Where Can I Learn More?
You might find the Physics of Racing web site interesting. It goes into these (and many other) topics in much greater depth. Check it out at http://members.home.net/rck/phor
What are Oversteer and Understeer?
[BSD] There is a technical definition and the real world one. The technical definition of understeer is when the front tires have a greater slip angle than the rears. Oversteer is when the rear tires have a greater slip angle than the fronts.
In real world terms, understeer is when the car won’t turn any sharper, even if you turn the steering wheel more. At some point, the front end may start to grip less even when the steering is turned sharply and the result is the car continues in more of a straight line than a sharp turn. That is understeer.
Oversteer is when the car’s rear tires lose grip in a turn while the front tires are still gripping. If the rear end starts to slide out from under you in a turn, that is oversteer.
A funny way to put it is if you smash into the wall with the front of your car, it was understeer. If you back into the wall, it was oversteer. Ok, so its not funny, maybe amusing.
Stock production cars are made to have understeer. The US legal system tends to blame the car manufacturer if the car does a 180 before smashing into a wall while if the car can’t turn sharp and crunches head long into the wall, then it is the driver’s fault.
Most front engine cars have understeer and kinda have to live with it. Most mid-engine and rear engine cars come from the showroom floor with understeer but can be made to handle more neutrally or even have oversteer with minor adjustments.
The following steps can be taken to decrease oversteer (increase understeer):
[PM]
The OPPOSITE of the above will decrease understeer and increase overrsteer.
What are "Trailing Throttle" and "Power On" Oversteer?
[BSD] There are several things to consider here. And, we can get way into driving styles and car dynamics. Therefore, I’ll mention just a few things that come to mind.
If you swing the rear end out either by trailing throttle or by power on oversteer, you are going to wear out the rear tires at a great rate. This is expensive. Further, you will heat up the rear tires even more which will slow you down in a few short laps. Most of us want to drive for 30 minutes or more on the track at a time. Full weight NSX with near stock size tires won’t be able to handle the abuse.
When you say trailing throttle, that must apply to the middle of a turn. Normally, you brake and then turn in to a turn (and opionally trail brake). As you get further into the turn, you hit your maximum turn and slowest speed somewhere along the way. After that point, you can eventually get back on the gas. If you use a lift of the throttle here to get your trailing throttle oversteer, you have missed your apex and track out points as you should be on the power more and more towards turn exit.
In some steady state turns, you have to be somewhat on the throttle for an extended period before increasing throttle and tracking out. In these turns, you definitely use the throttle to steer… give it more gas to turn less sharp and lift off the gas to turn sharper. These throttle movements should be in small increments and should be used to adjust the car a foot or so relative to its current line. If you lift so much as to get the a*s end to come out, you are way off the optimum line and doing almost panic action to get back to where you should have been already. I guess my point is that losing grip in the rear end (or front end, too) is a bad thing and is a sign of not getting the maximum use of your tires.
Some cars benefit from some power on oversteer. There is an optimum yaw angle for some cars (formula type) where they toss the a*s end out about 5 degrees while on the throttle. This gives them the maximum turn exit speed. These cars are race cars which drive short races so tire life is a secondary issue. I don’t know of anyone who regularly uses some sort of oversteer situation in their "street car" and likes it long run. It isn’t the fastest and the rear tires can’t take it.
What Is "Double Clutching"?
[BSD – 98/9/21] There is a countershaft in your transmission that double clutching is concerned with. Knowing tranny internals is required for this to make sense. The short of it is that you have synchros in your tranny that make double cluthing not required.
The basics of tranny internals are that you have an input shaft to the transmission. The clutch connects/disconnects the engine from this shaft. The input shaft drives the countershaft of the transmission via a gear. The countershaft spins the opposite direction of the engine and that is where "counter" comes from. The countershaft is next to the output shaft of the transmission and has a series of drive gears on it. The output shaft of the transmission has a series of the reciprocal driven gears. The countershaft, when engaged to a gear, drives the output shaft in the opposite direction of the countershaft, which is the same direction as the input shaft.
All the drive gears are more like rings with teeth on the outside that spin around the axis made by the countershaft. When stopped in neutral with the cluth engaged, the engine spins the clutch which spins the input shaft which spins the counter shaft. None of the drive gear rings on the counter shaft are engaged to the counter shaft so the countershaft spins inside the drive gears. The drive gears have their teeth engaged with the (driven) gears on the output shaft. The driven gears are always connected to the output shaft. In this case, all drive gears, driven gears and output shaft are not moving/spinning.
When you want to move, you need to connect one of the drive gears to the countershaft. This is done by disengaging the clutch, then using the gear shift lever to slide a gear along the countershaft until grooves on the inside of the drive gear match up with grooves on the countershaft. One way to get things lined up is to have everything stopped: input/countershaft not spinning and drive gears not spinning (which implies driven gears and output shaft not spinning.)
If you disengage the clutch, the engine will not force the input and countershafts to spin. No input force on inputshaft/countershaft (they are hooked together, remember) makes it much easier to get the gear ring (which ever you want) to side over the grooves on the countershaft.
Once the gear is selected and engaged, releasing the clutch allows the engine to spin the clutch, the input shaft, the countershaft, the drive gear, the driven gear and the output shaft.
Once rolling, keep in mind that the output shaft of the tranny is permanently fixed to the rear wheels of the car. If the rear wheels are moving, so is the output shaft of the tranny. If the output shaft is moving, so are the driven gears and the drive gears. Drive gears are always connected to the driven gears, just not necessarily connected to the countershaft.
Say we are rolling in 1st gear, with engine connected to clutch, connected to inputshaft, connected to countershaft, engaged to 1st drive gear, which does torque multiplication via some ratio to the 1st driven gear, connected to the output shaff, connected to the rear wheels.
If we want to shift to 2nd, we need to get the counter shaft disengaged from 1st and engaged to the 2nd drive gear. You have a spinning shaft (the countershaft) and a ring gear to engage (the one corresponding to 2nd gear). The problem is that, due to the different gear ratios between 1st and 2nd gear, the 2nd ring gear (drive gear) is spinning more slowly than the 1st driven gear. So, when you select the next gear, the countershaft has to be slowed so that the grooves on 2nd ring drive gear will line up with the grooves on the countershaft.
Normally, shifting is done by disengaging the clutch which lets the input and countershaft "free wheel" with respect to the engine, then de-selecting the current gear. By deselecting the current gear (neutral) and disengaging the clutch, the input and countershaft can spin freely, but they also slow down as there is tranny friction to slow them. At the same time, you can select 2nd gear with the shift lever. When the grooves on the countershaft line up with the grooves on the 2nd drive gear, it will "go into gear". The difference in spinning speed between the countershaft and drive gear can determine how long it takes for the grooves to line up and "fit in". Also, since the input and countershaft are just spinning freely, cramming it into gear just causes an abrupt slow down of the input and countershaft which don’t weight that much so there isn’t that much momentum. Upshifting is easy on the car because it isn’t hard to slow down the input and countershaft since they slow down due to friction anyway.
Now, let’s say we are driving along in 2nd and want to select 1st gear. The countershaft is engaged in 2nd gear which is driving the output shaft via the driven 2nd gear. The 1st gear driven gear, though, is spinning much faster around the countershaft, but is disengaged from it.
To downshift, you disengage the clutch which lets the input shaft and countershaft freewheel. They are spinning however fast they need to to keep up with the 2nd gear ratio because we were in 2nd gear. When we select neutral, the countershaft and input shaft get to free wheel again. However, the 1st gear pair is spinning like crazy (very fast) because were were going a speed which is way fast for 1st gear. To select first, the countershaft and input shaft have to speed way up to get the grooves on the countershaft to line up with the quickly spinning 1st gear drive gear. How is this accomplished?
In modern transmissions, the gear pairs and countershaft have synchronizers (synchros) which come into play as a gear is selected. As you select 1st, there is friction between the countershaft and the 1st gear drive gear which spins up the countershaft to the speed of the 1st gear drive gear. This makes alignment of grooves in the countershaft and drive gear easy, quiet and fast.
In old trannys, or in worn trannys, the synchros may not have much friction or none at all. Then, you need to spin up the input and countershaft manually. To spin them up manually, without being in a gear, you select neutral, then let the clutch out (engage it). If you blip the throttle, it will spin up the input/countershaft and, if you do it well, you will then be able to select the lower gear.
The idea of double clutching is to always select netural before a downshift, blip the throttle, then clutch in, then select the lower gear. This is mean to either save your synchros or to be used on trannys that don’t have synchros.
Common Problems Explained
[KS – 99/12/10] Someone said: "I had one major spin on turn 9, pinching the turn on too early an apex, and then compounding the error by keeping the throttle floored, putting two wheels off on the exit, and then deciding instead of going straight off and into the pits, to pull back onto the track, causing a high speed spin. I exited the track on the other side, going backwards."
This is a very common scenario, perhaps the most common error on a track. Just to analyze it further for the benefit of track novices (which does not include Rick)… What happens is, when you take an early apex through a turn, you turn in too early. Because you’re turning in early, you don’t have enough track exiting the turn for the speed the car is carrying, and so the outside tires drop off the edge of the track onto the dirt or grass.
At this point, you have much less traction on the outside tires than the inside tires, which are still on the pavement. Best recovery is to steer the car straight, ease gently off the throttle, thereby allowing the car to slow down gradually. Once the car is going slow enough that it is completely under control, you steer to ease the car back onto the pavement. Unfortunately, the natural tendency for most drivers is to think, "track good, pavement bad", and instead of continuing straight, to turn immediately back onto the track. Once those outside tires are back on the pavement, they grip RIGHT AWAY – but at this point they are turned to the inside of the turn, rather than straight down the track, and so the car goes shooting ACROSS the track to the inside. The trajectory then takes the car off the track on the inside, and it may or may not spin when doing so. Once you have four wheels off at this point, you have little or no traction or control, and if you’re unlucky, you may continue until a fixed barrier brings the car to a halt. 🙁 The key to avoiding this situation is to go straight when you put two wheels off, and not try to get them back onto the track too soon.