Suspension Technical Definitions

**McChook** · 24-06-2004 11:27 PM

Hey guys

As promised, I was going to explain castor for you, but, I haven't been able to find many websites that want to explain, so I was writing it myself until I realised there must be some hardcore articles out there... So, after a few hours of searching, this is it. Thanks to all the contributing sites

Camber, Toe & Castor - the difference
Most people hear these words but rarely have the opportunity to understand what they mean and do.

TOE
This is the amount that the wheels are pointed in or out EG often called "total toe in or out". On rear independant suspension cars this is also adjustable, Subaru, Daihatsu etc. NOT live axle cars though. Fords, Holdens etc. Often measured in mm this little change makes huge differences in handling. As a car moves forward the suspension often moves back reducing toe in, so cars are often set with 1 - 3 mm toe IN. If the car has toe out it often tends to wander on the road more.

On all our rally cars we run about 1 - 2 mm on the front and BACK.

Rear is less important as it tends to be less likely to be affected by knocks, pot holes and kerbs. BUT it is important to be correct

CAMBER
Think of the angle of most roads, look along it and it slopes to the side to make the water drain or is banked on fast freeway corners. This is camber, the angle your wheel sits in relation to vertical when pointed ahead and you look straight at the car from front or rear. Measured in degrees, most common road cars have 0 - .5 degree std. some more. Too much NEGATIVE camber will wear out tyres on the inside. POSITIVE wears out the outside. Look at really old cars they often have POSITIVE camber. (I do not know why).

The correct amount varies depending on CASTOR, (see follows) and how you drive your car. If you have little castor and you love driving fast through corners then you need more NEGATIVE camber, if you do heaps of freeway driving then less is better.

THE REASON? When you turn a corner the outside tyre tends to roll under the rim, causing it to wear on its outer edge. By laying it on its side you reduce this effect. Too much and it will wear on the inside, too little and wear on the outside.

NOTE this is often used to stop wide tyres rubbing on wheel arches or suspension points, this case tyres wear is not a focus! REMEMBER too much neg camber and you will lose traction in straight ahead driving as the tyre is not flat on the road.

CASTOR
This is the best of both! BUT is often not adjustable on modern cars.

Camber stays the same if the pivot (vertically) of the car suspension is zero. EG if you turn the wheel about its axis (steer not spin) it stays the same. BUT if the axis is at an angle (for and aft) then the more you steer the car, the more camber you get!

Its hard to relate, but if you imagine looking at the LHS of the cars wheel, with front to your left, if you grabbed the top of the axis and moved it back (to horizontal) with the wheel position staying still then this is castor, then imagine, if you turned the wheel to the right 90 degrees then the wheel will lay flat, this is obviously an extreme example but best explained.

SO, the more castor the more the wheel will increase negative camber the more you turn the wheel. BUT too much castor and the car will want to wander as it has less tendency to want to point straight ahead.

REMEMBER
Check your tyre pressures, over 80% of cars have UNDER inflated tyres AND most companies, TYRES AND CARS, suggest low, for better ride. On most Subaru's, Hyundai's Daihatsu's etc try 35 PSI it will steer better, ride a bit harder, but go HEAPS better!

On most cars these days we can supply camber kits to increase and allow adjustable camber, most Subarus have some adjustable limits. Castor well thats hard, but possible!

Remember that you pay for what you get, a cheap wheel alignment means just that!

MRT

Toe-In/Out is a slight steering angle that is preset into the suspension. Toe-in has the tires pointing slightly toward the center of the car's front. Toe-out has the cars pointing slightly away from the car. In the diagram above, there is zero toe-in/out. Toe-in/out is used to offset the natural change in toe position caused by braking and accelleration.

Unsprung Weight
Unsprung weight is a measurement of the weight of everything outboard of the wishbones or suspension links, plus 1/2 of the weight of the wishbones or links and spring/shock. It has a great effect on handling. The diagram below demonstrates why unsprung weight is so important:

The more weight outboard of the car, the more force bumps exert on the suspension (and ultimately the chassis). This force must be dealt with using springs, dampers and anti-roll bars (described below), and the more force, the more difficult it is to keep the tire planted on the road. This is especially true of lighter weight cars. In the example above, if the car weighs 1000 lbs, a 2G bump would result in a vertical force of 10% of the car's weight. This will at the very least reduce the grip of the car, because the weight of the car is what keeps the tire planted, and pushing a car up into the air with that much force will inevitably reduce the weight on the tire, and hence grip.

Tyres
As the first point of contact with the road, the tires work in conjunction with the suspension geometry and weight transfer dynamics to provide grip. Many different types of tires exist, but provided you are building for a specific class, you can easily select a particularly good or popular tire.

The grip provided by a tire is linked to the coefficient of friction (Cf) of the rubber compound and to the tire's construction (Radial/bias). This coefficient indicates the lateral grip the tire is capable of providing for a given weight being placed on it. Racing slicks are very high Cf tires, in the range of 1.0 or more. Street radials, on the other hand, rarely even approach 1.0. So what is in a number? If you were to place 500 lbs weight onto each of four tires with a Cf of 1.0, you could expect 2000 lbs (actually a little less) of lateral grip. Without aerodynamic aids to add to vehicle weight, the car would almost achieve a 1G turn.

Wheels
Of course, the wheel is what the tire mounts on. Wheels also come in a myriad of widths, sizes and materials.

The primary types of wheels used in racing are alloy and steel.

Alloy wheels can be constructed to very minimal weights, as alloying materials such as aluminum and magnesium can be used. They are also generally much more expensive than their steel counterparts, but they also lack the dent resistance of steel wheels. An alloy wheel, when struck by a curb will sometimes shatter, and possibly worse, crack (only later to fly apart!). Nonetheless, for most motorsports series, alloys are the choice.

Steel wheels can also be constructed to amazingly low weights. Their cost is quite a bit less than the alloys, due mostly to lower cost construction. Steel wheels are deformable when struck, and will usually allow air to leak out of the tire, as opposed to shattering. NASCAR, and the general stock car scene use steel wheels due to the extreme forces encountered by 2 ton cars.

Uprights (Wishbone suspension)/Knuckles
The upright or knuckle attaches the wheel, brake rotor, hub, brake caliper and steering arm to the car (of course, the wishbones and control arm(s) do the final attachment to the chassis)

The upright or knuckle determines the king-pin inclination, and the final camber, caster, and toe settings of the wheel and tire. These various factors are demonstrated in the diagram below.

Castor angle

If the lower swivel is further forward than the upper you have positive castor (aka trail). In this situation the centre of the tyre's contact patch trails behind the point where the king pin axis intercepts the ground. Thus
the steering self centres, the drag on the tyre will pull it back behind the king pin axis. Just like a bike (push or motor).

With things the other way round the steering would tend to un-centre itself (just like reversing).

The castor has no effect on the roll. Body roll is caused by the contact with the road being lower than the centre of mass of the car so when cornering the momentum of the vehicle trying to carry on in a straight line pulls the vehicle and tends to tilt it over. What we know as centrifugal force, but which doesn't really exist.

Experiments

"The factory race team drivers almost always use 25 or 30 degree castor blocks.

30 degree will add some turn-in steering, but it will cause understeering when exiting corners at speed or on high-speed sweepers. The 30 degree blocks will also stabilise the car on fast, bumpy conditions.

Using 20 or 25 degree castor will give the car more low speed steering and more corner exit steering. When using higher amounts of castor, raise the swivel hub by placing 2 'xyz' spacers under the outer ball joint to keep bump steer to a minimum"

I dunno if this helps at all? IIRC, castor also affects how much the wheel on the inside of a corner 'flicks out'. i.e. If you turn right to full lock, the right wheel will be tilted out a the top a little and it will be turned slightly more than the left."

also thanks to http://www.gmecca.com

here's some RWD assistance for toe..
Toe-in / Toe-out

I'm not the definitive expert on these matters, but AFAIK front toe is usually set so that in normal driving condition the wheels point straight ahead. A Caterham is RWD and so when driving forward the front wheels are pushed back (EDIT, THAT IS CASTOR) and a little toe in is needed to push front wheels to point straight ahead. A Mini is FWD and when driving forward the front wheels are pulled forward and thus a little toe out is needed to keep front wheels straight.

Changing toe settings will affect steering feel and not much else. adding toe-out will make a car turn in quicker and make the steering more sensitive. Too much toe-out will cause darting while braking heavily and an unsteady feel at high speeds. Too much toe-in will give the steering a very lazy feel. Toe settings also need to take into account the amount of Ackermann angle designed into the front steering arms. Bump steer is when the car darts to one side or the other when one wheel moves up or down more than the other wheel. This is usually a design of the suspension, or a bent steering arm, or modification (ie. lowering) that alters the geometry too much. Bump steer happens when the up and down arc of the steering arm and the suspension arms are of a different radius.

Toe-out in front wheel drive cars is to get the wheels to a close to zero toe setting when acceleration causes the wheels to move forward slightly due to suspension bushing deflection.

No toe-out in the rear!!!! If most cars have toe-out added to the rear it can cause serious oversteer, sometimes snap-oversteer when letting off of the throttle.

Source: Haynes Workshop Manual 1969 to 1996 up to P reg (UK) (added 25 11 02)

front 1.58mm toe out

3 +/- 1 degrees positive castor

2 +/- 1 degrees positive camber

rear 3.17 mm toe in

0.5 to 2.5 degrees positive camber

thanks to www.ime.co.uk

How can Whiteline HELP????
http://www.autospeed.com/cms/A_0134/article.html
Should get you guys started

Hope you can find some useful and intellectual assistance here, and please, do some of your own research on the subject, becuase it really is worth it

Supplementry articles may be found on Autospeed....
http://www.autospeed.com/cms/A_0913/article.html