$db_character_set = 'utf8'; ?> Geomertry lesson
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Author Topic: Geomertry lesson  (Read 10832 times)
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mr. warehouse
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« on: December 25, 2008, 09:59:52 pm »

I saw this on another forum a while back and thought it would be good to have here. I never knew where to put it so I made a new board just for this sort of thing.

Info below is courtesy of Sled...
I hope he's doesn't mind.


camber is the measurment, in degrees, of how much the top of your tire is leaning in or out from vertical. Positive camber is when the tire leans away from the center of the car, and negative camber is when the tire leans in towards the center. Almost ALL cars are set up with a slight amount of camber in the front to aid in handling characteristics (about 1 to 3 degrees of negative camber), while most vehicles have zero degrees of camber in the rear. In volkswagen applications, where we have a transaxle, the suspension must create camber as it is lowered or raised (IRS has much less camber change due to the relatively parallel pivot points). Excessive camber can hinder performance greatly and diminish the life of your tires. BUT the biggest misconception, is that camber alone is what destroys tires so quickly on the rear of a lowered swing axle car..this is not the case. Too much negative camber only contributes to the premature wear slightly.


Toe-in and toe-out is the measurement (in degrees) of the difference between the leading edge and trailing edge of your tires. If the leading edge measurment is smaller than that of the trailing edge measurement, you have toe in, and vice versa for toe out. Again, almost ALL cars are set up with a minimal amout of toe-in in the front and little if no toe-in the rear. Correct toe alignment can improve handling and help the vehicle track straighter. Excessive toe in will obviously eat up tires very quickly and cause very erratic handling. As a general rule, i set up my vehicles with 1/8" to 3/16" of toe-in in the front and zero to 1/8" of toe in the rear.


caster is the measurement in degrees, of how far forward or back the axis your spindle rotates on, is from vertical. ALL cars, expecially new cars and properly adjusted old cars, have a few degrees of positive caster (when the axis is leaning back). Caster is a very important part of suspension design and tuning. Caster is what makes the steering wheel return to center as you exit a turn, and it is also what make your vehicle track straight at high speed. Extreme caster has its advantages, but also great disadvantages. A good example is top fuel dragsters, they have a LARGE amount of positive caster (as seen when the wheels lean severely when turned side to side) to keep them going in a straight line at 300 mph+ speeds, BUT this cause very poor slow speed handling. For a daily driver (non high performance race car) there is a fine balance. Most cars should have 2-4 degrees of caster. I chose to setup my bus with about 7 degrees as i like my cars to track straight as an arrow at high speed and return to straight as I exit a turn. Also i plan to roll with a little bit of nose down rake from time to time, and this essentially reduces the amount of positive caster. THIS is one of the big contributing factors to buses not handling very well when lowered. Most people roll with a little bit of rake and a STOCK bus only has MAYBE one degree of caster. Negative caster is the biggest cause for wandering and flitty handling.


Ackerman angle is a VERY important and often overlooked part of suspension design and turning. Ackerman angle is what causes the inside wheel to turn sharper than that of the outside wheel during a turn. Go turn the wheel to either side then look at the wheels and you SHOULD see that the inside wheel (in the direction the vehicle would be traveling) should be turned sharper than the outside. This is required because as a vehicle turns, the inside wheel is circling in a tighter arc than the outside. IF your ackerman angle is wrong, and both wheels are parallel during a turn, it will cause excessive wear and a "push" feeling wheel the vehicle resists the turn.) The correct ackerman is set when you can draw an imaginary line from the center of the king pin (or axis of spindle rotation) through the tie rod end, into the very center of the rear axle (for volkswagens this would be the center of the transaxle) Here is a good diagram. Unfortunately as we modify our suspensions, the ackerman angle is changed, resulting in premature parts wear and ill handling (although generally very sligh problems)

ALL!!! narrowed beams cause incorrect ackerman, as well as lowering (causing the spindle and center of wheel to move forward) The ONLY way to fix this problem is to modify the spindle arm to bring it back in to alignment. You can see in my build thread where i dropped and also re-aligned the spindle arms on my bus.


Trail is an aspect of car suspension that is very important, but most vehicles have VERY little if any trail. Trail is when the rolling center of the wheel is BEHIND the axis of spindle rotation. What trail does, along with positive caster, is cause the wheels to naturally try to track in a straight line. The best example of trail is a grocery store shopping cart...if you were to deliberately spin the wheels so that the rotating axis was in FRONT of the turning axis, and then pushed the cart, the wheel would immediately try and swivel to follow. Excessive trail is a VERY bad thing as it makes steering very difficult. Trail is something that is not adjustable in about 95% of suspensions, it is set from the factory design.


bump steer is not a part of the design geometry of suspension, but it is the direct effect of having your tie rod outer ends being much higher or lower than that of the inner ends, As well as having the tie rods be longer or shorter than the suspensions arms (in the case of buses, this is irrelevant as they do not have A arm suspension). As the suspension compresses and extends, the outer ends of the tie rods travel in a large arc. The greater the angle of the tie rod, the greater its horizontal length changes as the tie rod end travels its arc. SO the more angle you have in your tie rods, the more bumpsteer you can expect to have, this is the reason for flipping the tie rod ends to enter from the bottom of your spindle arms.

As far as how all of this pertains to lowered buses (in most of our cases) it is important to consider these things when deciding to make suspension alterations. Some things you can get by with having incorrect, while others have a greatly adverse affect when they are wrong. The more you lower a swing axle suspension, the more camber, toe in, and lateral wheel movement at an exponential rate. Even with correctly extended spring plates, you will still encounter radical toe change due to the spring plates being at a more severe angle. In the front, the further the torsion arms are from horizontal, or angled down, the worse the ride quality will be. Greatly lowered buses ride poorly for this reason. As the torsion arms begin to approach 45 degrees (sometimes an even greater angle) the less the suspenion compresses when you hit a bump, and the slower it can react on up travel.

these are the biggest aspect of suspension geometry. Obviously it can be far more complicated and there are far more things to consider when building a race car, but we are not :D
Gadget Boy
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« Reply #1 on: July 20, 2009, 12:17:25 am »

This is a great Topic and one that Should be read by Any and All who are considering modifying there Cars , Thats means All of Em  thumbs up
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