Idea For Modified BMW Gearbox

oilburner · Post by **oilburner** » Thu Sep 06, 2007 12:39 pm

In a previous post I described the modification of a 1970-1973 4-speed BMW gearbox to alter the overall ratio from 1.5:1 to 1:1, making it suitable for use with low(er) speed engines e.g. 3600 rpm. The procedure is expensive, requiring relocation of the gearbox countershaft as well as the machining of one new gear.

Examining the 1974-1995 airhead BMW 5-speed gearboxes, I think it can be modified without relocating any shafts, by machining two new gears. The downside would be the loss of 5th gear. The upside is that there would be ample room for the new gears, permitting less complex gear design and a wide range of ratio options.

A question remains as to the suitability of the straight cut 4th gear as the new top gear. BMW gearboxes tend whine a bit in 4th gear. If money was no object, I suppose one could cut two new helical gears for 4th gear.

For my Daihatsu D950, a ratio of 1.41:1 for a new primary gearset along with a 2.91:1 BMW final drive would result in the following rpm/mph performance. At these speeds the engine is always operating below its rated horsepower at the rpm required for the indicated mph, suggesting this performance is possible while running the engine within the continuous hp/rpm range. The speed and fuel consumption figures are based on no wind lying on the tank formula (30 times the cube root of horsepower = max speed):

51 mph @ 2200 rpm - 180 mpg (British gallons)
56 mph @ 2400 rpm - 160 mpg
60 mph @ 3000 rpm - 139 mpg
65 mph @ 2800 rpm - 109 mpg
70 mph @ 3000 rpm - 100 mpg
75 mph @ 3200 rpm - 90 mpg
79 mph @ 3400 rpm - 81 mpg
81 mph @ 3600 rpm - 75 mpg

Even 70 - 80 percent of the above fuel consumption figures would be none too shabby!

andrewaust · Post by **andrewaust** » Fri Sep 07, 2007 9:59 am

Hi Avery

That's a good idea you have there, the torque of the diesel would handle a four speed box as long as the ratio's were spaced out well.

A few people I've spoken to think you need a heap of gears for a diesel engine, but they have a heavy vehicle in mind that relies strongly on keeping to a small torque band, whilst a motorcycle uses smaller high speed diesels which can get along well with lesser gearing having a larger band of power to use.

Cheers

Andrew

balboa_71 · Post by **balboa_71** » Fri Sep 07, 2007 11:15 pm

There's a good 10 minute video on YouTube someone made while riding around on his/her Enfield diesel. Because you can hear the engine quite well in the video, it's easy to see how much the 4 speed gear box hurts over all bike performance. The jump from 3rd to 4th is drastic and the little diesel strains to stay in 4th unless the rider can take the bike up to top speed and keep it there. Having a 5 speed box with a lower 4th and the same top ratio for the 5th speed would allow better over all riding and the new 4th gear would get a lot of use.

I don't know if there will ever be a good answer to getting higher speed out of a diesel bike until engine rpm can be increased to 5,000 rpm, which would solve a lot of problems. The 3,600 rpm limit is the real problem, not the lack of available gear boxes. Just my $.02 worth

oilburner · Post by **oilburner** » Sat Sep 08, 2007 8:59 am

The spread between 3rd and 4th gear in this scenario is about 600 rpm which is more than I prefer... I'd like about 250 rpm spread. I did a bit more number crunching and it appears it isn't the end of the world though. If you are cruising along at 50 miles per hour at 2200 rpm and drop down to 3rd gear, the revs increase to 2800 rpm. Also, there is plenty of power in reserve - at 50 mph for example, there is 25 percent more horsepower on tap if you whack open the throttle, meaning I shouldn't have to continually drop down to 3rd when there is a gust of wind. The chart below shows this. The columns on the left show miles per hour in each gear, the two columns on the right are continuous horsepower (hp-cont) and intermittent horsepower (hp-int) which is what you get when the engine is pulling max hp at a given rpm:

I've ridden my 600cc BMW with my modified 4-speed gearbox which has essentially the same torque and same ratios, keeping the rpms at/below 3600 while shifting in/out of 3rd gear and I feel there won't be a problem.

It's unfortunate Wolfgang Kayser in Germany retired. He made 6-speed conversions for BMW 5-speed gearboxes. Using that setup with my modification idea would provide a close ratio 5 speed suitable for use with a 3600 rpm engine.

Avery