Info wanted R65 vs R100

[MR.E]

Now then

I was on ADV rider talking to an aussie about the bike i'm building. most of the comments are others recommending me to scrap the 'flat R65' for a R100 as a race engine.

One fella commented
'Yes, the R65 has a shorter stroke, but when you look at bore:stroke ratio, the R65 is basically identical to the R100, 1.333 vs. 1.331 respectively (near enough to make no difference), so the R65 isn't really any more "revable" by nature than an R100. It's just geared a lot lower so you find yourself NEEDING those revs a lot more often. Nothing about the R100 makes it any more of a low RPM motor than the R65. R100s have more usable torque at lower RPMs than an R65 so some people choose to use the lower revs, but that doesn't mean they should'.


i was wondering where he's meaning when he states its geared lower than the 100??
is it final drive, main engine or gearbox??

Does anyone know the differences?

Thanks

[montmil]

Elliot,

Check item #48 on the Snowbum's site. I imagine your "advisor" is referring to final drive ratios.

http://bmwmotorcycletech.info/

[Motu]

They have the same bore/stroke ratio, but that doesn't make the R100 as revable as an R65.  That's the same as saying a 2,500cc 4cyl engine will rev like a 250cc 4 cyl engine with the same bore/stroke ratio - no way is a Subaru Legacy going to rev to 18,000rpm like a CBR250, no matter what you do with it.  The R100 has a LONGER STROKE, those huge pistons flying up and down their bores at higher speeds generate much more force than the smaller R65 pistons moving a shorter distance.Bore/stroke ratio is one thing, actual physical size is another.  

The twin shock R65 has lower gearing, the R65 the first poster is talking about is a Mono - it has higher gearing, the same as a R80RT.

[MR.E]

thanks

But where in the bike is the gearing lower??

Elliott

[montmil]

Elliot,  IIRC, there are/were some transmission gear clusters sold by BMW that could be swapped into certain Airhead gear boxes -primarily aimed at side car owners.

Get on the 'net and start researching but consider the "counsel" from your forum guy; he's off base on his comments. Caveat Emptor

[Barry]

But where in the bike is the gearing lower??

All unmodified airhead gearboxes have the same internal ratios with a overall reduction of 1.5 : 1 in 5th gear.  The only difference between an R65 and an R100 is in the final drive ratio. Thus an R65 with a 3.56 final drive has an overall reduction of 5.34 and an R100 with a 3.2 final drive has an overall reduction of 4.8

Note those final drive ratios are typical but not ever R65 had the same and R100 ditto.

[MR.E]

But where in the bike is the gearing lower??

All unmodified airhead gearboxes have the same internal ratios with a overall reduction of 1.5 : 1 in 5th gear.  The only difference between an R65 and an R100 is in the final drive ratio. Thus an R65 with a 3.56 final drive has an overall reduction of 5.34

So with me running a 1150rs final drive it really isn't anything to worry about.
It got me pondering really as to why most people say fit a 100 engine, but it seems the main difference is the R65 like to be revved.

Cheers

[Tony Smith]

They have the same bore/stroke ratio, but that doesn't make the R100 as revable as an R65.  


The early R100s with 40mm carbs and exhaust had the very heavy flywheel, whereas the late R100s were strangled by 32mm carbs and small exhausts. My own observation based on owning a 1978 R100RS, spending a fair bit of time riding my brother's last model R100RS, my wife's R65/80 and my own 1982 R65 is as follows.
 
They all rev more willingly than the old RS. The R80 runs the most "sweetly" of them all and revs a bit more willingly than either of the R100s, but the r65 feels like it wants to rev and spins up more readily than the rest, albeit with more vibration than the R80.  
 
Bear in mind that the above is entirely subjective, my wife once grizzled about how "slow' her r65 was after the R80 engine went in. after much head-scratching stretching the carb return springs to lower their tension cured all her complaints, indeed she commented on how much "faster" it was after a decent tune-up (she maintains, with some degree of justification that her bike sucks hind tit as far as maintenance hours go.

Last comment. The last model R100RS is just plain sad, I don't know if it was the same in other countries, but here in Australia it was strangled by small carbs, small exhausts and small valves and had cam and ignition timing played with to meet emission targets, a good honda 90 step-thru would give it curry up to 60kph or so. I have been at my brother for years to return his to an earlier specification, to date he has not been interested.

[Barry]

The carb return springs thing is entirely believable. I found mine to be stronger than they need to be and gave them a careful stretch making sure they finished up both the same with a spring balance.

Someone somewhere is making and selling substitute springs with a lower rate.

[nhmaf]

Less reciprocating mass with smaller, lighter pistons,pins, and connecting rods, shorter stroke= higher top RPM for the same piston speed = the R65 engine IS revvier than the R100.   The R100 does have more bottom end grunt (it should with such a displacement advantage) but on a racetrack setting it is often important for the engine to rev up more quickly.    With its admittedly shorter final drive gearing, a well ridden R65 can keep up with an R100 at least up to 100 km/h coming out of corners.

[Ed Miller]

I certainly have no trouble keeping up with a buddy on his R100GSPD.  Now an R100S would likely be a different story.

[MR.E]

Less reciprocating mass with smaller, lighter pistons,pins, and connecting rods, shorter stroke= higher top RPM for the same piston speed = the R65 engine IS revvier than the R100.   The R100 does have more bottom end grunt (it should with such a displacement advantage) but on a racetrack setting it is often important for the engine to rev up more quickly.    With its admittedly shorter final drive gearing, a well ridden R65 can keep up with an R100 at least up to 100 km/h coming out of corners.

I've never had a problem with the power of the r65, it really made me laugh how nippy it was for a 45bhp bike, and i like to hear a revving engine so am happy keeping it mid range.

I've built it on the 1150rs final drive to hopefully gain a little more top end for use on the uk race tracks.

All the best

[livingdeadhead]

i knew chain and sprockets had a use! [smiley=3stooges.gif]

[Tony Smith]

i knew chain and sprockets had a use! [smiley=3stooges.gif]

The young bloke who lives four doors down from me would agree with you. On the weekend he fed two fingers through the rear sprocket of his Aprilia road bike.

I heard the howl and raced down the street to see what happened and supervised his mother wrapping his hand in cloth while waiting for the ambulance. I scouted about and found most of one finger and the tip of the other and put them in a take away food container on ice, and gave them to the Ambulnace officers when they arrived.

Visited the mother yesterday, the finger that lost the tip still has the joint below the tip, but the finger tip was too damaged to re-attach.

The next finger was severed between hand and first joint and has been re-attached but is unlikely to ever have function. Luckily (if such a term is appropriate in the circumstances) it is his non-dominant hand.
 
How did this happen? He started the bike on the stand, then leaned it over to one side so the rear wheel was off the ground and then put it in gear and whilst using his shoulder to balance it, was dribbling chain lube onto the chain. Evidently he saw some debris on the lower chain run and reached out to grab it.

The rest, as they say, is history. He is a book keeper so the loss of function in his non-dominant hand is not going to be life changing for him.

[Barry]

We've probably all been there lubing a chain while rotating the rear wheel. I don't believe I ever did it with the engine running though.