What can I do to lower r.p.m.'s of a ST? At hiway speeds of 70 m.p.h., the engine is spinning about 1,000 more r.p.m.'s then my R100. I know of two ways to change the ratio of the final drive on a G/S - ST. One is with the higher 5th gear and the other is the optional final drive. The higher 5th gear is expensive from what I've read, the cost of the gear plus the time and expense of a rebuild. I thinK I've read it drops r.p.m.'s by 400-500. The optional final drive is NLA I believe, but if it is still available I'm guessing it's unbelievably expensive and I'm not sure how much it drops r.p.m.'s. Is there another option? Is it possible to use a different FD and re-lace the rear hub to a ST rim? Any other way to replace the final drive with another to drop engine speeds?
The issue is the three bolt hub. I dont think there is a spoke hub that will fit a four bolt FD with a higher ratio. You can't just change out the crown gear either, cause that's where the bolts fit. By far, the best option is to add the tall 5th to the tranny. It's actually a very easy job. As I remember, its the low 1st that is a bitch because it goes into the intermediate shaft which requires many tons of force to press.
...or the higher 32:10 ratio 3 lug mono final drive is still available new. They're basically non-existent on the used market.
Most of the hi 5 th gears are 5% higher - on a decent running motor you wouldnt know the difference unless someone told you. Just fork out for a new 32/10 as AW says, and sell the three stud 37/11 - they are hard to find and sell for reasonable money.
I concur with airhead wrangler.... I've been trying to find a 32:10 bevel for two years now... Not even a sniff. I've just had a higher 5th installed but if you are planning for only road use, Richie Moore of morespeed does a +10. Check out his site and email him.
Actually, I think the 32:10 works out to be about a 5% difference. It's 3-4 mph higher at 5000rpm. 78mph instead of 75mph. If you add a 5% taller 5th on top of that you'll get a 5th gear that's nearly identical to a R100GS final drive ratio, but the rest of your gears would be a bit lower.
AW -you are right on at five+ percent - somehow I had it in my head that 37/11 was 3.56 , but I just booted the Cray and you are correct. BT, there is some tricky engineering in these gears - remember you are only changing one of a pair, so a change in gearing can only be accommodated in the size and profile of the tooth. This is normal gear cutting practice, of course, as the sum of the number of teeth in the pairs in a normal box is rarely the same for all pairs, but there is a limit to how far you can go with any given gear, and it seems that most folk are happiest at 5% difference from the stock OEM gear.
moorespeed.uk I think. However, I would carefully consider your riding style before going to a drastically taller 5th gear. I personally like the "Four and a half speed" Airhead gearbox because it lets me downshift on the highway without changing the RPM all that much. The ratio that I've seen that's higher than 5% involved all three helicals. I can get that gearset also, but I've only been asked for it one once.
Correct anton... the +10 is a 3 cog jobbie and it alters the spread of the whole box. It is a specialist item and probably only really useful for someone who really needs it i.e. racing.
Looking at the forest and not the trees: Don't be tempted to gear your 800 like it is a 1000. If you want lower 1000cc rpms, get a 1000cc engine. 800's are geared for more rpm for a very good reason: Less power. The rpms compensate.
They can go a LITTLE higher, but depending on the use, maybe shouldn't. Agreed, gearing it higher than a 32:10 is asking for a lifeless throttle in 5th. The decrease in revs probably wouldn't result in better gas mileage either as you'd need to have it wide open to push that taller gear.
I recently bought a taller 5th (5%) from Anton for my 81 R80 g/s and am happy with the results. I just recently installed a tach so I have some numbers written down. Using a GPS for speed, the engine is turning at 4,000 rpm for 62 mph. Every 500 rpm's increase provides 8 mph. (4,500 is 70mph, 5,000 is 78mph, and at 6,000 rpm I was travelling.. no wait.. I would never break the speed limit so disregard that) . It's enough of a difference that 5th now feels like an overdrive when loafing along the highway.
100kph / 61.2 mph @4000 rpm is factory spec for an R80G/S with a 37/11 drive & 4:00 /18 tyre. What rear tyre are you running? Does the tacho need calibrating?
Hmmm.... Perhaps the factory spec was for the OEM speedometer, which isn't accurate? I'm running a Michelin Pilot Activ in a 4.00-18. If anything it looks big compared to the 4.00-18 Conti Go I had on there previously. As for the tach, it's a new digital tach designed for two cylinder dual fire ignitions. I would suspect it is pretty accurate. And I just checked to make sure and my g/s is a 37/11, so the previous owner hadn't changed it.
Here's a pic installed in a temporary location. Since I own a shop posting details and price is frowned upon. Check your PM's.
There is a solution that hasn't been mentioned yet: the adaptation of a 88-95 RxxGS rear wheel to a monolever 4-lug FD. That gives you three FD ratios from factory:37:11 (R80RT Mono), 32:10 (R100RT Mono) or 33:11 (R100RS Mono) . Details are here: http://www.stephenbottcher.net/BMW/rearconv.htm Hope that helps, Stephen
it's the best option for your bike.just lowers the rpm in 5th enough to make freeway riding a bit nicer.
I have been converting a r100 mono 33/11 bevel box to 3 stud. My dad has a st with the siebenrock 1000cc kit and with the 37/11 bevel it's very under geared. I work the difference in gearing to be about 12%. Not sure this is the easiest way to get taller gearing but at least it keeps the bike looking std and retains the original wheel. Any questions?
