AMRs. Granted my car will be a DD/Road race...and (occasionally) local track racing vehicle, but I care about precision, and I plan to own the car a very long time so there is no other choice than to buy what I want/need for the "long haul".
pretty serious for a daily. might be worth running some koni yellows and see what you think before spending big. Plus on a street car they are going to get a lot of abuse, and they should be plenty for a weekend track day. If not, you can always upgrade down the road. just my .02 but penskes would be nice
 
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