Sunday, 25 January 2015

Suspension Geometry - Advanced

So, you're looking for some advanced suspension geometry? That must mean that you've either read my Introduction or have enough knowledge on the basics already. In either case, welcome friend!

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The introduction to suspension geometry finishes with toe so we'll move off from there. The next angle is Ackerman.

(Image source: Auto-Ware)


The Ackerman steering principle states that when a car travels around a corner, the inside wheel follows tighter curve than the outside wheel shown by the image above. Most manufacturers design their cars to work with this principle; this gives the best control and traction around the corner as well as minimalised tyre wear. But we are drifting.

Most RC drift chassis' have a predetermined Ackerman angle and is adjustable but the question is how much?

(Image source: Turbo Bricks)

This image shows a standard left-hand turn and the angle of each wheel. Notice how the rear wheels are perfectly inline on their solid axle and the steered wheels (front wheels) are both pointing at different angles. This is so that each wheel sits at 90° to the center of the circle created by the turn. With the wheels facing away from each other on lock, this is referred to as positive Ackerman.

(Image source: Turbo Bricks)

This second is however, shows the position of all wheels when drifting around the same corner. What we see is that the rear wheels no longer follow a radius; they can be as far out as you can manage. More importantly, notice that the steered wheels are still at a 90° angle to the center of the created circle but are now facing towards each other. When the wheels are facing towards each other on lock, this is referred to as negative Ackerman.

With our drift cars, we ideally want as close to 0 Ackerman (neutral Ackerman) or slightly negative. With a positive Ackerman setup, the wheels can end up fighting against each other.

How to adjust? There are different means of adjusting depending on what chassis you run. I have seen front hubs (knuckles, uprights) have multiple holes offering various Ackerman angles. There are also some chassis' that supply a turnbuckle that joins the two wipers instead of a solid arm and that is how you make adjustments.

(Image source: Broadtech)

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The next three are a pretty complex subject but are directly related. Steering Axis Inclination (SAI) or King Pin Inclination (KPI) as it's popularly known, Included Angle and Scrub Radius.

(Image source: Institute of the Motor Industry)

The SAI angle is created by drawing (an imaginary) line through the upper and lower arm ball joints when looking from the front or the rear of the chassis. I am still yet to see a RC drift car that has negative SAI where the angle through the ball joints leans outwards, they all come with positive SAI where the angle leans inwards towards the center of the chassis.

The main reason that drifters adjust their SAI angle is so that they can make the SAI line and the camber line intersect at the road surface (refer to the image above), this is known as the Included Angle.

The SAI and the camber lines create the sides of the included angle and where they meet is called the vertex. The closer the vertex is to the road surface, the less effort is required to steer and the better the steering response is. This is due to the tyre being able to rotate at the point of contact which, if set correctly, should be where the tyre meets the road. Should the point of intersection be above or below road surface, the tyre will end up "scubbing" as it goes from lock to lock; the difference between the two lines will create the Scrub Radius.

(Image source: Institute of the Motor Industry)

Of the large offset image, the left line is the pivot point for the wheel. As the steering is operated, the wheel and hub assembly will all use this left line (SAI) to pivot around forcing the wheel to scrub through it's travel. The small offset image shows a small positive where the vertex is slightly below road contact and the final negative offset being slightly above.

(Image source: RE-Extreme)

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(Image source: CompetitionX)

Ride height is another tuneable item considered when setting suspension geometry. Not so much an angle but literally the clearance the chassis has from the ground measured in mm (or inches if you like) and has more to do with weight distribution. Another term to mention here is Rake.

Rake angle is created by the height of the front of the chassis to the height of the rear; if the front is lower than the rear, this gives you a Normal Rake Angle. If the rear is lower, you get Reverse Rake Angle and if the front is equal to the rear, you get 0 Rake.

(Image source: Broadtech)

The simple and easiest way to adjust ride height is by adjusting the shock collars. These are the rings found on the body of the shock absorber and most drift chassis' will come with the adjustability whether they are plastic or aluminium. Adjusting ride height will effect spring preload so do it moderately or change springs if required.

The effects of rake or reverse rake will differ from chassis to chassis but as a general rule, remember that a higher front will shift the weight towards the rear and higher rear will shift the weight towards the front.

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(Image source: Town Fair Tire)

The last item on this post will be Thrust Angle. The reason I decided to include this is because I have personally suffered from thrust (lol).

Thrust angle is created by the rear wheels and although the front wheels are affected, until you correct the rear wheels, the fronts will disagree with anything you tell them. Notice on the image on the left, the two rear wheels are pointing to the right instead of straight ahead or toeing in slightly. Any forward movement will result in the rear wheels "pushing" the rear end of the car to the right and (gradually) forcing the front of the car to the left.

This is something that I see happening on many chassis' due to cheap or faulty plastic or even incorrect settings but is an easy fix. Simply by making sure that all the parts are in correct working order and are fitted correctly with minimal play as well as making sure that the rear toe is set correctly. Aluminium upgrades will also reduce the chances of having an incorrect thrust angle as they are better machined and don't easily wear.

So there you have it (again); this time, it's advanced or complex suspension geometry although the same golden rule applies:

Only ever change one thing at a time

And as always, make sure you are having fun while doing it. After all, this is a fun hobby, let's keep it that way.