Understanding vehicle suspension dynamics...Part One
12-11-2001, 10:23 AM
#1
Honda-Tech Member
Thread Starter
Understanding vehicle suspension dynamics...Part One
Well here is a first contribution to the FAQ page... 
It was not written by me, but I feel the content is valid and have edited it with regards to this board.
Understanding Vehicle Dynamics
Before we start we have to have an explanation of common vehicle dynamics terms. We had better understand the common ways to describe the different aspects of vehicle dynamics before we even turn a wrench, so here we go:
<u>Friction circle</u>: This is basically a vehicles performance envelope. It's expressed in lateral G’s, accelerating and braking G’s. When graphed, the friction circle looks like an egg with the X axis lateral G’s and the Y access braking and accelerating G’s.
<u>Understeer</u>: This is when, at the limit of vehicle traction, the front of the car slides first before the rear. Race car drivers call this "push". This is the way that many cars come set up to behave from the factory as it is the most predictable for average drivers (like the Type R). The crash mode for understeer is that when the limit of adhesion is exceeded, the car will plow strait ahead off the road nose first. This is not the fast way to have your car set up, but is best if you are an inexperienced driver. When the car understeers you should regain control if you let off the gas, unless of course you run out of road first. (That is what air bags are for.) It is not efficient for extracting maximum lateral G’s because the car will dynamically use the front tires excessively for turning, overloading them while the rear tires basically just hold the back of the car up - scrubbing off significant speed. Front wheel drive cars tend to exhibit understeer as the final terminal mode of balance.
<u>Oversteer</u>: This is when, at the limit of vehicle traction, the rear of the car slides first before the front. Race car drivers call this "loose". The rest of us call this "spinning out", "spinning a shitty", "doing a brodie" or even crashing. The final crash mode of oversteer is backwards, tail first into the woods or in the worst case spinning round and round with the driver as a helpless passenger. Since the infamous days of Ralph Nader and the Corvair, most auto manufactures avoid oversteer like the plague. Oversteer is difficult for an inexperienced driver to handle because recovery requires judicious use of countersteering and throttle feathering to control; fine motor skills that only some of us can deal with. Although oversteer looks neat and macho it is really a slow way to drive except in maybe pro-rally on the dirt . Oversteer is slow on the pavement because hanging the tail out bleeds off a great deal of speed going through a corner. Conserving the momentum is the fast way around as turn.
<u>Neutral</u>: This is the fast way around a turn where all four wheels slide evenly. Since the total friction circle traction of each tire is being used, all the available grip that the tires have is being put to the ground. Racers call this "drifting". This not to be mistaken for the idiotic Japanese Option Magazine video stuff which makes a mockery of proper driving technique. Neutral is the fast way around a corner most of the time. Neutral is also the hardest handling mode to achieve for the suspension tuner.
<u>Polar Moment of Inertia</u>: Or PMI as we will refer to it, is a description of how a cars mass is distributed along the length of the vehicle. A car with a high PMI is like a rear engine, rear drive car like a Porsche 911 or a front engine, front wheel drive car like our beloved Type R, same thing only the poles are different, so to speak. A car with a low PMI would be a mid engine car like a Boxster. Low PMI cars have most of their mass about the middle, high PMI cars have the mass at one end or another. Low PMI cars are the easiest to get a neutral balance out of due to the balanced, centralized mass. High PMI cars like to oversteer, in the case of the 911 or understeer like our cars. To get a feel for this phenomena, hold a bowling ball in one hand and rotate it back and forth by twisting your wrist. Now get a set of dumbbells of the same weight, grab the middle of the bar and do the same thing. Bet the bowling ball wants to rotate easier right? Guess what type of car will be easier to get neutral!
<u>Slip Angle</u>: This is the wonderful thing that allows us to tune our cars suspensions despite the design limitations caused by the PMI. Proper manipulation of slip angle is the great equalizer and is what suspension tuning is all about. Slip angle is the difference in which a cars wheels are pointed vs the angle that the tires contact patch is placed on the road. The main thing that affects slip angle is the manipulation of the individual load placed on each wheel while cornering. This is the key for suspension tuning. A front wheel drive car has most of the weight on the front wheels. So the front wheels run at higher slip angles and develop understeer. Conversly the same for a rear wheel drive, rear engine car developing oversteer. That is also a reason why a mid engine car with equally loaded tires will be more or less neutral. Slip angles, weight distribution and PMI are the main factors in how a vehicle will handle.
Because Type Rs are front heavy, front tire overloaded, front wheel drive cars, does that mean that we are condemned to econobox hell for driving fun? Heck no! By design we can not change the basic layout of our cars to significantly change the PMI or weight distribution but we can sure tweek the slip angles of the tires to achieve world class handling out of our killer compact sport transportation units.
The easy way to tweek the slip angles are with anti-sway bars and springs. Shock absorbers, going against what people think that they do, are not really for changing the handling balance. Shocks mostly act as spring dampers and affect understeer/oversteer balance mostly only in transient (which is big word for a change from straight line travel to turning) maneuvers like initial turn-in and zig zaging around slalom cones.
Changing to heavier springs changes the slip angle differential by resisting the cars tendency to roll on the end of the car that they are installed on. The resistance of the heavier spring to compression causes more weight to be transferred to the outside wheel of the end of the car that they are installed on as the car tries to lean over in a corner. This causes that wheel to proportionally run at a higher slip angle than it normally would. If you put heavier than stock springs in the rear of your Type R while not changing the spring rate of the front, the car would tend to understeer less.
<u>Anti-roll bars</u> work in much the same way. Antiroll bars are torsion bars attached to the cars chassis and are linked to the right and left control arms. Antiroll bars offer resistance to independent side to side wheel movement. This is how these bars limit sway in the turns and hence their name. While limiting roll, the bars also cause weight transfer to the outside wheels. By altering the diameter of the antiroll bars or installing them where there were none before adds yet another chassis tuning element. If you were to increase the size of the rear antisway bar on an Type R you would be increasing the amount of weight transfer to the outside rear wheel, thus causing it to run a bigger slip angle. This would give you more oversteer.
<u>Tire pressure </u>also can affect the slip angle. Higher pressures reduce the slip angle and lower pressures increase it. A great deal of suspension tuning can be done for free by adjusting the tires pressure.
<u>Alignment</u> also has a great deal of effect on a vehicles handling balance. Caster and camber affect how a tires contact patch is positioned on the ground by compensating for a tires tendency to flex and lift the inside tread while cornering, By helping keep the tread flat, these settings can increase or decrease the available friction circle traction on an end of a car thus affecting balance. Toe in or out can affect balance also by changing how a vehicle turns in.
Suspension adjustment-->Affect on vehicle balance--> Symptom of TOO MUCH adjustment
Front spring rate increase. More understeer .Terminal understeer, front of car hops in corners, excess wheelspin in FWD car
Front spring rate decrease. Less understeer. Too much oversteer, oversteer then understeer if spring is so soft that the car bottoms under lean, car bottom excessively with a jolting ride
Rear spring rate increase. More oversteer .Too much oversteer, hop in corners, twitchy
Rear spring rate decrease. Less oversteer. Car understeers, if way too soft car understeers then oversteers as car bottoms out under lean, car bottoms out excessively with a jolting ride
Front antisway bar stiffer. More understeer. Terminal understeer, Lifts inside front tire off the ground which can cause massive wheelspin, also not good for most effective tire usage as inside wheel is now doing nothing
Front antisway bar softer. Less understeer. Oversteer
Rear antisway bar stiffer. More oversteer. Big time oversteer, Can cause the inside rear tie to lift off the ground which is not two bad on a FWD car.
Rear antisway bar softer. Less oversteer. Understeer
Front tire pressure higher. Less understeer. No traction as tire is crowned so more understeer, bad wheel spin, jarring ride, center of tires wears out
Front tire pressure lower. More understeer. Edges of tires wear quickly because tire is folding over, feels mushy, tires chunk because low pressure means more heat build up
Rear tire pressures higher. Less oversteer. No traction as tire is crowned so more oversteer, bad wheel spin on RWD cars, jarring ride, center of tire wears out
Rear tire pressures lower. More oversteer. Edges of tires wear quickly because tire is folding over and cupping upward, feels loose in back, tires chunk because low pressure means more heat build up
More negative camber (Front) Less understeer .Poor braking, car is road crown sensitive, twitchy, tires wear out on the inside edge
Positive camber (F) More understeer .Poor braking, car is road crown sensitive, twitchy, tires wear out on the outside edge You almost never want to have positive camber unless you are a dweeb
More negative camber (R).Less oversteer, more rear grip, less breakaway warning when limit is exceeded/-3 degrees. More oversteer, car feels twitchy in back, tires wear out on inside edge
More positive camber (R) More oversteer Car feels twichy in the back, tires wear out on outside edge
Ride height too low Car twitchy with unpredictable dynamics
Toe-in front. Car is stable while going straight. Turn in is average. Car has slow twichyness under braking, feels odd, kills the outside edge of tires.
Toe-in rear car is less likely to suddenly oversteer when throttle is lifted. Weird slow rocking movement in back, feels slow but still unstable, wears the outside edge of tires.
Toe-out front. Car turns in well, works pretty good in FWD cars as they tend to toe-in under load. Car is real twitchy under braking, car is very road crown sensitive, car wanders on straight road, kills inside edge of tires.
Toe-out rear. Helps the car rotate, useful on tight low speed courses and slalom events. Not to good for street driving, causes lift throttle oversteer, car makes violent side to side rocking motions in rear, tires wear more on insides .
[Modified by Big Phat R, 2:06 PM 12/11/2001]
It was not written by me, but I feel the content is valid and have edited it with regards to this board.Understanding Vehicle Dynamics
Before we start we have to have an explanation of common vehicle dynamics terms. We had better understand the common ways to describe the different aspects of vehicle dynamics before we even turn a wrench, so here we go:
<u>Friction circle</u>: This is basically a vehicles performance envelope. It's expressed in lateral G’s, accelerating and braking G’s. When graphed, the friction circle looks like an egg with the X axis lateral G’s and the Y access braking and accelerating G’s.
<u>Understeer</u>: This is when, at the limit of vehicle traction, the front of the car slides first before the rear. Race car drivers call this "push". This is the way that many cars come set up to behave from the factory as it is the most predictable for average drivers (like the Type R). The crash mode for understeer is that when the limit of adhesion is exceeded, the car will plow strait ahead off the road nose first. This is not the fast way to have your car set up, but is best if you are an inexperienced driver. When the car understeers you should regain control if you let off the gas, unless of course you run out of road first. (That is what air bags are for.) It is not efficient for extracting maximum lateral G’s because the car will dynamically use the front tires excessively for turning, overloading them while the rear tires basically just hold the back of the car up - scrubbing off significant speed. Front wheel drive cars tend to exhibit understeer as the final terminal mode of balance.
<u>Oversteer</u>: This is when, at the limit of vehicle traction, the rear of the car slides first before the front. Race car drivers call this "loose". The rest of us call this "spinning out", "spinning a shitty", "doing a brodie" or even crashing. The final crash mode of oversteer is backwards, tail first into the woods or in the worst case spinning round and round with the driver as a helpless passenger. Since the infamous days of Ralph Nader and the Corvair, most auto manufactures avoid oversteer like the plague. Oversteer is difficult for an inexperienced driver to handle because recovery requires judicious use of countersteering and throttle feathering to control; fine motor skills that only some of us can deal with. Although oversteer looks neat and macho it is really a slow way to drive except in maybe pro-rally on the dirt . Oversteer is slow on the pavement because hanging the tail out bleeds off a great deal of speed going through a corner. Conserving the momentum is the fast way around as turn.
<u>Neutral</u>: This is the fast way around a turn where all four wheels slide evenly. Since the total friction circle traction of each tire is being used, all the available grip that the tires have is being put to the ground. Racers call this "drifting". This not to be mistaken for the idiotic Japanese Option Magazine video stuff which makes a mockery of proper driving technique. Neutral is the fast way around a corner most of the time. Neutral is also the hardest handling mode to achieve for the suspension tuner.
<u>Polar Moment of Inertia</u>: Or PMI as we will refer to it, is a description of how a cars mass is distributed along the length of the vehicle. A car with a high PMI is like a rear engine, rear drive car like a Porsche 911 or a front engine, front wheel drive car like our beloved Type R, same thing only the poles are different, so to speak. A car with a low PMI would be a mid engine car like a Boxster. Low PMI cars have most of their mass about the middle, high PMI cars have the mass at one end or another. Low PMI cars are the easiest to get a neutral balance out of due to the balanced, centralized mass. High PMI cars like to oversteer, in the case of the 911 or understeer like our cars. To get a feel for this phenomena, hold a bowling ball in one hand and rotate it back and forth by twisting your wrist. Now get a set of dumbbells of the same weight, grab the middle of the bar and do the same thing. Bet the bowling ball wants to rotate easier right? Guess what type of car will be easier to get neutral!
<u>Slip Angle</u>: This is the wonderful thing that allows us to tune our cars suspensions despite the design limitations caused by the PMI. Proper manipulation of slip angle is the great equalizer and is what suspension tuning is all about. Slip angle is the difference in which a cars wheels are pointed vs the angle that the tires contact patch is placed on the road. The main thing that affects slip angle is the manipulation of the individual load placed on each wheel while cornering. This is the key for suspension tuning. A front wheel drive car has most of the weight on the front wheels. So the front wheels run at higher slip angles and develop understeer. Conversly the same for a rear wheel drive, rear engine car developing oversteer. That is also a reason why a mid engine car with equally loaded tires will be more or less neutral. Slip angles, weight distribution and PMI are the main factors in how a vehicle will handle.
Because Type Rs are front heavy, front tire overloaded, front wheel drive cars, does that mean that we are condemned to econobox hell for driving fun? Heck no! By design we can not change the basic layout of our cars to significantly change the PMI or weight distribution but we can sure tweek the slip angles of the tires to achieve world class handling out of our killer compact sport transportation units.
The easy way to tweek the slip angles are with anti-sway bars and springs. Shock absorbers, going against what people think that they do, are not really for changing the handling balance. Shocks mostly act as spring dampers and affect understeer/oversteer balance mostly only in transient (which is big word for a change from straight line travel to turning) maneuvers like initial turn-in and zig zaging around slalom cones.
Changing to heavier springs changes the slip angle differential by resisting the cars tendency to roll on the end of the car that they are installed on. The resistance of the heavier spring to compression causes more weight to be transferred to the outside wheel of the end of the car that they are installed on as the car tries to lean over in a corner. This causes that wheel to proportionally run at a higher slip angle than it normally would. If you put heavier than stock springs in the rear of your Type R while not changing the spring rate of the front, the car would tend to understeer less.
<u>Anti-roll bars</u> work in much the same way. Antiroll bars are torsion bars attached to the cars chassis and are linked to the right and left control arms. Antiroll bars offer resistance to independent side to side wheel movement. This is how these bars limit sway in the turns and hence their name. While limiting roll, the bars also cause weight transfer to the outside wheels. By altering the diameter of the antiroll bars or installing them where there were none before adds yet another chassis tuning element. If you were to increase the size of the rear antisway bar on an Type R you would be increasing the amount of weight transfer to the outside rear wheel, thus causing it to run a bigger slip angle. This would give you more oversteer.
<u>Tire pressure </u>also can affect the slip angle. Higher pressures reduce the slip angle and lower pressures increase it. A great deal of suspension tuning can be done for free by adjusting the tires pressure.
<u>Alignment</u> also has a great deal of effect on a vehicles handling balance. Caster and camber affect how a tires contact patch is positioned on the ground by compensating for a tires tendency to flex and lift the inside tread while cornering, By helping keep the tread flat, these settings can increase or decrease the available friction circle traction on an end of a car thus affecting balance. Toe in or out can affect balance also by changing how a vehicle turns in.
Suspension adjustment-->Affect on vehicle balance--> Symptom of TOO MUCH adjustment
Front spring rate increase. More understeer .Terminal understeer, front of car hops in corners, excess wheelspin in FWD car
Front spring rate decrease. Less understeer. Too much oversteer, oversteer then understeer if spring is so soft that the car bottoms under lean, car bottom excessively with a jolting ride
Rear spring rate increase. More oversteer .Too much oversteer, hop in corners, twitchy
Rear spring rate decrease. Less oversteer. Car understeers, if way too soft car understeers then oversteers as car bottoms out under lean, car bottoms out excessively with a jolting ride
Front antisway bar stiffer. More understeer. Terminal understeer, Lifts inside front tire off the ground which can cause massive wheelspin, also not good for most effective tire usage as inside wheel is now doing nothing
Front antisway bar softer. Less understeer. Oversteer
Rear antisway bar stiffer. More oversteer. Big time oversteer, Can cause the inside rear tie to lift off the ground which is not two bad on a FWD car.
Rear antisway bar softer. Less oversteer. Understeer
Front tire pressure higher. Less understeer. No traction as tire is crowned so more understeer, bad wheel spin, jarring ride, center of tires wears out
Front tire pressure lower. More understeer. Edges of tires wear quickly because tire is folding over, feels mushy, tires chunk because low pressure means more heat build up
Rear tire pressures higher. Less oversteer. No traction as tire is crowned so more oversteer, bad wheel spin on RWD cars, jarring ride, center of tire wears out
Rear tire pressures lower. More oversteer. Edges of tires wear quickly because tire is folding over and cupping upward, feels loose in back, tires chunk because low pressure means more heat build up
More negative camber (Front) Less understeer .Poor braking, car is road crown sensitive, twitchy, tires wear out on the inside edge
Positive camber (F) More understeer .Poor braking, car is road crown sensitive, twitchy, tires wear out on the outside edge You almost never want to have positive camber unless you are a dweeb
More negative camber (R).Less oversteer, more rear grip, less breakaway warning when limit is exceeded/-3 degrees. More oversteer, car feels twitchy in back, tires wear out on inside edge
More positive camber (R) More oversteer Car feels twichy in the back, tires wear out on outside edge
Ride height too low Car twitchy with unpredictable dynamics
Toe-in front. Car is stable while going straight. Turn in is average. Car has slow twichyness under braking, feels odd, kills the outside edge of tires.
Toe-in rear car is less likely to suddenly oversteer when throttle is lifted. Weird slow rocking movement in back, feels slow but still unstable, wears the outside edge of tires.
Toe-out front. Car turns in well, works pretty good in FWD cars as they tend to toe-in under load. Car is real twitchy under braking, car is very road crown sensitive, car wanders on straight road, kills inside edge of tires.
Toe-out rear. Helps the car rotate, useful on tight low speed courses and slalom events. Not to good for street driving, causes lift throttle oversteer, car makes violent side to side rocking motions in rear, tires wear more on insides .
[Modified by Big Phat R, 2:06 PM 12/11/2001]
12-11-2001, 11:40 AM
#4
Honda-Tech Member
Re: Understanding vehicle suspension dynamics...Part One (tjtruong)
Reminder to everyone:
You can use Zeroforum's notepad feature to save a pot. Just clik on the
under the name of the poster. 
You can use Zeroforum's notepad feature to save a pot. Just clik on the
under the name of the poster.
12-11-2001, 11:44 AM
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Re: Understanding vehicle suspension dynamics...Part One (Big Phat R)
[Mr. Burns Voice] Excellent [/Mr. Burns Voice]
Where do you get this stuff? Good read'n.
Thanks
Where do you get this stuff? Good read'n.
Thanks
12-11-2001, 11:50 AM
#7
Honda-Tech Member
Re: Understanding vehicle suspension dynamics...Part One (IntegraT)
First off, it is certainly not my intention to argue anything you have said, but to clarify some concepts which to me are still vauge and inconsistant with some things I've read.
More tire pressure in rear BPR said decreases oversteer UNTIL ADJUSTED TOO FAR. However I've also heard, which makes sense to me, that increase rear pressure increases oversteer, because of a smaller contact patch on the rear tires, which will help the *** come out more easily.
Is this to say that only if they are underinflated at operating temps will an increase cause less oversteer, up until the peak as far as tire crown (perfectly flat)???
Please clarify.
More tire pressure in rear BPR said decreases oversteer UNTIL ADJUSTED TOO FAR. However I've also heard, which makes sense to me, that increase rear pressure increases oversteer, because of a smaller contact patch on the rear tires, which will help the *** come out more easily.
Is this to say that only if they are underinflated at operating temps will an increase cause less oversteer, up until the peak as far as tire crown (perfectly flat)???
Please clarify.
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12-11-2001, 11:57 AM
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Re: Understanding vehicle suspension dynamics...Part One (sackdz)
More tire pressure in rear BPR said decreases oversteer UNTIL ADJUSTED TOO FAR. However I've also heard, which makes sense to me, that increase rear pressure increases oversteer, because of a smaller contact patch on the rear tires, which will help the *** come out more easily.
.
.
Drinker
12-11-2001, 12:12 PM
#12
Honda-Tech Member
Thread Starter
Re: Understanding vehicle suspension dynamics...Part One (sackdz)
The size of a contact patch doesn't change - only it's shape (just ask nsxtcjr 
). This is because a) the force of gravity is constant and b) the weight of the car is constant. This is of course true until you lift one of the inside tires - which would greatly increase the weight over the outside tire.
Using stock tire pressures as a starting point, increasing the rear tire pressure will cause the sidewall to flex/roll over less which would decrease the oversteer tendency of this tire until it's limit of adhesion is reached. Note: most "competition" tire pressure settings are higher than OEM standards.
The other confusing factor is how tire pressure dynamically changes as you heat up the tire compound - which can have a great effect on adhesion.
This is not a simple science - and I meant in no way to imply that it was - only to provide a basic starting point for some good tech discussion.
BPR - who is sick of people asking for emergency photoshop pics of rims on cars
). This is because a) the force of gravity is constant and b) the weight of the car is constant. This is of course true until you lift one of the inside tires - which would greatly increase the weight over the outside tire.Using stock tire pressures as a starting point, increasing the rear tire pressure will cause the sidewall to flex/roll over less which would decrease the oversteer tendency of this tire until it's limit of adhesion is reached. Note: most "competition" tire pressure settings are higher than OEM standards.
The other confusing factor is how tire pressure dynamically changes as you heat up the tire compound - which can have a great effect on adhesion.
This is not a simple science - and I meant in no way to imply that it was - only to provide a basic starting point for some good tech discussion.
BPR - who is sick of people asking for emergency photoshop pics of rims on cars
12-11-2001, 12:12 PM
#13
Honda-Tech Member
Re: Understanding vehicle suspension dynamics...Part One (itr009)
For more good info, buy the book "Going Faster". Good post, should be required reading for the forum
12-11-2001, 12:21 PM
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Re: Understanding vehicle suspension dynamics...Part One (Big Phat R)
For the most part the information is spot on. A great read, however there is one correction I'd like to see be made. The "anti-sway bars" are incorrectly named, and instead should be reffered to by their proper name "anti-roll bars".
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