Aero mods to dual sport or dirt bike, fully retaining off-road function

[Otto]

My son just bought a BMW F650GS Dakar dual sport motorcycle:

https://encrypted-tbn0.gstatic.com/i...Wek6CGEtSrfJrs

http://www.sierrabmwonline.com/image...indssplate.jpg

This single cylinder 4 valve thumper gets ~63 mpg cruising at 70-75 mph on the highway, which is excellent, due to fuel injection, decent gearing, etc. It's 50 hp engine and fuel economy is as good as otherwise similar bikes with much less displacement and power, i.e. similar to ~400cc bikes with ~35 hp, despite terrible aerodynamics typical of motorcycles. OTOH, it's a target-rich environment.

Notice the beak, basically unfaired frame, fork, etc.

The beak is basically useless from a functional standpoint, and draggy. The front wheel and fork is an aero abortion. The windscreen is cosmetic and causes annoying buffeting at speed.

This bike weighs ~390 lbs. dry/empty. By stripping off the useless crap like the beak, replacing the battery with a smaller/lighter one, dumping the effectively worthless ABS, etc. some owners have reduced the bike to ~340 lbs. dry/empty, thereby improving acceleration, handling, and fuel economy.

Question: What lightweight aero mods can be made which would improve ~75mph highway cruise fuel economy, yet not compromise off-road handling, ability to cross shallow streams, etc.?

I'd bet that ~100 mpg is well within reach, while retaining off-road function in conventional upright riding position.

How best to do it? I have some ideas including a lightweight fairing, but would like to get your perspective.

[alvaro84]

Well... I hope someone will pop up to answer your questions, I only have some experience with the same engine in a different model. The F650CS I own (look for the bike named Teresa in fuel logs, right below my avatar) is like a lowered, street-oriented version of your son's bike. The seat is 9cm (3.5") lower (so much for the wheel travel) there's no beak and there's a belt drive instead of chain. But the engine, and the frame is the same, so possibly my experience still apply somewhat.

I'd say these changes make FE a bit better, but nothing stellar, so you'd need something more serious. Yes, you can see 80-90mpg tanks in my logs, even a few over 90, but those are not at 70-75mph, more like 50. I tried different angle setting on the windscreen (with a home brew little gadget from an f650.com mate), but that didn't change anything either, I could only make things worse. A more complete fairing would probably help. Or an aerotrunk, low&slow seems to get good results with it: http://ecomodder.com/forum/showthrea...ect-27216.html

It might be the best option because large, protruding side fairings are the best way to make a bike useless on the trail where you'll fall and get up a lot, and break the expensive plastic in the process.

If you plan to keep cruising at 70-75 mph, taller gearing might help too. Teresa revs 4000 at indicated 100km/h (62mph) and I'm absolutely sure she could easily handle 70+ mph at that range if I could change the gearing. You have chain drive so it's way easier for you. It might qualify as compromising off-road function too, though. And it might be a drawback FE-wise in the city too, as you'd probably have to use a lower gear - I'm at the very bottom of 4th at 50-something indicated km/h (30-something indicated mph).

Shaving off that weight won't help FE too much (possibly you won't be able to measure the difference at all), but it can surely help handling and won't make anything worse, so why not. Except for removing ABS, that helped me out a few times. Might be bad off the road, though, so it's your choice again.

[Otto]

Thanks, Alvaro.

Do you know if the belt drive can be easily used on the F650GS Dakar bikes? In other words, could the GS sprockets be unbolted and the CS cog pulleys bolted on? What ratio does Teresa have between the teeth on the front and rear?

Thanks again.

[alvaro84]

Probably can be done, but surely not that easy. The CS has a completely different swing arm, probably because belts can't have the chains' slack. So I guess it would be even more difficult with a Dakar's longer wheel travel.

About the teeth - it might have been better to take a torx wrench and count it myself, googling took a great deal of time Well, the final ratio is 2.94:1, which is the same as the GS' default 16:47, and AFAIK the CS' gearbox has a 1 tooth taller 5th (data sheets say 2.75/1.75/1.31/1.05/0.84:1 instead of 2.75/1.75/1.31/1.05/0.88:1 in the GS/Dakar's case).

[redyaris]

otto
I suspect that streamlining an off road bike for riding in the back country would be very difficult, due to the amount of moving around on the bike that is needed for keeping the bike on its wheels. You would need to make some kind of crash protection for the body work, as well as compromise the body work for rider movement and control...
I have started work on streamlining my 2007 Kawasaki KLR650 which I plan to use on gravel and dirt roads as well as highways, but I will not be riding it "off road".
Like alvaro84 suggests I doubt that there is much that can be done that would get major improvements in fuel economy on off road bikes, used in off road riding.

[TedV]

take a look at the Craig Vetter Alcan fairing project. Vetter-Alcan-fairing I spoke to him about it when I rode in the 2011 Mid-Ohio challenge. I was 7th and the best stock bike there. I also was the only one carrying anything resembling the 4 bags of groceries. I had 5 days worth of camping gear as I was on my way to the BMW rally in PA. 2011 Mid Ohio Craig Vetter Fuel Economy Challenge results

Craig had made the prototype Alcan out of aluminium. He said it reflected engine noise, and since you were out of the air stream you felt more heat. Temps were over 100 degrees (hit 104 one day) and riding on the bike in the air was grueling that weekend headed to PA and back to TN. I gave up the idea of a high MPG bike since I have to be socially acceptable when I get to my destination most days. Dual sport you are not going fast enough to need the aero, unless you are traveling a long distance to hit a trail, like riding from central USA to the north of Alaska on the Al-Can HWY.

I think someone in the North East got the fairing from Craig and was going to work on it. Craig and I discussed the use of sound deadening foam to cut down the noise reflection. Think about being in the focal point of a parabolic mirror. it would need it. Streamlined panniers with a Kamm tail might help a bit. Like has been mentioned, you will need to leave room for body english when you off road the bike. Also remember the crash bars and panniers strong enough to support the weight. Mine has taken a few dirt naps without damage.

I have tried all kinds of screens trying to find something that didn't buffet. Being in the wind has been the best so far, altho the Ztechniques VStream® Tall Touring Screen for F650GS/G650GS Single held the most promise, but my handlebar risers hit the screen. National Cycle

I started to fabricate a way to mount a K1200rs screen but some health issues have nipped my bike riding for a bit. Notice the lower wings the VStream screen has. Similar wings, as well as vents to let some air in the rider zone are on factory BMW's that seem to not have the buffeting most larger screens have.

To find all kind of info on the 650 thumpers, F650.com Home

so, yeah, there has been some thoughts on this subject.
good luck, have fun

[Otto]

Thanks, all, this is helping a bunch.

Ted, you got 73.6 mpg on your Dakar loaded as in the picture in the linky? That's really good! Have you looked into Madstad or similar adjustable screens? I've heard they're pretty good, adjustable to allow wind up the back side to relieve the pressure bubble which contributes to buffeting. I'd like to be able to cruise comfortably at 80.

What I have in mind are some subtle and not so subtle changes to a Dakar. As you know, its engine fuel management is excellent, but aero could stand some tweaking. My thoughts:

1. Remove beak, consolidate hand protectors, turn signals, mirrors into one airfoil leading edge D section-shaped unit only as thick as needed to protect hands from wind (and bushes during off road use). Much less stuff hanging in the breeze that way.

2. Add front fender which hugs the tire, has airfoil D section "horn" to fair the airflow past the fork. I did this on my BMW R1100RSL using a flat wrap piece of Corplast, as the gap in the front of the fairing and behind the fork is surely a huge drag source. With the Coro bent into a D section and taped to the fork legs, I got significantly better fuel economy and less buffeting even though this mod is much lower on the bike than the windscreen.

3. Teardrop sorta wasp tail fairing as top bag/rider backrest, instead of sleeping bag, tent, etc. just strapped on and hanging in the breeze. This tail fairing would contain cargo, and also house the turn and brake lights, as well as license plate. Remove the existing license plate holder.

4. Fairing and windscreen conceptually similar to Craig Vetter's Alcan fairing, to shield the rider's legs and torso, which aerodynamically are part of the machine and so must be faired.

5. Hard bags thinner and longer, and tapered and much less protruding into the slipstream than conventional hard bags. Touratech makes expensive aluminum hard bags which are functional, but are aero speed brakes. The same or greater volume could be carried with conformal and streamlined hard bags closer to centerline, with lower weight and center of gravity,yet also protect the bike in tipovers. The conceptual inspiration for this idea is conformal fuel tanks on fighter planes. When first used on the F-15, designers were reportedly shocked to learn the plane had lower drag with the conformal tanks than without.

6. Use tough, forgiving plastic for these mods, designed to take tipovers. That's how modern kayaks are made, to take major rock hits, bounce off, while retaining shape, strength, and beauty.

I like the BMW F650GS Dakar because it is a dual sport bike, mainly for highway use, but suitable for dirt or gravel fire and logging roads, crossing some streams, etc. On YouTube are videos of mc tours in the beautiful Altai Mountains of Siberia, and evidently the tour companies all use BMW F650GS Dakar bikes in this very remote and primitive area. So, I presume these bikes to be reliable and efficient.

[TedV]

Yup I got that with all the gear, the speeds were down a little bit compared to other competitions Craig has had. Jack McCornack had higher than usual mileage and thought he under filled his tank. I over filled mine and still got good results. I rowed the gears to keep RPM in the 4500 to shift point at 5000rpm. After the event on the hwy with the heat I went a bit faster than the limit to get where I had to go. Mileage went down to the mid to upper 50’s at the speed you would like to go, stopping every 150ish miles for fuel and to soak my riding gear in water for airconditionin effect. A local friend has noticed she gets better mpg on pure gas than when she runs 10% ethanol. I just run whatever the fuel station has and usually get mid to upper 60’s where she gets closer to the 73mpg.

I have seen the Madstad screen online but not in person. Like I said my riding time has suffered from some issues.

The F650 forum will have some heads up on maintenance issues. Like wires to fuel pump inside the tank are too short and might break loose leaving you stopped. Inspect your pivot bearings-BMW is stingy on the grease. AGM battery is preferred over the wet cell as the liquid acid will dribble out. Depending on your year, watch for cracks on the front forks close to the axle bolt. After my bike sat for a while, the fuel hose loosened on the clamps somewhere above the engine. Gas dripped all over, I turned the bike off, hosed it down. When I got it apart to troubleshoot where the leak was, it no longer leaked. General thought was the hose expanded again after soaking in fuel. Old gas in the tank might not let the bike idle correctly. If you ride it regular, not an issue on either count.

If you ride in mud, watch out getting the front fender too close to the tire or it may cake with mud. The beak does work when riding through water to cut down the spray that gets launched off the top of the tire that you will ride back into. Imagine a spitting into the wind effect. Be careful blocking the air flow to the radiator in that area. There are a few stories on the chain gang forum about overheating from putting farkles down there. During summer rides I have found myself splaying my knees way wide to get out of the hot air blast from the radiator. So keep the idea of how to route that air out too. I usually wear mesh riding pants for some abrasion protection, but they do allow both hot and cold air to flow through.

Your thoughts on the subject are pretty close to what I considered. Form meets function. I was thinking to make the panniers out of Kevlar and high density core composite. I knew the bike will meet the ground, trees, rocks, etc. and wanted the impact resistance on easily formed cores for a one off. Also make the front crash bars as supports for the front side fairing.

Good luck with the project and I hope life doesn’t get in the way of you getting it done.

[bikepilot]

Cool project. #2 and #3 won't work off road. Need to be able to stand up and move around a lot off road (interferes with 3) and mud will stop the front wheel and/or break off the fender (#2). Have a look at Dakar racebikes. They get the aero pretty good (given off road requirements )b/c they have to do fairly high speeds on low HP (450cc limit, so about 55-60hp).

Depending on how much stuff you carry, a soft off road oriented tail bag might be a good substitute for panniers. Something like one of the smaller Giant Loop bags would probably be reasonably aero (in draft of rider's body/legs). A tank bag is also great for extra storage for street riding (gets in the way off road though).

[PeterS]

The G650 is a bike I'm quite interested in too but not the GS ,rather the Cross Country which is a really stripped down version. It's about 325 pound and doesn't have the silly beak. The seating position is quite good and I'd say it's quite a good basis for a reasonably economical ride .Not exceptional but I'd say 95 mpg US is possible at 55 mph with a complete Zero Vetter type fairing....the actual swept volume of the engine being the real limitation..

The attraction of a bike like this is of course the 50 hp! Anyone who has been riding for a while will find it very difficult to go to a low hp bike. I know. I'm tossing the possibilities around every day while I try to sell my old BMW boxer.