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Old 04-24-2009, 10:15 AM   #1
Desert Dave OP
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Joined: Aug 2004
Location: Tracy, CA
Oddometer: 2,767
F800GS 15 Tooth CS Impressions & Questions

****YES, I did search and checked the index for this topic and only found a number of references on the subject. If I missed a major thread go ahead an merge this in...if not, post your experiences here so others can make an educated descision.****

Many have noted the tall first gear on the 800, while others say it's fine. If you're in the "fine" category, good for you, go on to fixing the suspension, or seat or whatever. For me 1st is fine most of the time, until I get into 1st gear, feet paddling rock step ups that the stock suspension simply won't allow me to just gas it an hang on like I would a lighter dirt bike. So I end up with both feet off the pegs, doing a balancing act, at less then walking speeds, hoping I just slide on the skid plate and don't get hung up on it (as both feet are often airborne now), all the while I'm smoking the clutch and thinking of the thousands of miles of street use I may of just burned up . I need a lower 1st. Truthfully I really don't enjoy that kind of riding, but it's often necessary to get through. I'll leave the hero stuff for trials riders if I see an easier route.

Many of the fire roads I ride turn into creekbeds or are just rutted so bad from runoff and jeep tracks that could swallow a bike that I often find myself wanting a gear between 1st an 2nd. I really enjoy this sort of riding, but first is to low as I'd be screaming the tach into grenade territory , and having to watch throttle response. 2nd works well as the speeds keep the Rs' under redline, and makes throttle response much easier to control. However, I often need to chop the throttle for a rut and the R's drop so low, around 2K, that there isn't any instant snap for clearing an object, so again I'm playing the clutch hard (two stroke style) in this sort of stuff, as a downshift to first isn't practical. I'm amazed at from how low the twin will pull clean, but I don't like asking this much from the motor to often. A slightly lower second would be perfect for the more technical fire roads/trails.

A 15 tooth CS should solve that, my only concern is what happens on top out on long highway hauls. To test before I commited I tried a number of highway rides using only 5th gear. With stock gearing at 70 mph (GPS) 6th gear was around 4700 rpm and 5th was about 5100, these numbers are close as it's hard to see 100s of rpms accurately with just the tach needle. I figured with the new gearing my rpms would be right inbetween, so if I could deal with 5th on the highway, then 6th with the new gearing would be no problem. I found it it quite doable, an liked the increased passing power, even if I missed the overdrive a bit, try it yourself an see what you think. Most important was my fuel mileage didn't change out of the range I normally got on the highway. So no major downside.

So I finally got around to putting on the 15 tooth sprocket (and new chain). Took her for a 300+ mile test ride yesterday, of which over 90 miles of that was dirt. Mostly 2nd/3rd gear dirt (the stuff I like) with a little high speed stuff thrown in for good measure . My first impression was the gearing did just what I wanted, an works much better for dirt. I was able to lug second like I did before with much better response and it's now excellent for rutted climbs, with little clutching an by the time I shift to 3rd that means the trail has opened up enough to sustain speed. 1st became a MUCH better rock crawling gear as I had a few sections of very washed out trail to try it on (still not sure how I kept balance in some of that stuff, an no my shorts aren't clean ) and was nicer on regular starts on a hill. Other than that I no longer spend any time in 1st. On both dirt and street the easier to access power on tap is nice, although I find myself short shifting if I don't want that much compression braking .....pretty much exactley what you'd expect from this sort of gearing change.

On open highway 6th was doing as I expected, turning just a hair under 5K rpm at 70 mph. I'd prefer the higher sixth but this was worth the trade-off for me in the dirt. The crusing rpm is fine but I really don't prefer the added compression for slight throttle adjustments, and you can't short shift above 6th. Don't get me started on how much I wish BMW made this more of a WR transmission, but I'll have to deal with the cards as they're dealt.

So here's the down side, as well as my questions. My mileage took an absolute dump, I'm talking like 5 mpg or more. Now I did only test three tank fill ups so far, resetting the MPG computer an checking with a calculator, and there was one really windy section so I'm going to ride more and get more results to make sure that ride wasn't a fluke before I cast judgement.

Here's where I'm confused. When I tested highway MPG with stock gearing riding in 5th gear, I noticed no major changes, and that was slightly lower overall gearing than what I have now with the new gearing and riding in 6th.

So I'm wondering if it's possible if the ECU is doing something different with fuel delivery. I understand most maps are based on gear and rpm (I know, throttle position, an other sensors as well) If the ECU "knows" the gear and rpm speed is a given, but knowing the way BMW engineers is it possible the computer sees a different speed (from the speedo sensor) and it doesn't match what gear/rpm are telling it? And this causes it to react differently? Or does actual speed play a part at all in the fuel map?

For those that have done a gearing change what have your experiences been regarding mileage?

For that matter chime in with your 2 cents on how you ride and how the gearing worked (or didn't) for you.
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