Darkside conversion for R1200 BMW bikes & late K's with newer Final Drives

Sold out, thanks to you for your interest, and to those of you who purchased!

Interesting idea. Since they're not made for turning in the same manner as a motorcycle, do you have any issues while leaning the bike over? Either in traction or from the sudden change from that wide flat area?

Good question!
It is really not any different than a m/c tire that has been worn flat in the center from lots of highway miles.

Some of the high-falutin' hardcore corner carvers claim that they don't like the transition over the corner of the tire, but you get the same effect on a set of worn bike tires.

Here is a video of a dude on a Supercharged Triumph Rocket 3 running a 245/45R18 car tire on the back of his bike running with some other sport bikes on a nice set of roads in Western Wisconsin.
(I apologize for the music...)

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I personally did about 6,500 miles of solo riding on my R1200RT with my car tire setup and never had any concerns. Those miles included a trip through the Black Hills of SD, the dirt/gravel/mud of the back entrance to Badlands NP, and some roads similar to the video in my part of Wisconsin.

I will admit to starting out a bit slowly at first, but soon gained enough confidence that I was leaning the bike pretty well.

Here are a couple of shots of the latest bike to enter the darkside...


^^With my rig in the background...


^^You can see that the profile is not totally flat, so there is still a smooth transition while leaning into a corner.


^^Plenty of caliper clearance.

Mike reports that it handles very well, and has been increasing his lean angles as he builds confidence with the grip.

.

Hi Karl,
Love your posts and was wondering if you are still selling the kit?

I have a r1200rt and wanted to convert the rear tire.

Thank you,

Roundworld

i don't know why a new wheel is required. GOLDWINGS have been running car tires for a few years now. good reports from all that do...BAD reports from those who won't try.

There's no car tire in the US that fits the BMW wheels that's why the adapter is needed. If you find one, please let us know!

Well the Vstrom guys that have went to the darkside are running either 205/50-17 (a tad shorter than stock) or a 205/55-17 tire (a tad taller than stock, but harder to find in this size). Either way, that tire is pretty squished on a rim meant for 150/70-17 tire. And on top of that, most tires in that size range are pretty high performance tires, although guys are still getting 15-20k+ out of them. The most popular tire most of the guys started with was the General Exclaim UHP in the 205/55-17 size, but its discontinued now and hard to find. Others have found other tires that work well for them including snow tires for some of the guys up north in Canada. I have a UHP tire sitting here for my Vstrom haven't installed yet, but will be putting it on to see how it works for big trips where I don't want to have to swap out a rear tire.

I don't own a GS, but I really like this idea, especially how easy it is to swap out a rear wheel on that single sided swing arm and the 15" rim gives you a much better selection of tires in that size to run and cheaper than 17" tires too. The other nice thing is it allows you to keep a regular MC tire on your normal rim with either a knobby if you like to play off road or maybe just a normal MC tire when you really want to play.

I give a big thumbs up to the vendor here!

Does this work on the 1200 gs ?

And I wonder if the modification will be harder on the final drive, considering its more weight and the spacer possibly moving some of the load farther outwards ??

Greetings, and thanks for the kudos.
Sorry for the delay in responding, but sometimes life gets in the way of the stuff you WANT to do...

Right now I don't have any of the adapter hubs made up, and I was actually trying to decide if there has been enough demand to justify doing another run of them.
It does tie up some capitol to have a batch of them machined and anodized and then leave them sitting around waiting for someone who is interested. But it does bring the cost down to have them made in batches rather than one-off as I get orders.

I suppose that I could probably be talked into doing another batch of them if I knew that I had a couple of them pre-sold to offset some of the initial investment cost of making up some more adapters...

Hello GAS GUY,
Yes, this is a direct bolt-on conversion for the R1200GS/GSA bikes as well.

It will also fit any of the newer K-bikes which use the same style of final drive. (You just need to stand on your head when installing because the shaft is on the wrong side... )

As far as changing the suspension or effect on the final drive:
Based upon my abuse of one of these setups on my sidecar rig, I don't see it being any problem.

1st; The centerline of the wheel assembly is NOT offset vs. the stock wheel setup.
The wheel used for the conversion has more backspace to it, which is what allows there to be enough space for the adapter, but the thickness of the adapter brings the new wheel right back to the original wheel/tire centerline of the bike!

2nd; The unsprung mass of the wheel assembly doesn't really change much.
If you compare the weight of the stock GS/GSA wheel and tire to the total weight of the new setup (adapter/wheel/tire) they are right in the same ballpark. The exact figures depend somewhat on which tires you are comparing.

-The stock GS/GSA spoked rear wheel is listed as being 13.01 pounds.
-A 150/70R17 Metzeler Tourance rear tire has a listed weight of 15.43 pounds.
That gives a total wheel/tire package weight of 28.46-lb (including valve stem).




- As you can see from this photo of an actual complete wheel/adapter assembly, including the lug nuts, stem, etc., the total weight of the new wheel AND adapter assembly is only 14.38 pounds.
- The typical tire used with this setup on the GS bikes is a 165/80R15 all-season radial. This size tire weighs in at about 17.0 pounds when new.
That gives a total package weight for the new setup of 31.38 pounds.
Less than 3 pounds difference between the two wheel/tire assemblies, and you can easily pick up more mud than that.

On my sidecar rig, I am using a different size tire with the new setup to change my overall gearing. That tire is only 13.5 pounds when new, for an total wheel/tire assembly weight of only 27.88, which is actually LESS than the stock setup!
(My winter tire is even another 1/2 pound lighter still...)


3rd; The unsprung mass of the wheel is not even CLOSE to the greatest force/load on the final drive.
The weight of the bike bearing down on the rear wheel is far and away more of a factor to loading on the final drive bearings. The unsprung mass will simply affect the reaction rate of the rear suspension.
And even there, the wheel is less than half of the total unsprung mass when you start adding up the weight of the final drive assembly itself (18.21-lb), the swingarm and torque links (8.79-lb), the driveshaft (4.65-lb), and the various pivot bearings, bolts, drive fluid, etc which adds and additional 1.86 pounds. For a total unsprung weigh of the swingarm/final drive of 33.51 pounds.


4th; I believe that I have "torture tested" this setup with loads transmitted to the final drive above anything that you will be able to dish out. And my original final drive is still humming along happily with nearly 50,000 miles on it. (about 42,000 of those with this wheel/tire conversion installed)

When fully loaded for a trip my sidecar weighs in at close to 1,400 pounds with just myself aboard. That equates to an actual measured static rear wheel loading of about 660-lb. More than the entire weight of a stock GS bearing down JUST on the rear wheel!
Add in that a sidecar doesn't lean into turns, so that my final drive has been subjected to side-thrust loading in addition to the normal radial loads on the support bearings and you can see that I have been testing the final drive of my rig pretty hard...


(I DO like to sometimes ride the rig at a bit more 'spirited' pace...)

In addition to the on-road miles, I have ridden this rig over quite a few dusty miles of rutted/washboard gravel at speed, run it through mud, and hit whoops big enough to bottom out the suspension on more than one occation.






AND, I don't own a car, so my sidecar is my only winter transportation. Meaning my final drive has been subjected to running with below-zero lube in it, road salt thrown at the seals, and all sorts of other mean, nasty stuff.



I don't baby my bikes, and so far I have not detected any issues with this conversion!

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Sorry if this reply got a bit long, but I just wanted to give you a thorough answer to your question.

It all depends on what the bike's stock wheel/tire size is as to wether there is a safe car tire substitute without doing a wheel conversion.

The modern BMW's run a 17" rear wheel with a single-sided, double-pivoted swingarm. That swingarm is pretty beefy and doesn't offer much extra clearance for an oversized tire. That is true both in the overall width, as well as the height of alternate tires.
There is a big radius brace molded into the corner of the swingarm that limits the size of a square profile tire.

Add in that there just aren't that many 17" car tires available in smaller widths in the United States, and those few which are made are on the pricey side; and the cost of a conversion quickly becomes practical.

There is one inmate over in the Hacks forum who recently fitted a 17" car tire to the stock wheel of his K-bike sidecar rig (Yokahoma S-Drive 195/40-17 ~$110). The first size he bought didn't fit (rubbing), and the smaller one he bought second required THREE wheel spacer shims before it would clear the swingarm, which altered the centerline of the wheel.
(And he recently reported that he is still getting rubbing at speed due to tire growth when it is spinning at highway speeds.)

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On the other hand, there is a relatively wide selection of 15" car tires which easily fit into the space available, and they are MUCH less expensive ($50-70).
And if on a trip these tires are readily available from nearly any tire outlet, service station, etc. to keep you rolling.
(In my researching this conversion I found that most of my local car tire outlets will not mount a car tire to your bike rim for you, but if you have a car rim - no problem!)

So for half the tire cost, you get wider selection, availability, and easy servicing if you have an issue out on the road.
Just from the cost of the tires alone the conversion can end up paying for itself in the long-run.

Is there a height difference? Is it possible to use this method to gain or lose height?

It all depends upon the tire you select to mount on the new 15" rim.
With a 15" car rim there is a wide range of tire sizes which will fit in the space between the swingarm and exhaust can.
How that will affect ride height is that it will raise/lower the rear axle by 1/2 of the difference in tire diameter. And you will probably notice about 1/2 of THAT difference at the seat/pegs. So going with a tire that is 1" smaller diameter would result in a change at the seat of around 1/4".
If you also go with a smaller section tire up front or do something else to lower the front ride height, then the entire change in height would be seen at the seat.

Be aware that changing the rear tire size too much without also altering the ride height at the front will change your steering head angle and trail figures. Which might affect how the bike handles.

The stock rear tire on the R1200GS is a 150/70R17.
According to my Metzeler catalog the Tourance in that size has a new tire diameter of 650mm (25.6" dia.)
The more dirt oriented Metzeler Karoo II in that size is listed at 667mm (26.3" dia.), so it will raise the rear axle by .35" (8.5mm).

The tire that most of the GS riders who convert to a car tire setup use is the 165/80R15.
Depending upon manufacturer, that tire is listed as being 25.0-25.5" in diameter when new.

On my sidecar rig I run a smaller diamter rear tire in order to drop the overall gearing of the drivetrain.
It would be equivelant to changing the gearing in the final drive unit, but much less expensive to try...

My current tire choices for my bike is a 165/60R15 for the summer. Diameter 22.8"
And a 145/65R15 for the winter, with a diameter of 22.4".

There is also a 155/60R15 which is slightly smaller at 22.3" diameter if you really want to get maximum drop and lower gearing in your bike.
That tire would put the rear axle about 1.65" (42mm) lower than stock. But remember by dropping the rear axle that far without altering the front ride height it will also result in the head angle changing by around 1.6-degrees (reduced steering angle, and increased trail).

{ tan(angle) = rise/run = (change in axle height)/(wheelbase) }

Wasn't that simple...

There is also a 155/60R15 which is slightly smaller at 22.3" diameter if you really want to get maximum drop and lower gearing in your bike.
That tire would put the rear axle about 1.65" (42mm) lower than stock. But remember by dropping the rear axle that far without altering the front ride height it will also result in the head angle changing by around 1.6-degrees (reduced steering angle, and increased trail).


If I put the car tire and used the 155/60R15 and increased the trail would that also then make the steering better as if I put an easey steer? Just a thouhgt or am I off base

No worries, looks like you had a blast. I have two questions one how soon can I get the conversion with the rim? and two, If I put the car tire and used the 155/60R15 and increased the trail would that also then make the steering better as if I put an easy steer? Just a thought or am I off base. Also I saw that you make the trail reduction bracket. I wanted that as well. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
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Someone told me that while it would help, that while one is in a turn the trail is reduced do to the compression of the front spring and there by negate having it. Is this true?<o></o><!-- / message --><!-- / message --><!-- sig -->

Roundworld

For easier steering with a sidecar rig you want to REDUCE the trail of the front wheel.
That is what the various "easy steer" modifications do.
(also known as trail reducers...)


Trail is what makes the front wheel want to stay going in a straight line because the contact patch literally "trails" behind the point where the steering axis intersects the ground.
In the following diagram the red dashed line is the steering axis, and you can see how dropping a line from the front axle shows that the wheel is "trailing" behind the point where the steering axis intersects the ground.



For a two-wheeled vehicle more trail means that the bike is more stable because the front wheel doesn't want to flop over as readily when the bike is tilted off of vertical at speed. For a sidecar more trail simply means that the friction of the tire has a longer lever arm to resist steering inputs at the handlebar. Reducing the trail shortens the lever arm of the centering force on the front wheel, making it easier to turn.
The diagram shows how the trail is usually reduced for a TeleLever front suspension by the addition of an adapter to push the wheel farther forward in the suspension.

Some high-performance sidecars have center-hub steering with extremely small (or no) trail for a lighter feel at the bars with the wide car tires that they use up front.

So in the case of a sidecar rig, increasing trail by going to a much smaller wheel on the back without also reducing the ride height at the front will make it HARDER to steer.

Does that explain it adequately?

As you can see in the diagram in my previous reply, if you compress the front suspension (drop the front end), you would increase the steepness of the steering axis. And yes, that would reduce trail.
Of course, for a sidecar this is not necessarily a bad thing...

Of course, even with a sidecar, if the trail figure gets too low the steering can begin to feel a bit vauge. I like to have some trail left so that I am still getting feedback from the front wheel.
(center hub steering is a whole 'nuther kettle-o-fish, so the trail figures for those are not related to what we are talking about.)

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As far as the trail reducer brackets that I made for myself, it was designed to work with the R1200RT/ST/Roadster.

The R1150GS & R1200GS has a different type of lower steering cross-member. Rather than one which bolts to tabs on the slider, the later model GS lower bridges have a clamp ring which the fork sliders slip into. So my adapter would not work for one of those bikes.
I have not taken the measurements to see if my trail reducer plate will fit on the R1100GS or any of the other Telever from ends which use a bolted on lower bridge...

When I designed the bracket I went to a place which does laser and water-jet cutting to cut my blank, and I had a couple of extra blanks made so that I could experiment with less offset/reduction if I wanted to.
However, the first offset chosen worked so well that I never did anything else with the extra blanks.
So I would be able to offer a reducer for a R1200RT or one of the stable-mates.

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As far as the rear wheel conversions, I don't currently have any of the adapter hubs on hand, so it would take several weeks to get all the parts together, the anodizing done, etc.

To be honest, it has been months since anyone has shown any interest, and I have been trying to decide if it would be worth the investment of having another batch of hubs machined up only to have them sitting gathering dust for who knows how long.
I haven't even checked to see if the prices at my anodizing shop, machine shop, and parts suppliers have changed since the first batch was made.

Ideally, I would like to know that I have at least 2 people interested/commited before investing the time/money to do another batch of hubs.
Even then, I will be sitting on part of my investment while I wait for other buyers.
SantaRosa?
GAS GUY?
Anyone else with a serious interest???

Great thanks, I think I finally understand how all this works. by moving the front wheel back that moves the bars forward and so the plate moves the bars back to the original place?<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
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If you are selling the trail reducer then I would like to get one form you. You said that yours was perfect so one just like yours. Do you have all the pieces for the conversion or do I need to get some other pieces? Also did you compress the front spring or replace it? And how much?

I have a 2005 R1200RT with a Hannigan sidecar<o></o>