Recent Modifications January 2011

I'm just going to bullet list these changes. I believe that I have acquired enough knowledge about the workings of shifter karts to finally start to write some more technical posts about the 'whys' of karting. After flipping through a good majority of Carrol Smith's three most popular books and after hours of trying to figure out every angle and dimension oh the chassis of this Tony Kart, I feel that it is all starting to make some very time-tested sense. So today's post is mostly about some changes that I have made to the setup of the kart, with a belief reason why I made each of the changes.

Most importantly I changed the steering wheel back to the original. This was for the simple reason that now I will be able to grip the thing correctly. In addition, I can now actually see my gauges. I still don't look at the gauges very often, but now that it is more in my natural line of sight, I might actually be able to keep an eye on the water temperature and thus know if I've sprung a leak again.

Relocated the water temperature sensor. Again nothing huge here, but monitoring the temperature over here places it at one of the highest points in the system. This way if the water levels drop, I will be able to tell much earlier than when the sensor was on the other side right off the radiator. This also organized the wires a bit.

A close look will reveal that two of these tubes are actually venting to open air now. Still have not spent the time to investigate where these two ports lead to, but they are intended to vent to ambient pressure. This will allow for a more tuned fuel flow at higher RPMs and thus allow a greater power band (more power for more rpms means I get to shift less often and that helps me go faster)

Relocated the fuel pump to a more vibrationally isolated position. That is a piece of rubber hose cut along its length to act as a rubber pad. The pump is vacuum driven (blue line) from a port behind the carb (low pressure). The 'fuel-in' is on the bottom left, 'fuel-out' is bottom right. The reasoning behind moving this here is for both greater service life (vibrations have a habit of breaking things, especially things that move/rotate for a living), as well as smoother performance. Since this pump is a turning wheel on some bearings, if I can reduce the amount of shaking it experiences, the pump will do its job much more smoothly and deliver power to me in a much more predictable and reliable fashion. Again, a small change, but there are many small things to update on this.

This is where the fuel pump was moved from. It is right next to the engine and directly under the exhaust header (the port coming right out of the engine, seen in the top right of the picture). What you see here is the ignition computer. The pump was right on-top of it and the two were held in place with the bolts you see. The white metal plate was added to maintain the location of the ignition computer. This too would have been moved if the cables were not so short. These electronics should not suffer too greatly from the vibration of the motor.

Now for some real changes:

Front and rear wheels moved INWARD 0.8 inches on each side. This is a major tuning change to the setup. While it may not sound like much, most real racing cars might be able to move their wheels in and out by this much as most, this kart is 1/4 their size.

Discussion about why:

Pushing the front wheels in will shorten the lever arm from the center line of the kart to the point of contact with the road. By shortening this length the normal force (the amount of force going directly into the road, as opposed to the force going perpendicular) will have to be increased to accommodate the rolling moment of the kart in a turn. Normally people try to push the tires as far OUT as they can to prevent the car from rolling over. Makes sense, but in this case I couldn't make this kart flip over if I tried. They are SO wide that I can afford to narrow up the wheels and not roll over.

What does all this mean? When anyone turns a car, you probably feel the car want to roll (lean over) towards the outward direction of the turn. The force that causes this leaning, is known as the rolling moment (a moment is a force that causes a rotation, as opposed to moving in a straight line. You might know this term as 'torque'). This rolling moment is countered by the normal force on the tires. Otherwise you would just keep rolling over, this is what is happening when a car rolls over, the tires have lost the power to push back against the rolling moment of the car. By pushing the wheeling INWARD, the force on the tires to push back against this rolling moment will have to be GREATER since the 'arm' is shorter.

With this greater normal force on the tire, the kart will GAIN traction since friction depends how hard you push down against the surface (normal force). With more traction, the harder I can corner.

The rear tires also were moved in for similar reasons, but also to help the body of the kart roll more easily. I will discuss the body roll in a kart some other time as well.

Edit: Another side effect of pushing in these tires will be that the inside rear tire will not be unloaded as effectively on the initial turn in. The original plan was to also reduce the stiffness of the rear chassis (via a small bar you can twist to increase or decrease the twist about the axial). However, since I moved in these tires, I am going to have a lot of trouble making the initial turn-in if I don't leave the rear end stiff enough to unload the inside rear tire. So that will be moved from 100% stiffness to only 75% stiffness initially. I will certainly discuss my findings after the next practice session. 

Steering arms were moved to a lower gain position, the height of the arms was lowered to flatten out the link between the steering column and the pinon arms. Leveling this link and lowering the steering gain, fixed a problem before of a non-linear response at high turn angles. While unlikely to impact normal conditions, fixing these issues could only improve the over-all performance.

*** A very important draw back to the last two changes. The draw back to moving the tires inward, and reducing the gain on the steering column will be that I will indeed LOSE some of the feel with what is happening at the front tires for certain 'kinds' of disturbances, while other will remain the same. I will describe the different forces involved in this 'feel' in my first technical post which will deal with nothing but tires. However, for now I can tell you that the reduced returning moment (the force that makes the tires want to point 'straight ahead') felt in the tires as they start to break lose from turning too hard (too high a 'slip angle' or 'alpha') will not, or at least should not, be reduced since it acts very nearly perpendicular to the rolling plane of the tire. Therefore, the distance this plane has from the axis of the king-pin is immaterial since it is parallel to the moment arm. However, the 'feel' under breaking of one tire giving way before the other, will become less sensitive. I will elaborate on these forces in my next post, which is taking quite a bit of time. I have been deriving, simulating, then confirming my results against various publications. This takes time. But for the true nerds out there, I hope we can all rejoice.***



That's the changes made so far. Before the next race though I still need to do the following:

-Fix brakes, or maybe learn how to use them correctly (bleeding completed, they are now as hard as a rock. I am going to give a 'vacuum bleeder' a shot, and if that doesn't work, I'm going to make my own damn tool because this should take 5minutes at best and be near perfect every time)
-Fix shifting lever, it is really loose and difficult to reliably shift
-Insert fuel return line
-Acquire and install new radiator assembly (new radiator acquired)
-Lower camber angle?


So until next time,

-S