Adjusting & Understanding the New E.C. Distributing Mower Front-end Geometry and why it works so well on your mower.
Front-end geometry correctly and accurately set is the key to get the most out of any mower as is with all racing vehicles. Tire wear handling, drivability, and even engine performance can be seriously affected by poor settings whether it’s caused by neglect and or damaged or worn components. A mower chassis is totally different than a car chassis, and so are the setups required to make the mower work properly. While they share the same steering geometries as a racecar, a mower's setup is lets say for a lack of better words exaggerated. A mower is combining a solid rear axle (no differential) with a much shorter wheelbase as compared to the very wide track of a racecar, which presents very odd problems for front-end geometry. Now lets say we set up a mower like we would a racecar set up for a road course. You would experience severe under steer at the moment of turn in. Now some of you may have heard me explain that if you blow the initial turn in, you are playing catch up to the next turn. Now back to the subject, this under steer you would experience is because the combined grip of the rear tires would simply push the front wheels straight. Now to overcome this problem, a mower needs to have the steering geometry set up so that it will lower the inside front wheel and raise the outside front wheel in relation to the chassis at corner turn-in. This is very important. Now this changing of the front wheel heights will cause a weight transfer from the inside rear wheel to the outside rear wheel and the inside front wheel. And this will mechanically lift the inside rear wheel off, or nearly off the racing surface at the moment of turn-in. Now once your mower is turned into the corner, this mechanical weight transfer now is no longer important and is superseded by weight being transferred due to forces of cornering. I cannot begin to tell you how important the transferring weight from the inside rear tire of the mower at corner turn-in is. It is so important that almost every steering geometry angle and setting is designed around this. And another thing is a mower that does not do this enough, or does it too much, will not handle well. If it don’t lift the inside rear wheel enough, then the front wheels of the mower will tend to slide at turn-in, then all of a sudden gain front grip and throw the rear end into a slide. Now this can be one of the hardest and most difficult ill handling issues to drive with and 9 times out of 10 is mistook as a rear grip issue because the rear end slide is such a strong feeling. Now the opposite if it lifts the inside rear too much, it could cause the mower to feel twitchy and hard to drive it smoothly. Now again this initial turn-in weight transfer is one of the most important factors in making the chassis turn and is mostly all a function of the correct front-end set-up. So getting this correct without a doubt very important. And although it is very important to drive mowers smoothly, would you believe it’s possible to be too smooth when turning into a corner. Yes that’s right it is possible. Let me explain that one, if when you turn the wheel too gently at the initial turn-in, the mower can act just as though it won't transfer weight from the inside rear wheel. Think about it but this article is not about how to drive it is all about the front end. With that being said lets look at the individual settings and how they work.
1. Toe is how much the front tires point toward or away from one another. If they are point towards each other this is called toe in. This can make a mower more directionally stable but at the same time make for poor turn in into the corners. If they are pointing away from themselves this is called toe out and is the opposite of toe in. it will make a mower directionally unstable but yet help the mower to initially turn in better. With it toed out, the inner front tire will move down relative to the chassis more so than it would with toe in or no toe in. Toe in any direction causes friction t the point the tire touches the racing surface. This heats the tires that in some cases could result in over heating and also leads to excessive tire wear. This is all generated from the engine power that is not used to accelerate the mower down the straight. Lengthening or shortening the tie-rods adjusts toe.
2. Camber is the amount the front tires lean in or out from each other. With the tops leaning in this is negative camber. With them leaning out this is positive camber. In order to gain the most grip from your tires you want the largest amount of the tire patch as possible on the ground while cornering. This is obtained mostly with camber adjustments. Camber is adjusted using the camber adjusters at the stub axle mounting (screwing the ½ rod ends in or out) But if the king-pin bearings are housed in the stub axle, caster and king-pin inclination will also change when you adjust camber settings (mowers with the king-pin bearings housed in the chassis (not using ½ rod ends) have no adjustment for caster or king-pin inclination). Not all mower front ends have adjustable camber, caster or kingpin inclination.
3. Caster angle is the leaning of the tops of the king pins (the bolts that the stub-axles pivot around) rear ward. Caster is responsible for the self-centering action of the steering and is also an important factor in lowering the inside front wheel and raising the outside front wheel of the mower at corner turn-in. The greater the angle, the greater the changes of the front wheels heights. Caster also causes change of camber when the steering is turned, which results in more negative camber on the outside front wheel and more positive camber on the inside front wheel. Caster is adjusted using the 'camber' adjustable plate ends on the axle by loosening the jam nuts on the ½ rod ends.
4. King-pin inclination is the inward lean of the king-pin (up, towards the centerline of the mower). It causes some of the self-centering of the steering also. It also modifies the amount of camber change caused by the caster angle when the steering is turned, lessening negative camber gain on the outside front wheel and increasing positive camber gain on the inside front wheel. It would be unusual to deliberately alter KPI, but it can be adjusted.
5. Scrub radius is the distance from the center of the tire at ground level to the point where a line drawn through the kingpin axis intersects the ground. Scrub radius works with the caster angle to alter front wheel heights in relation to the chassis. The greater the scrub radius, the greater the front wheel height changes. Increasing scrub radius will also widen the front track. This track increase effectively softens the front end of the chassis, possibly increasing grip. Scrub radius is adjusted using the track spacers on the stub axles.
6. Ackerman effect is caused by the relationship between the position of the kingpins and the position of the outer tie-rod ends (the inward angle of the steering arms) and by the use of two separate inner tie-rod end mounting positions. Ackerman effect causes the inside front wheel to turn substantially more than the outside front wheel. Mowers employ far more Ackerman effect than almost any other type of vehicle except karts and it is used for somewhat different purposes than on a car.
A car uses Ackerman to minimize the effects of tire scrub when the vehicle is turning a corner. A mower (mostly) uses Ackerman effect (in conjunction with caster angle and scrub radius), to make the inside front wheel move downwards as much as possible how!! (By making it turn more), in order to raise the inside rear wheel at turn-in. Some mowers have adjustable Ackerman, involving the use of different length tie-rods, and mounting them in different holes on the steering arms and/or steering column.
SOME MORE FOOD FOR THOUGHT
The next time you are at the races look around the pits and you will probably see quite a few mowers using noticeably negative camber settings. Some of these mowers will be this way through neglect, but many will have been deliberately set-up this way. Some mower racers see many full sized racing cars using pronounced negative camber settings and conclude that if it works for them, it should work for me too. Unfortunately the only problem with this theory is that most racing car tires use radial construction and have very soft sidewalls, while most mower tires we use are as low a profile as possible and mounted on wider than normal rims which help provide stiffer sidewalls, and doesn't work as well with the large camber settings for this reason.
Many racers will spend large sums of money having their engines blueprinted and getting the latest new pipe for their mower, in the belief that the only way to go any faster is to get more out of the motor. Yet many of these racers are simply wasting some of the engine power they already have, and every little bit counts! Engine power can be unnecessarily wasted in two ways, friction in the wheel bearings, and incorrect wheel alignment. Both of these problems cause an increase in the rolling resistance of the mower, which means more power is required to allow the mower to maximize it's speed and acceleration. Since the engine doesn't magically gain power because the mower has more rolling resistance, it goes just that bit slower, and will probably handle poorly. Try thinking about it this way, if you're losing only ½% per lap to the mower in front due to poor alignment (or any other reason) then in ten laps, on a 256-foot track you will lose 12.8 feet. Think about it. It doesn't even take a mower length to lose a race!
So what is good alignment? A well aligned mower will have the toe and camber settings at, or close to zero toe and camber. This will ensure that the tires are being used as they were designed, not slowing the mower on the straights and also maintaining a wide patch of rubber on the track in corners. It will also have enough scrub radius and caster to adequately lose grip from the inside rear tire (a mower should effectively be almost a three wheeled vehicle in corners). Most mowers, most of the time, will handle and accelerate better with toe set to absolute zero. Sometimes slight toe-out will help turn-in to corners, but rarely more than 1/8” except in wet conditions, when larger toe-out settings can be helpful). Setting camber to zero will nearly always be the best starting point, and can be fine tuned using tire wear as a guide, or tire temperatures across the tread which on the loose dirt or grass tracks we run isn’t that noticeable.
A general rule of thumb is the less available grip; the more scrub radius, caster and tire pressure should be used.
NOW the rest is up to you!!!