BMW R1200R R1150R/T F800 K1200R R1100R/T R1250R RnineT Message Board

i guess i don't know how 'overdrive' is defined.
on my 5 speed '87 r80, in fourth gear the tac and speedo needles would be parallel and in 5th, the tac would drop below the speed, which seemed to indicate overdrive to me.
any inputs appreciated

goo wrote:i guess i don't know how 'overdrive' is defined.
on my 5 speed '87 r80, in fourth gear the tac and speedo needles would be parallel and in 5th, the tac would drop below the speed, which seemed to indicate overdrive to me.
any inputs appreciated

I understand that in the R1200R version of the bike, the 6th gear ratio was refined to be more of a driving gear, not so much an overdrive gear.

"Overdrive" to me would mean the output shaft of the transmission is turning slower then the transmission input shaft.

Such is the case on our bikes - in both 5th and 6th: http://www.wolfbmw.com/R1200R.html

"Gear ratios I 2.28; II 1.58; III 1.26; IV 1.03; V 0.90; VI 0.80" - meaning that in 5th, the output shaft turns 0.90 revolutions for every revolution of the engine, and in 6th, 0.80 revs per engine rev.

deilenberger wrote:"Overdrive" to me would mean the output shaft of the transmission is turning slower then the transmission input shaft.

Such is the case on our bikes - in both 5th and 6th: http://www.wolfbmw.com/R1200R.html

"Gear ratios I 2.28; II 1.58; III 1.26; IV 1.03; V 0.90; VI 0.80" - meaning that in 5th, the output shaft turns 0.90 revolutions for every revolution of the engine, and in 6th, 0.80 revs per engine rev.

Calculate 1/X function on everything for internal overdrive (output shaft of the transmission is turning slower then the transmission input shaft only on first four gears [underdrive]) with Overdrive (Output shaft turning faster than the input shaft for five and six) - ignoring the Primary Drive ratio of 1:1.882 for a moment it's;

I 2.28; (0.438 turns of the output shaft for every one turn of the input shaft)
II 1.58; (0.632 turns of the output shaft for every one turn of the input shaft)
III 1.26; (0.793 turns of the output shaft for every one turn of the input shaft)
IV 1.03; (0.970 turns of the output shaft for every one turn of the input shaft)
V 0.90; (1.11 turns of the output shaft for every one turn of the input shaft)
VI 0.80 (1.25 turns of the output shaft for every one turn of the input shaft)

But since the transmission on our bikes also contains a Primary Drive ratio of 1:1.882, technically everything is underdrive, even though internally the last two gearsets (5/6) are overdriven.

For example at 4000 (engine) rpm in VI, the input shaft is turning at 4000 rpm, the (internal) output shaft at 5000 (4000*1.25 - hence the term "over" drive), the Primary Drive is 1:1.882 (5000/1.882 = 2656rpm) and the FD ratio is 2.75 (2656/2.75) = 965.8 rpm at the rear wheel. Calculated Revs per mile for the 180/55 ZR 17 tire is (7.8" total sidewall height + 17" rim = 24.8" tire diameter = 6.5 feet per rev or 812.3 revs per mile. Therefore, 965.8/812.3 = 1.188*60 = 71.28 mph @ 4000 rpm in sixth gear. Actual tire rev's per mile will be a bit more as the tire grows at speed, or could be less if underinflated.

There was a 1000 rpm drop going from 5th to 6th on my '02RT. This was very noticeable on the RT as an overdrive. On the R, there is only a 500 rpm drop from 5th to 6th, so, no 6th is not a so called overdrive on the R.

blueviewlaguna - Excellent post! Thanks for that detail, I appreciate it.

WOW --------------- I never knew

Good job Laguna!

Dave

I don't think of the R12R's 6th gear as a an overdrive in the traditional sense where:

"overdrive = A gearing mechanism of a motor vehicle engine that reduces the power output required to maintain driving speed in a specific range by lowering the gear ratio."

As shown in the BMW R12E catalog, 6th gear is intended to be a driving gear and increased the top speed range.



http://www.kivas.com/BMW/r12r_gears.png

Of course you can use 6th at cruising speeds to reduce rpms and conserve fuel, but you can do that with any gear depending on your cruising speed.

Mark_1