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 :eek1) 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.

I can almost guarantee its not fueling.

Are you sure its not just the fact that you changed the gearing, so its throwing off the speedo/odo? That's what typically happens in bikes that don't have front wheel speed sensors. I suggest you check your gauges against a GPS.

All of my speed measurements were taken from GPS, the stock speedo is inaccurate. But even the stock speedo reading didn't change from what it was before as the speed measurement is taken from the rear wheel sensor, which doesn't change no matter what gearing you put on, it will be actual speed. So the odometer etc. will read as it always has.

I'm wondering if the bike uses this as input in the mapping at all? If not it's a non issue.

I went the other direction. I changed to the 17 tooth(650 gearing), to get better fuel economy. It got worse. A friend and I went on a Baja trip, where he constantly got better mileage with the stock gearing then I. It would be nice to have a wider box. Go figure.
Cisco

Dave,

I too went with a smaller front sprocket for the same reasons. I'm very happy with the changes for the dirt. Slow technical stuff is much more manageable and getting restarted on steep hills doesn't require you to slip the clutch much.

6th gear, in my opinion, was too low even before the gearing change.

I've put about 2200 miles on it since I put the sprocket on, but haven't paid attention to the mileage.

I'll be doing some riding next week and see what I'm getting.

Hi Dave,
I changed to a 15 front some time ago and like you, we (wife and I and others) do a lot of dirt riding and the difference the 1 tooth made to the ability to rdie the bike easier (for a 207kg bike) in the dirt and technical stuff far outweighed any fuel consumption figures.
I found the acceleration in 6th gear is far better for us now, but our speed limit is only 100 - 110 kmh. The ability to just blast past a tintop on open road without having to change down to 5th or even 4th to get past quickly is not longer an issue.
I find that i am constantly riding in 4th and even 5th gear on 4WD tracks and using the engines low RPM torque to smooth out the ride. It is amazing how much low end this engine really has. Where as before I was riding in lower gears and the slightest throttle twist would have the TKC break traction, I can now just ease the throttle open and attack the snotty stuff without losing momentum through wheel spin. Of course the lower 1st gear is great for just crawling when required without slipping the clutch to much.
Downer for me...
One thing I have found with my bike is that the clutch will grab if slipped too much. I have duplicated this many times and dont know why it happens, but if I'm slipping the clutch to get traction on very loose steep terrain the clutch will suddenly grap and stall the engine. Not good.

Anyway put me down for a +1 on the 15 tooth front. I like it.
That doesn't mean that I dont wish BMW had made the gear ratios different though, but we use what we have.

Cheers from downunder..

Crock

Cisco:
:huh I would not have thought going from 16T to 17T would reduce MPG....
Any chance you can do an A<-->B (swap) test and have your buddy ride your bike and see if the results are the same? Maybe it is riding style, differences in gear+rider weight?

Jim

The speedo is driven by the wheel sensor and as such is unaffected by any gear change. Does not matter what the gearing is, the wheel will be doing the same number of turns at a given speed. Only changing the wheel size would throw the speedo off. Thus the change in fuel economy would have to be due to the engine load changing.

Yea that was the last thing I expected myself. His bike had more miles then mine, that could be it, as the moving parts were more friendly with each other. I could blame the computer, but he put in less gas each time we stopped. I am hauling more lard then he. The 17 tooth is nice on the road, as I wasn't looking for that 7th gear on the tar.

Thinking about this for a minute...
(something I try to avoid ... ... )
While riding yesterday I do seem to have noticed that the "instant MPG" seems to be a higher value in 5th than 6th gear on my 658 ... but it's hard to keep everything else constant...

So........... maybe you're on to something?

Next time I'm on a straight flat stretch of slab with minimal wind I'm gonna mess with it again..........

Jim

Yeah I've found that mode of the computer to be almost useless, except for curiosity, as the slightest changes really make the numbers jump around. You may want to try resetting the average mpg while holding a constant speed for a mile or so and that would give you a quick average for that mile, and then reset and do the same for another gear/or another speed.

But there's really no load change.

It takes the same amount of poop from the engine to go a steady wheel speed, regardless of what gear you're in. So it really boils down to the efficiency of the engine at those different engine speeds.

I'll agree it only takes 'x' amount of hp to maintain a given speed on a level road but a taller gear (if taken too far) will make the engine 'think' it's working harder and will require greater throttle for the given speed.
Sometimes I think you have to give the engineers credit for for picking the correct gearing, although I can see a case for going lower for off-road use.
Jim

Sometimes it's hard to out think the design engineer.

In my experience, that lower gearing, along with the newfound acceleration that is a byproduct of it, causes the throttle hand to be a bit more enthusiastic than before. Result: increased fuel consumption.

Yes the sportier gearing does cause a hooligan side to appear more often and make the power easier to tap into. Guilty as charged

BUT, that would explain backroad an offroad MPG which stayed closer to normal as the lower gearing often allowed me to smoothly pull a higher gear as often as I whacked open a now torqueier (is that a word ) lower gear. On open highway where it seems the worst I still pretty much hold steady throttle, besides I ride like an old man on the open highway....no sense wasting a ticket on straight road.

Does anybody know torque for front sprocket bolt/nut?

Just did it today.

25nm

You can find most of the torque values here:

http://f800riders.org/HoW/showentry.php?e=43&catid=5

Blasphemy! My handlebars and pegs are in the wrong spots, seat is unfit for human use, transmission gearing is all wrong, wheels to soft, engine low on power and the throttle is absolutely to sensitive to any input. All hope is not lost however, we can fix all that BMW has gotten wrong. Now where is that Touratech catalog...

On a slightly more positive note. DD I feel exactly the same as you. 1st is a little to tall and a slightly lower second, even third, would suit me fine on my sandy fire roads.

Is there enough adjustment to pop on a smaller CS cog without changing the chain?

jim