1800 gearbox ratios

what is 3rd gear + final drive ratios.

so I dont have to think about it...
how do I calclulate my revs at a given speed.
Step 1 measure tyre diameter.
Step 2... divide n multiply all heaps of stuff yeah? Which way?

ok so I just let wes work it out using his calculator thingo.

According to that and the SAS dyno I make 69.1hp @ about 3800 rpm. So what is that roughly at the flywheel?

1800 ratios (from Stephen Foster's gear calculator):

3.80
2.06
1.26
0.82

final drive: 4.867

calculate like this:

multiply SPEED by 1000 (convert to meters)
divide this by tyre outer circumference
multiply this by final drive ratio (ie. 4.867 for 1800 diff)
multiply this by gear ratio (ie. 1.26 for 3rd gear)
divide this by 60 (convert to minutes)

…and you have RPM

eg.....

80 (kph) * 1000 = 80000
80000 / 2.0 (meters circumference) = 40000
40000 * 4.867 = 194680
194680 * 1.26 = 245297
245297 / 60 = 4088 (rpm)

I don’t know how much you lose through the gearbox, can’t help with flywheel horsepower.

cool thanks. That looks about the same as wes's calculator. tyre diameter 2020mm. approximate speed reading off the dyno graph 75.5. came out at 3820.

thanks again.

Engineers correct me if I’m wrong but loses have more to do with friction than anything else, hence difficult to calculate.
I’ve seen estimates of 15 to 20% if that helps.

The driveline efficiency is mainly to do with friction and the helical gears used in the gearbox. The helical gears are strong and quite but not 100% efficient. Hence drag cars use straight cut gears. Type 4 match gearbox's (later type 2's) also have a hypoid ring and pinion which is less efficient than the beetle, but much stronger.

Late model FWD cars seem to be close to 85% efficient (15% loss). But I have also seen a range from 15% to 30%. Bit of a range huh. The bosch automotive handbook says about 24% losses is normal, but it was written a while ago.

As you would have noticed, my calc spreadsheet has all the kombi gearbox ratios. You just need Excel 2000 to run it.
http://www.offroadvw.net/tech/wes/calc.zip

The latest one tells you the RPM in any given gear at any given speed. I used it to calc my 4th gear ratio for highway cruising.

OK. well I will use 35% loss then cos mine box has some dodgy bearings and a lot of metal circulating around :P

keeeeweeeellll. 106.4hp. reads much better.
:party
need to edit my profile again.

A couple points of interest:

1) Measuring the circumference of the tyre will not work. The weight of the car distorts the tyres, decreasing the effective radius and hence the effective circumference. Try measuring from the ground (on a flat surface) up to the centre of the hub and multiplying by Pi and doubling it to get the correct circumference.🙂

2) Driveline friction does not increase as the horsepower of the engine does. If your stock motor makes 70 Hp at the flywheel and loses say 35% in drivetrain losses, then drivetrain loss will be 24.5 Hp. If you are getting 75.5 on the chassis dyno then it would be fair to assume that you have exactly 100 Hp at the flywheel.🙂

Col,

(1) - It is actually a mix of the two. At the leading edge of the contact patch, the radius is the full radius of the tyre, in the middle it is the reduced radius you described, and at the back of the contact patch it is the full radius again. Therefore a certain amount of slip and tyre squirming goes on in the contact patch. I would use the full radius. The best way is to draw a white line on the tyre. Roll the car forward on flat ground so the tyre rotates say 10 time. measure the distance it travelled and divide it by 10. That is the true effective tyre circumference.

(2) - No that's not right. For one, friction is a coefficient, not an amount. It is a something you multiply by your driving force to get a loss.
Also, most of the losses are not friction as such, but are due to the inefficient nature of helical gears. The helix angle of the gear determines it's efficiency.

With a helical gear, a certain amount of torque gets transmitted to that gearset through the driving shaft. The torque then gets transmitted from one gear to the next by a force at the gear teeth. The force is perpendicular to the line of contact between the two gears. With a helical gear that force is at an angle to the shaft centerline (the helix angle). Therefore some of the driving force gets transmitted to the driven gear as a torque, and some gets transmitted between the gears as an axial thrust on them. This force is wasted. It is a percentage of the driving torque, dependant on the helix angle.

So a 15% driveline loss is a 15% driveline loss no matter what the engine size.