Thanks for asking MaxiMan. Sorry all, haven't posted anything for a while. Have done very limited work on the car since I lasted posted up pics. The summer rains well and truly arrived mid January and brought progress to a halt. I had hoped that the weather would have dried up by the Easter weekend but it didn't. And even when it wasn't raining the humidity has been too high to allow for painting (the compressor just fills with water). It seems the winter months are the only time I can get any painting done on the car. Luckily there really is very little to finish, so given the first opportunity you can expect to see her finished.
Not satisfied to simply sit on my arse while it was raining though, I set about finding something mechanical to do on the car. Something that has bothered me about building a track car from parts that originate from daily-drivers is the gear-ratios. Track cars simply don't use the same gear ratios as road going cars, and no matter how you adjust your final-drive ratio (by fiddling with the diff) you simply cannot bring your ratios close enough together to make all gears useable on the track. For example, shortening the final drive ratio may bring 5th gear into reach on the fairly short straightaways (typical of most "local" tracks) but almost always at the expense of 1st and 2nd gear, which land up confined to the pits and paddock. Shifting the final drive ratio the other way has the opposite effect, and can render 4th and 5th gear redundant (other than on a very long straightaway). No matter how you look at the problem, you are typically limited to three gears if you use a road-going gearbox on track.
And so I set about designing my own gearbox :shock:
To cut a long story short, I bought a spare Toyota W55 manual gearbox at Christmas, stripped it down, and measured
everything as I went.
A few specialised tools were required to shift some of the more stubborn gears and bearings. Luckily I found a Toyota W57 S&R manual on the net, which is identical to the W55 in all respects except for the ratios, and which includes some info on the specialised tools required to disassemble and reassemble the box.
With the box disassembled and measured up I was able to commence the design of my own box. The idea was to keep the alloy casing, the selector rods and selector forks from the original W55, but to redesign the spigot shaft, countershaft and all gears and selector rings to suit my own track requirement. The original W55 output shaft is also retained. Given that I know (very roughly) the mass of the car, coefficient of aerodynamic drag and engine torque as a function of RPM, it was not that difficult to calculate my ideal ratios. After a fair bit of calculation using MATLAB, and simulation using GTR2 (around both Kyalami and the Nurburgring
) I arrived at the following gears: 29, 32, 31, 31, 32 and 29 teeth on the gear clusters (1st through 5th) and 19, 27, 33, 42 and 45 teeth on the counter shaft (also 1st through 5th). These gears define the ratios:
1st = 2.00:1
2nd = 1.55:1
3rd = 1.23:1
4th = 1.00:1
5th = 0.85:1
Notice how low the first gear ratio is. A daily driver will have a first gear ratio of around 3.5:1. Although my off-the-line acceleration will be poor, This gearbox will actually give me five usable gears on track.
Once the ratios were determined, the next step was to design the gears and shafts, and then send the designs off to a gear cutter. I located a gear cutter in Germiston (just east of Johannesburg) to manufacture the gears and shafts for me. As is typical of a race only gearbox, the gears are straight-cut (spur gears) and engage to the selector rings via dog-teeth. I will not be using synchroniser rings in my gearbox. Spur gears are significantly noisier than helical gears, but are also substantially more efficient, and thus result in substantially reduced transmission losses. The fact that I am using spur gears also removes the axial forces placed on the various shafts, and thus alleviates the need for tapered bearings.
The percentage change in ratio between the respective gears are:
1st to 2nd : 22.2%
2nd to 3rd : 20.7%
3rd to 4th : 18.9%
4th to 5th : 15.4%.
The percentage change between gears marginally decreases with increased speed, but by nowhere near as much as one would expect from a road-going gear-box. The fact that each % shift is below 25% defines this box as "close ratio".
Interestingly (and I am afraid you will just have to take my word that this is purely coincidental) my 1st through to 5th ratios correspond fairly closely to the 2nd through to 6th ratios of the Quaife 60G 6-speed box. My 5-speed box will, therefore, effectively function as a 6-speed box with 1st gear removed, as was the case with the older 5-speed LeMans racers. If I had this “extra” gear in my box it would effectively function as a “crawler” gear to make negotiating the pits and paddock easier (1st gear will require some slipping of the clutch at slow speed). One thing is for sure though, with a 1st gear ratio of 2.00:1, my problems with wheel-spin in 1st gear will be eradicated.
Shown below is a graph of engine revs (rpm) as a function of speed (km/h) while shifting up through the gears. The shift is executed each time at exactly 6000rpm. The final drive has a ratio of 41:10, and the rear wheels are shod with 245/40/17’s.
Finally, shown below is the assembly drawing of my new gearbox. Needless to say this little sub-project has kept me very busy, and is the reason I haven’t posted up on the forum as regularly as I used.
My gears and shafts are currently being manufactured by ARMA gears of Germiston, and should be ready in about 2 weeks time.
I won't know what to do with myself when this project is over.
