Basic princeples of vehicle dynamics

I am writing this because I have found the misinformation of driving to be running rampant and so many people saying silly things and asking silly questions. Now instead of trying to explain in a few words peolpe can just point out this thread. If the write up is good enough maybe we can make it a sticky.
I am no master of driving, I have not done much racing, I haven't drifted much on pavement but I have done my fair share on dirt. If anyone thinks I made a mistake or if you would care to add to this, the more knowlege shared the better.

The basic dynamics of a car work like this.

Step on the gas and the weight of the car is shifted to the rear tires.

Step on the brakes and the weight of the car is shifted to the front tires.

Turn the wheel while in motion and the weight of the car is shifted to the tires on the outside. For example if you turn the wheel to the right your left tires will be supporting the majority of the cars weight ie heavy, the inside tires will be light.

The more weight a tire has on it the better the traction will be, the less weight the less traction.

Now we move into complex dynamics.

If you step on the brakes and turn the wheel right the weight will shift to the left front tire giving this tire the most traction. The right rear will have the least traction.
This is when the rear end of the car is most likley to want to oversteer, or slide twords the outside of the corner.

If you turn the wheel and step on the gas the reverse will happen and the front will want to understeer, basicly go straight despite the wheel being turned.

every car is different so you must apply that to any situation.


So lets start on the road. most people know when you start to slide turn into the slide right. Well thats a good start but which way into the slide. And if you are sliding twards a cliff do you really want to steer into it :confused: At the driving school I worked at we always said look where you want to go and steer in that direction. No matter which way your car is pointed you should still be trying to get where you want to go, and that will almost always be steering into the slide. Of course if your car spins past 90 deg to the road you want to straighten out your wheel. Past 90 degrees this is the fastest way to get your car straightend out.
It is normal when people loose controll of their car they get the strong urge to stop, step on the brakes and what happens.
If the car was oversteering the front tires grab, and it worsens the problem, or said another way you start to spin faster.
If the car is understeering you have already lost traction with your steering and best stopping wheels so it will just make the problem worse, you will continue to slide straight into the snow bank. The best method for the average driver is to let off the gas, step on the clutch, and focus on steering it where you want it to go. This evens out the vehicle dynamics and frees up 100 percent of your traction for steering. If you are a skilled driver and are very familiar with the car you can use some of the techniqes shown below to help bring your car back under controll.


There are a couple different ways to drive close to the threshold. In most road racing you want to slide very little.
The difference between the direction the vehicle is pointing and the direction it is traveling is known as slip angle. In most road racing your tires will slide a little but the slip angle will usually be under 5 or 10 degrees, where as in rally and drifting it can excede 45 degrees.

Since most people seem to ask about drifting I will touch on vehicle dynamics in this situation, it also applys to initiating the proper slip angle on other racing .
In a performance situation there are also differences between front wheel drive, rear wheel drive, and all wheel drive.

In a front wheel drive coming into a corner as you step on the brakes the weight shifts to the front of the car. As you initiate the corner the car will be more likley to oversteer, as the back starts to slide out application of the gas pedal will shift weight to the back tires and bring the slide under controll. Too much gas and the car will start to understeer. A controlled drift is the proper amount of gas brake and steering that can only be attained through experience.

In a rear wheel drive you enter the corner the same getting the weight on the front tires and getting the rear to step out. the difference here is since the drive wheels are already sliding if you give it gas it will only worsen the slide, of course if you give it brakes you will also worsen the slide. I have by far the least experience in a rear wheel drive so all I can say is it is fine controll over all the inputs to avoid excessive oversteer. The basic theory still applies to reduce oversteer you want to get the weight shifted back to the rear tires, you just can't gas it like a FWD.

The AWD has the good characteristics of the two as well as the bad.
I would like to clarify something here. So many people talk about how much AWD improves the handling of their vehicle. Untill you know how to throw your AWD sideways in a controlled drift the only benifit you will get from your AWD is accelerating traction. Other than that stopping and turning characteristics and overall acceleration can even be worse in an AWD due to the added weight, more rotating mass, and more unsprung weight.
When entering the corner you can throw it sideways in the same manner as the other two but now you have all four tires pulling you into the apex of the corner. Simalar input techniques are used to keep the vehicle balanced but once you get it set up the way you want it and start aproaching the apex you can give it more gas and the car will continue to pull you twards the inside of the corner.

There is another technique used in rally I don't remember what it is called but it is used by FWD, RWD, and AWD, drivers.

If you are aproaching a sharp right hand corner you will get the car over to the right side of the road, next you swerve over to the left side of the road. Now you can picture as you do this the suspension is compressed on the right side of the vehicle, now as you apply brakes and start to turn into the corner the energy that has been built up in the right side suspension unloads and pushes the car into a quicker more extreme oversteer than otherwise possible.


This ended up being far longer than I expected but it is still about one percent of the information required to understand the effects of vehicle dynamics.
Which is maybe ten percent of understanding how to drive.
This is not a how to guide to teach you to drift or take a corner at speed.
If you want to learn how to drive ( I know everyone thinks they know how to drive but if you want to learn to drift, or race, or just improve your driving) do it in a safe place. There are auto crosses, Rally crosses, and TSDs that anyone can partisipate in. There are driving schools across the country that teach top level driving techniques. This I highly recomend, If you want to learn right do it from the pros.If you can't do that find someone who knows more than you, but if they are not a professional driver, professional instructor, or well educated by one of the above it is quite likely they will teach you as many bad habits as good.

And Dagnabbit everyone stop talking like you know how to drive. Show me a top three finish in any high level race with corners and I might start to think twice about your abilities.

Wow, brilliant arrogance on your part. Not only did you completely ignore the technical forum that explains this, but you took it upon yourself to answer a question that in fact no one on this forum has asked nor that I believe needed to be answered.

Not to mention you have GROSSLY over-simplified vehcle dynamics in failing to introduce the variables of roll, adherence, arc of travel, rise versus droop, or even the basics of chassis grip versus tire grip.

For instance......

If you step on the brakes and turn the wheel right the weight will shift to the left front tire giving this tire the most traction. The right rear will have the least traction. This is when the rear end of the car is most likley to want to oversteer, or slide twords the outside of the corner.

This, in fact, does not equal a prelude to an oversteer situation. Only if the chassis rolls enough at the front to lessen the contact patch of the inside rear but not enough at the rear to compensate by loading the outside rear will you have oversteer (and this is common to front or rear wheel drive cars).

Also, the front end must dive, or have enough inherent "rise" in the control arm arc, to allow the rear to "droop" and thus unload. If the rear suspension still has preload, it will still track well enough to keep the tail in line.

The example I can give is to compare an IMSA GTU Beretta to its SCCA Showroom Stock companion. In the GTU car, the suspension is very rigid and allows for very little droop or rise. The arc of travel, ergo, is very small. Thus, under braking, the car's chassis prohibits weight shift, in the exaggerated simplified version you spoke of, and keeps the car neutral through the corner. The Showroom Stock car, however, has the long control arms and soft suspension of its street version, thus allowing the front end to dive under braking while the rear end kicks up, unloading the rear causing it to step out while the increased weight on the nose overstresses the tires and pushes the car offline.

In other words, the chassis and tires first determine the physics of the car. Simplistic "this is what happens when you do this" statements are best left to the instruction booklets of video games.

And as far as my credentials, I was one of the design team that built this car and I guarantee you that a chassis man knows more about vehicle dynamics than a driver.

http://www.ismasupers.com/Roster%20Pages/7-700-C-Tim-Jedrzejek.jpg

I appoligize for oversimplifying my examples I was trying to keep my post short enough that someone could possibly make it to the end. I also missed the technical forum writeup, I would love a link to it. If this is completly incorrect or less than the current writeup then delete it or let it fall. I wrote this because so few people know how the dynamics of their vehicle effect their driving experience. If this interests them as I said at the bottom this is only a small percentage of what there is to learn. And yes it is oversimplified, Thats why I said basic princeples. I also said it is effected by the individual vehicle maybe I should have gone into a little more detail. I already learned more than I knew about how suspension effects dynamics. How many people do you know who are going to start out auto crossing or or rallying a IMSA GTU Beretta, or maybe an f1 car. I didn't really go into how they would handle differently, in fact I can't really say other than that I know it's a lot differently. I also don't figure too many people driving f1 cars are going to need my advice on how they handle.
For example someone said they thought you had to spin your back tires to initiate a drift. I could have just awnserd it with a yes or no awnser but I thought it might help someone like this understand a little more what is going on. I think every thing you added makes my post ten times more technical and gives a deeper understanding of the mechanics and dynamics. I didn't mean to upset anyone.






Wow, brilliant arrogance on your part. Not only did you completely ignore the technical forum that explains this, but you took it upon yourself to answer a question that in fact no one on this forum has asked nor that I believe needed to be answered.

Not to mention you have GROSSLY over-simplified vehcle dynamics in failing to introduce the variables of roll, adherence, arc of travel, rise versus droop, or even the basics of chassis grip versus tire grip.

For instance......



This, in fact, does not equal a prelude to an oversteer situation. Only if the chassis rolls enough at the front to lessen the contact patch of the inside rear but not enough at the rear to compensate by loading the outside rear will you have oversteer (and this is common to front or rear wheel drive cars).

Also, the front end must dive, or have enough inherent "rise" in the control arm arc, to allow the rear to "droop" and thus unload. If the rear suspension still has preload, it will still track well enough to keep the tail in line.

The example I can give is to compare an IMSA GTU Beretta to its SCCA Showroom Stock companion. In the GTU car, the suspension is very rigid and allows for very little droop or rise. The arc of travel, ergo, is very small. Thus, under braking, the car's chassis prohibits weight shift, in the exaggerated simplified version you spoke of, and keeps the car neutral through the corner. The Showroom Stock car, however, has the long control arms and soft suspension of its street version, thus allowing the front end to dive under braking while the rear end kicks up, unloading the rear causing it to step out while the increased weight on the nose overstresses the tires and pushes the car offline.

In other words, the chassis and tires first determine the physics of the car. Simplistic "this is what happens when you do this" statements are best left to the instruction booklets of video games.

And as far as my credentials, I was one of the design team that built this car and I guarantee you that a chassis man knows more about vehicle dynamics than a driver.

http://www.ismasupers.com/Roster%20Pages/7-700-C-Tim-Jedrzejek.jpg

Duplicate posts deleted. Please refresh your understanding of the AF User Guidelines (http://www.automotiveforums.com/vbulletin/guidelines.html), you've already agreed to abide by them.

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And Dagnabbit everyone stop talking like you know how to drive. Show me a top three finish in any high level race with corners and I might start to think twice about your abilities.

For driving, granted, not top level, but here are the NESCCA 2003 Points. Check out SSB.

http://www.ner.org/RR/2003/points%202002-2003.xls

For a car and team I worked on, here's a race winner for you

http://www.world-challenge.com/2001/spt/gt-results.html

I have also done setup and am crew cheif on a former SCCA T1 lap record holder at NHIS and Lime Rock, the current Spec Miata track record holder at NHIS, and have crew chiefed the ITS Miata that at the 12 hours of Summit Point, has finished 3rd, 1st, and 2nd in ITS, and 5th, 2nd, and 4th overall. My specialty is suspension setup and vehicle dynamics, and I have been involved directly in road course racing for 9 years. I could go on, but that would make the post really long.

Keep in mind that this is an AUTOMOTIVE ehthusiast forum, and as such, there are some automotive enthusiasts involved in fairly high professional levels of racing that do keep an eye on this board.

So yoshimitsuspeed, what's your background that makes you an expert in vehicle dynamics?

Sorry to take this off-topic, but kfoote I was wondering if you are an engineer. Actually just wondering how you became a crew chief, and what recommendations would you give someone trying to obtain a similar position?

TIA

Basically, I'm a crew chief at club level, standard mechanic at Pro level. Educationally, I have a BS in Mathematics, and was sort of drafted into the postiotion when the team owners and drivers thought I could do a better job setting up their cars and making adjustments on them than they could. The first major race I crew chiefed was the 24 hours of Moroso, mostly because I had worked as general crew and suspension consultant the two previous years at the 24 hours of Daytona, and I was one of only three people on the team that had ever been involved in a race longer than 45 minutes (the other two were drivers). In the first attempt, we were leading ITS 8 hours in and running top 10 overall when the engine blew. The best bet is to start off at an SCCA race, there's always people looking for help there.

As I said I am no master of driving. And probably know a good deal less than kfoote and Laylas keeper about vehicle dynamics. I also know more than I wrote but I was trying to get a basic understanding across. I have worked at a driving school in colorado started by a very sucessful porsche cup racer, where we used top level techniques to teach begining drivers how to drive. It is a situation much like this, since you don't have the time to teach them all the finer points of driving you give them as much basic information as possible that will help thier knowlede grow with personal experience of the finer points of driving. If they are interested to learn more then they have a good platform to do so from.

Just next time, use this as an answer to someone's question. Not to be rude but nobody asked for an explanation of vehicle dynamics. I dont know if someone said this earlier or not, but in addition to the fact that there is already a forum about this on AF, people who have questions can also use a search engine to get their answers.

But i do appreciate your purpose for doing this (i understand why as well).

wow u immpressed me w/ your massive amounts of common sense that u illustrated here today