Opinion needed: 32/11 vs 33/11 final drive on R100RS

[davidh21500]

Hi -

The previous owner of my '83 R100RS didn't lubricate the rear drive splines. The Good News is, I was able to source a replacement rear end from the same year/model bike with approximately 10K miles on it – the splines are pristine. The Bad News is that it is a 33/11 (3.0) ratio as opposed to the 32/11 (2.91) currently on the bike.

I know I'll need to have the speedo re-calibrated (needs rebuilding anyway), and that I am looking at roughly an additional 500 RPM at 75MPH. An important consideration is that I now live in the great state of Wyoming, where we have open roads and 80 MPH posted speed limits. That would seem to indicate the original 32/11 (2.91) is preferable.

I'd like to hear real-world experiences from riders who have ridden with both ratios on the same/similar bike. I'm looking for both empirical (measured observations) and subjective ('seat of the pants') experiences. I'm especially interested in the effect on high-speed (75+ MPH) cruising, as that will comprise the majority of my riding. Long-distance is implied; after all it is a BMW …

Bottom Line, I need to decide to go with the 33/11, or sell/trade the 33/11 for another 32/11. I'm leaning slightly towards having another 32/11; however, I'd like to better understand the real differences so I can make a more informed decision.

Thanks in advance for any comments, recommendations, opinions, etc.

David

 

[20774]

David -

Welcome to the forum! I don't have any experience with anything other than the 33/11 rear end on my /7. On my '78 R100/7, I read about 4K RPM at 70 mph, so that is a point of reference. The slight difference in the ratio means you'll get a little more uumphf off the line but you might run out of RPM at the top end. Giving up top end might not really be that big of a consideration for most.

The 33/11 would mean a slight reduction in gas mileage I suppose...when you're running at those kind of speeds in WY you're getting pretty close to being on the main jet...so some improvement to gas mileage could be had to dropping the main jet down one notch...would need to confirm with plug readings to be sure it's OK.

Seems like for all the other work you'd have to do, I wonder if getting the rear end rebuilt would be a consideration. Get to keep the 32/11 and everything that does with it. There are resources for that if it makes sense. Hansen BMW in Oregon is one of the better places.

 

[happy wanderer]

Good place to start right here courtesy of Anton Largiader: http://www.largiader.com/articles/gearing/

 

[20774]

Good place to start right here courtesy of Anton Largiader: http://www.largiader.com/articles/gearing/

I had thought about that, but the graphic doesn't render for me. I get lines on the screen but no axes for the graph. Maybe just my computer.

 

[lmo1131]

I get lines on the screen but no axes for the graph. Maybe just my computer.

... then we've got the same computer.

 

[dbrick]

Here's a screenshot of the top part of Anton's page. It shows how the page looks, but doesn't allow the user to click and view and compare road speeds.

 

[AntonLargiader]

If you select either 1200 PM or 5000 RPM from the legend it will work. The 1200 RPM should come up by default but it hasn't for a long time. I'll have another look at it.

EDIT: I think I fixed it. Let me know how it works for you.

 

[6322]

If you travel two up or loaded the slightly lower ratio may be appreciated. There are also some great mountain passes in WY. Again that new ratio may be a plus. All things being the same, I doubt you'll notice that big of change in your mileage. In my '77 R100/7, came a 32/10. With that ratio I could smoke the '77 R100RS's well over 100mph. It was all about final drive ratio. Also if it reduces your top end slightly, how often are you running top end and getting there quicker may be more advantageous (?). Regarding your speedo calibration, I run a gps and pay little attention to the speedo. I like to know exactly how far over the limit I am... Anyway, good luck with your decsion and ha a great riding season in WY.
gp

 

[AntonLargiader]

I don't have a real basis for comparison but everyone I have talked to over the years has said that 3.0 is really the perfect FD ratio for the R100 engine. From the riding I've done on my own bikes (2.91 on the R75/5, 3.09 on the GS and R) I have no problem believing it. The 2.91 is very tall. I prefer the 3.09 on the GS and R, even for street riding. And the R100R is not by any means a slow bike; I've held 100-ish on I95 in Florida and pushed well past that on other roads. With the slightly better pull-away that the 3.0 gives you over the 2.91, I suspect you'll like it. BMW abandoned the 2.91 ratio, and I suspect they did it for a good reason.

 

[20774]

If you select either 1200 PM or 5000 RPM from the legend it will work. The 1200 RPM should come up by default but it hasn't for a long time. I'll have another look at it.

EDIT: I think I fixed it. Let me know how it works for you.

Anton -

It still reacts the same. Initially, no grid even though I select a rear end ratio. But if I select 1200 or 5000 RPM I do get the grid. Then if I select Clear All Curves, the grid remains and then I can select anything I want to see the curves.

BTW...could a couple more RPMs be added? Say 3000 and 4000?

Thanks...

 

[AntonLargiader]

BTW...could a couple more RPMs be added? Say 3000 and 4000?

Could... would it really be useful? I picked those two because they were typical offroad crawling and highway cruising speeds. There's a limit to how accurate the whole thing is going to be anyway, since it assumes the same wheel diameter for every situation.

 

[jagarra]

Here is a 32/11 that is on e-bay https://www.ebay.com/itm/BMW-Motorc...280116?hash=item3d5f15f734:g:WywAAOSwjH5axUvi

This seller is a great source for parts, bought many items from him.

 

[lmo1131]

This seller is a great source for parts, bought many items from him.

lamontsanford is a reputable seller, I've bought stuff from him as well.

 

[davidh21500]

Hi -

Thanks; that's exactly the type of info I'm looking for. Your charts and graphs were the first things I consulted when I discovered the FD was 33/11 - thanks for providing them.

So, can I assume cruising at speed with the 3.0 FD is not appreciably less 'pleasant' than with the 2.91? From what I recall from cruising at the same (projected) RPMs with the 2.91 FD the engine still liked it (ran good, felt good); however, wanted to validate my memory.

RE: BMW abandoning the 2.91: Didn't they do that when they revived the 247 engine in '87/'88, which they re-tuned (smaller valves, etc.) for more low-end torque? As I recall, the '88+ airheads were quicker (acceleration) but slower (top speed); I wonder how much of that was because they also went to the 3.0 FD.

Again, many thanks!

David 21500

 

[AntonLargiader]

RE: BMW abandoning the 2.91: Didn't they do that when they revived the 247 engine in '87/'88, which they re-tuned (smaller valves, etc.) for more low-end torque?

When BMW went all-Monolever in 1985 they also dropped the R100 models, so there were no R100 Monolevers to make gears for. The Monolevers needed new gearsets and there would have been no point making a 2.91. A 3.0 shows for certain special R80 models in that time frame but I don't know if that gearset was actually produced in 1985 or if it was only introduced for '88 when the R100s came back. It makes sense to me that they had no reason to make a 2.91 in '85 and no interest in making one in '88 as they were just biding their time until the Oilheads came out. That said, they did make the 3.09 gearset that was R100-specific, so I'm leaning toward thinking that they didn't see a 2.91 as a good decision. The R80s were mostly 3.36 and 3.20, and the R100s were 3.09 and 3.0.

And don't lose sight of the fact that BMW is all over the place on what ratio they put on what bike. The R80GS was geared taller than the R80RT, but the R100GS was geared shorter than the R100RT. The R1100GS was geared shorter than the R1100RT, but the R1150GS was geared taller than the R1150RT. Got it?

 

[Rinty]

I'd like to hear real-world experiences from riders who have ridden with both ratios on the same/similar bike. I'm looking for both empirical (measured observations) and subjective ('seat of the pants') experiences. I'm especially interested in the effect on high-speed (75+ MPH) cruising, as that will comprise the majority of my riding...

David:

I don't have experience with the 3.0 rear end, but I had an '82 RS for many years, and in my opinion the gearing was perfect for cruising at 80 mph. I forget the rpm at that speed, but it was in the 4 - 5,000 rpm sweet spot. For riding on the western plains, I wouldn't have shorter gearing.

Good luck with this.

P.S.

Looking at Anton's chart, 5,000 rpm with the 2.91 gives 85 miles per hour, so for your 75 mph cruise, you should be at 4,000 rpm or higher. IIRC, on my (2.91) bike 4,500 rpm was 80 mph (130 kph); a nice, smooth cruise speed.

 

[Na Cl K9]

Ratios are selected to be compatable with modern highway speeds.

People talk about top speed and fuel economy when discussing FD ratios. I think about it in terms of accessing the power band. The FD ratio choice moves the entry and exit points for the power band around depending on your power requirements and displacement. You all can recall grabbing a big hand-full of throttle with too few RPM's listed and winding up with a lot of manifold pressure and a flat spot in the acceleration until the revs come up. Managing ones momentum on an airhead means keeping the RPM at around 4200 on my 2.91 equipped 83 R100RS - solo, (4400 on my 3.0 equipped 77 R100RS - solo) in order to avoid that condition. The point in the power band where I like to be at any given loading would be the point at which rolling on the throttle to pass a vehicle on the road becomes seamless acceleration. A racer will hold the RPM closer to the middle of the power band so that snapping the throttle open produces more immediate results. The 'useful' RPM range varies with terrain, load, speed limit and so on. So the factory thoughtfully included an infinitely adjustable throttle and a transmission with complimentary gear ratios to allow us to haul-ass or what ever else. 'Good fuel economy is the direct result of proper power management'

 

[snowbum1]

rear drive ratios

Here is another article on rear drive ratios, with both Km and Mi, speedometer ratios to match, road and tach readings for certain speeds, ETC.
http://bmwmotorcycletech.info/ringgears.htm

IMO, if the original poster already has the higher ratio rear drive, he may well want to install it, and also may well NOT want to recalibrate the present speedometer.
The slightly higher ratio will enable being able to come closer +-, to yellow or red line rpm in 5th gear (great for capturing new Citations from the Highway Patrol Officers), because BMW originally selected the rear drive ratios to prevent getting to redline in top gear, in effect, BMW gives you top gear as somewhat of an "overdrive". You will probably not really notice much, the over-all effects, no matter what gear you are in; perhaps the more noticeable effect will be a slightly lowered tendency to have to shift from 5th down to 4th at higher speeds.

The reason I say you may not want to recalibrate the speedometer unit is that BMW speedometers (except for a few 0-85 mph ones from back in the National 55 speed limits days) all read faster than your actual road speed. BMW did that on purpose. There are some little considered effects, such as rear tire slippage, that makes things a tad worse under some circumstances (BTW, tire size has only a quite small effect). You may find that the over-all effect has the speedometer reading accurate enough, or, put an arrow on it at some convenient speed. GPS's are great for calibrating speedometers and odometers. The other item is the ODOMETER. BMW odometers are usually spot-on in accuracy. Yours will read a bit faster.

 

[Rinty]

Here is another article on rear drive ratios...

Thanks for weighing in on this, Robert.

And to Anton.

 

[8ninety8]

FWIW, '74 R90S 33/11, shows, as old Cycle World test, 16.8 mph per 1000 RPM, give or take air resistance, HP, state of tune, exact diameter of rear tire, grit of rider. A little over 4K is about exactly 70MPH, with a re-calibrated speedometer. And multiplication tells the rest of the story. The old KRS has a 2.82 which, in IMHO, is exactly about perfect. For that bike. The older it gets, the faster it was. I remember the time.......