THSCC Forums

Time to open up a big can o' worms.

Back when I was learning how to design a suspension for pavement surfaces, I read Fred Puhn's book "How to make your car handle". It completely demystified the topic for me. Here's what I got out of it:

1. Do whatever you need to in order to heat up the entire cross-sectional area of the tire consistently (ie camber, tire pressures)

2. Weight transfer split front-to-rear should be changed with springs to affect balance. (This is also sometimes called LLTD or Lateral Load Transfer Distribution) The reason this works is because of the nonlinear coefficient of friction of the tires -- Lateral force vs Normal force is NOT a straight line and lateral force falls off with increasing normal force. So increasing weight transfer at one end decreases grip at that end and *increases* grip at the other.

So far so good -- all the above is for steady-state cornering. Shocks are a different story since they should only affect transitions. Just extrapolate #2 and imagine the shock doing the work of the spring in load transfer during transitions. This understanding has served me very well for almost 10 years of competitive driving on paved surfaces.

Now on to dirt.

Most of the underlying assumptions are no longer true. Tires don't heat up due to constant slip angle with a flat surface. Shocks are almost always in transitional state but the springs may still be steady state. Is the grip curve the same as tarmac tires? Does lateral force fall off as normal force is increased? Or could it actually increase due to the "digging in" factor? Also, what is the optimal slip angle for lateral force?

Among some of the rally suspension tuning "advice" I have heard:

1. You want shocks with really stiff compression damping
2. Stay away from shocks with really stiff compression damping
3. Stiffening swaybars will behave just like on pavement - stiffen the end you want to slide more
4. Stiffening swaybars behaves exactly opposite as on pavement
5. It's best to run with no swaybars
6. Stiffen springs on the end that you want to slide more
7. Stiffen springs on the end that you want to slide less

No one is in agreement on anything it seems. Any reputable sources of information?

Discuss

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

I've heard all these things before, too. You just have to look these people up and find out whether they're dominating their region's events. If they're not, I ignore everything they have to say. Just like in autox, there are lots of people out there who swear that X modification really screwed up their handling, or that "it really works great!!!" when they're finishing mid-pack or lower at every event.

Having used more suspension setups than I can remember, I don't really think it's that important, as long as the car isn't bouncing off the bumpstops, incapable of turning, or doing something else that makes it extremely hard to control.

The most important thing you can do to the car is reduce the weight by a significant amount. I'd love to have a crappy-looking 2.5RS that runs just like my current one, that I could drop a few hundred pounds off of. THAT would make a noticeable difference. Running the WRX and the RS (which is 400 lbs lighter) back-to-back at the last rallyx made it obvious which one was better for rallyx & why.

Hence the reason that I am looking for theory right now and not personal experience.

Fred Puhn did wonders for me in demystifying suspension tuning. I don't care if he never turned a wheel, as long as he could explain the physics behind it. He did, and it's pretty simple.

Yeah, I'm an engineer

_________________
Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

Well, I've seen numerous threads on several different sites where people are asking for rally or rallyx suspension tuning theory, but I've never seen any answers that involve books or other resources where you can go read about it. It's always been people's personal experience that ends up taking over the thread. I don't think this book exists, unless it's written in a non-English language or something.

Interesting link.

http://homepage.virgin.net/shalco.com/subaru_setup.htm

Here a small quote from it
" The more grip you've got, the more important it is to run low and the less grip you have means you need as much suspension travel as possible and to keep the springs soft. More grip equals low and stiff, less grip equals high and soft. "


Patrice

I watched that interview somewhere. The main thing I got out of it is...



Not much setup detail there, though.

Mike,

You might try to find current or archive issues of Racecar Engineering and Race Tech magazines (published in England) but distributed here through a "model company" in NJ whose name I forget. (Some Borders and Barnes & Nobels carry current issues). Rally Cars are covered fairly frequently and you probably can pick up their design goals from the articles. NOBODY will understand the issues better than the teams involved in WRC and similar professional rallying.

EDIT: Actually the people doing professional off road and stadium cross type car/truck/buggy and "ATV/Buggy/bike" dirt racing probably understand pretty well also. Given the US interest in these sports there may be some books available with good engineering.

EDIT Again: Have you checked out any of the books on oval dirt track racing or even the Hoosier Tire dirt tire tech area on their website (assuming there is such an area)? Some of the dirt oval classes use tires that are not particularly sticky as I recall.

Some of the TV coverage and/or DVD's may also include technical info that is actually credible.

I'm sure you have googled for books.

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Dick Rasmussen

FS 50 2018 Mustang GT

I think this is the biggest thing.

While I have never rallied I used to work (back in hight school) on a race car that ran dirt ovals. My Friend and his father were front runners pretty consistantly. The Cars generally sat a little higher than the other competitors. He wanted more travel so hit did not hit the bumpstops when the track got rutted up as the races were ran on it.

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David Teague
2015 Lexus IS 250c
1994 Honda Del Sol HS 39
2009 Dodge Journey R/T
http://teaguefamily.us

If you find a book on rally suspension design, that would hopefully have enough details in it that you can pick out what things they do to deal with specific issues. Many of the issues that performance rally teams deal with are not ones you'll expect to encounter in rallycross. For example, very high speeds, sudden surface changes, large jumps, dips and compressions, etc. Mostly we're driving around at very low speeds on a pretty flat, semi-smooth surface.

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Carl Fisher

Be Cool to the Pizza Dude:
http://www.npr.org/templates/story/stor ... Id=4651531

the thing with round track you could also use weight(which most tracks have a required weight of the car) to make it turn also and going in one direction it could be used as a benefit. we also used a limiting strap on the rear end(like monster trucks) so if we hit a big dip or somethig like that, that it woud not pull the car off the ground and thus upsetting it ALOT more than just boudnign


I just wish i could figure something out whether it is me or the car. i got the forester sti suspension and love it but i felt like i wsa either on or off where i should have been. i know that i hit the bumpstops a few times being in the wrong spot and finding some rutts. David if you want you are more than welcome to drive my car at the next rally and lend some input!

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[color=#FFFFFF][/color]http://www.usedmiatapartsonline.com for all your used miata parts needs
http://www.wagnersauto.com for your salvage needs
2000 miata se Estreet,going to rally
2004 mazdaspeed miata bsp #77
shifter kart also(someone buy this $1500)

Kevin,does this mean you finally realize that you need to quit buying cars and develop the RS? With all the money you have spent messing witht hte STi and the WRX you could have a really well prepped RS.

Mike, you should give some of the rally prep companies a call. Vermont sports car, Primitive, and Paul Richard's company in canada, Rocket something or other.I remember there was a guy that posted in the Nabisco motorsports forum that was selling a rally prep manual. It was kind of expensive but it supposedly had a lot of good info.

_________________
Silver Honda
Green Ford
Blue/White Suzuki
Red Triumph
Grey BMW

"Never let life be shaped by fear of its end"

No, you're a towel!

The manual Marcus is talking about was by Randy Zimmer, and it's around $200 or something. Goes into detail about how to prep a 2.5RS for RALLY, including info about what's going to break & how often. If you plan to rally an Impreza, you should buy it.

And it's Pat Richard, not Paul Richard. Noob.