Sunday, December 4, 2016

Front End

The front end design takes some explanation. When I started this hobby it did not seem apparent to me which type of suspension was necessary for which surface or vehicle dynamics. As time went by I learned exactly what type of suspension is necessary for the type of dynamics that a vehicle of this weight class and speed is acceptable - and preferable.

Questions to ask yourself:
  • Will I be OK driving my kart in mud/snow/ice/sand
  • Am I only OK driving my kart on smooth pavement
  • Do I have dentures or will I want them in the future
  • Will I ever consider titling this as either an ORV (off-road-vehicle) or pseudo motorcycle
  • How fast do I really feel it is safe to go before increasing my term life-insurance policy
Ideally one suspension setup is good for all conditions. Sadly this is never the case. If you plan on driving on rough bumpy surfaces, like dirt/gravel/mud it is important to have alot of travel. This is not that type of front suspension. This is a hybrid of old and new, using just enough structure and shear dynamics to avoid bump steer.

This suspension is an inverted version of the classic cradle design used in pre war race cars, and inspired by horse-drawn buggy construction. Back then, a half round leaf spring with an axle suspended under the chassis, and a few 'dampers' or spring joints to smooth things out worked like a charm and was cheap & easy to fix. The problem with doing that style on this type of vehicle is that it's heavy. Plus there are alot of moving parts that dont contribute to the overall enjoyment of the ride. Add to that the complexity of fabricating an axle with delicate geometry angles, and potential for error and it all adds up to driving displeasure in my humble "Average Joe" opinion.

Remember that this blog is an editorial blog so my opinions are loud and clear. In my opinion using that type of suspension on anything but a horse-drawn buggy does not make sense in the modern era no matter how 'period' it looks.

Meanwhile in Paris (at that time) Edmond De Marcay built a Cyclecar:

which started up recently as filmed here on this nice video:

Which used the same principle of suspension that I am using on this kart. The chassis IS the spring. If you lift a single wheel on the back end of my Type-69 the front wheels stay planted, as in they dont move. If you stand on the front end of the chassis and jump up and down, it flexes. The wood and the steel move together, and allow each wheel to move independently without complex parts.

Another useful item that seems more expensive than it's worth is the ARC racing rack&pinion steering box. This beautiful aluminum billet design is light and powerful and solves the age old problem with any 'pitman arm' steering setup which is over-center-lock that happens when you travel over center with the linkages. It also is buttery smooth and makes steering feel responsive and intentional. If I'm to put my life in the hands of a welded pitman arm or a billet aluminum rack & pinion steering block, the choice is simple.

BMI karts was also key here. The connecting rods they sell (also Aluminum) are inexpensive and come in almost any length. It took a few tries to get the right size but I finally settled on 13.5" rods.

Azusa 'kingpins' or steering spindles come in 5/8" and 3/4" (your preference) and can be mounted easily at any angle. I cut and re-welded the angles of the link on the kingpin to follow the instructions for proper 'Ackerman' angle. Which is where the angle of the links point to the center of the rear axle in a resting position. I also rotated the entire axle at 13.5ยบ for proper caster, allowing the turned wheels to decrease bump-steer and to increase stability at speed. I will show a detailed drawing of this later.

The Axle is a straight Aluminum tube (smooth bore) that measures 1-1/4" outside and 5/8" inside. This is a strong front axle and is as light as I dare. The bits welded on the ends were done by a professional welder for the bargain price of $60. I was planning on trying some garage 'brazing' using 'ALUWELD' rods but then thought it a terrible idea. 3/8" wall stock Aluminum needs proper TIG welding. This is the single part of this design I could not rationalize using simple techniques. So when it comes time, and you need welding done, there are hundreds of fabricators around town willing to spend an hour or so creating professional results for about $60 an hour. I prefer that option to a potential broken weld, or a bent tube, or even worse a 30-lbs front axle. The front axle as shown here weighs roughly 10 lbs. ...with the springs and hardware.

I was willing to compromise the statement of 'cheap and simple' to realize something that was light and effective. Maybe in the future I will come up with a simple mechanical method of creating the front axle, but for now this is the only 'specialty' part on the design. The brackets holding the kingpins are TIG welded in place and are square. It might have been a better idea to utilize negative Camber here, but there is not much weight on the front end so my theory is that most times the wheels will sit square - thus the Camber angle is neutral.

Lastly there is a large wide plate connecting the two 'buggy' springs. This plate serves a dual purpose: 1. to apply a friction surface to the front axle and prevent twisting or rotating and 2. to keep an equidistant relationship between the tips of the leaf springs. As a result the front end is VERY robust. The wheels are able to move independently and as a consequence of the geometry are able to move along the rotation pivot of the rear axle by design. The entire front end pivots from the back of the leaf springs and all angles are meant to move with the flex of the springs. -CW

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