Question 1: High performance gearboxes have gears with straight teeth while in ordinary gearboxes, the gear teeth are set at an angle relative to their axis. What is the advantage of straight toothed gears?
Question 2: I have a Best Motoring video featuring a battle between a Porsche and some other cars. The narrator says that the Porsche’s “gear ratio is crossed from third gear and above”. Later in the battle he again mentions something about Porsche’s “crossed gearbox”. What is a crossed gearbox/crossed ratio gearbox?
On straight-cut gears the full length of the teeth are against each other when they are meshed. In helical-cut gears, the face of the gears where they touch varied across the tooth and is not as great at any one time. This means that there is more area supporting the power being driven through the gears on straight-cut.
Most passenger cars use helical-cut gears because they are quieter than straight-cut, which tend to whine more. Helical gears also put side-thrust loads on the gears - converting horsepower into friction and heat. Straight-cut gears don’t have this problem and are therefore more efficient at transfering power.
I can’t help you on the second question. I have never heard of the term “cross gearing” before.
IAmAME, so, here goes…
Racecar 'boxes have straight-cut “spur” gears with a simple near-involute profile on the teeth as this provides a larger average contact area, allowing for smaller static loads through the 'box. This is necessary because racecar gearboxes see a lot of huge static loads - firing a heavy, traction-optomized car on sticky rubber off the line or slamming downshifts through without using the clutch.
However, there are a couple downsides. Straight spur gears generate very large dynamic loads as teeth are slamming into and out of contact; consequently the gears can’t turn anywhere near as fast as helical gears. Since they can’t turn as fast, they have to transmit more torque to transfer a given amount of power, which means that the gears themselves either have to be bigger or not designed to last as long. Racing gearboxes in some applications require $25,000 rebuilds between each race. That’s not a misprint…
The other, somewhat less-obvious problem with straight-cut gears is that they are MUCH more prone to resonance and vibration. All those teeth slamming into and out of contact produce noise; they’re louder than an unmuffled small-block and that’s through thick gear oil and a solid casing. Noise means vibration. Vibration means cyclic loading. Cyclic loading leads to fatigue failure. In addition, the friction advantage of not having side loading and tooth-slip is cancelled out and then some by the energy dissipated by the flexing and relaxation of gear teeth.
“Crossed” gears means that they built a 6-speed gearbox by building a three-speed one and a two-speed one, and providing six gears as follows : 1,1 1,2 1,3 2,1 2,2 2,3 where the number before the comma is the gear the two-speed box is in and the one after is the gear the three-speed one is in.
Nope. Race boxes are unsynchronized because you’re just ramming the lever around; if there were synchros there they’d wear themselves to death and slow down the shifting anyway.
I drove an old Crosley from Tacoma to the Washington coast once. You had to double clutch to put the car in first. It was a pain in the ass. Even worse, drive a car with top speed of 57 mph on an divided state highway with a 65 mph speed limit. I was about run over a number of times.