Converting an F800GS to belt drive.

Nick V

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Greets, everyone. :bounce1

I normally post over in the 1200 GS Hexhead and 1200 GS Watercooled sections, but I'm in need of a new project.
An idea came up the other day, and I'd like to reach out to anyone who's done what I'm proposing to do - IF anyone has done what I propose to do.

I want to convert an F800GS to belt drive. But not by using an aftermarket belt and bespoke belt pulleys.
The way I want to do it is by transplanting the engine/gearbox and rear swingarm from a crashed F800S, ST or GT into an F800GS frame. While I'm at it, the converted GS will also be fitted with the S, ST or GT 17" wheels and four-piston front calipers.
The idea is to take the GS (which is already a better-than-average road bike) and maximize it's strengths on tarmac. I have no interest in taking an F800GS off-road.

I've already done a good deal of research on this, and I see no major technical reason why it can't be done. The biggest issue so far is that the rear of the F800GS frame will need to be modified to accommodate the new swingarm mountings. (For those who don't know: while the 798cm3 Rotax twin has the same basic dimensions and produces identical power and torque in all '800' iterations (chain- and belt-drive), the swingarm attaches to the rear of the engine on all alloy-framed variants. On the F800GS, the swingarm attaches to the rear of the main frame.)

In the unlikely event that someone else has done this, how did you solve problems with regard to:
  • Modifying the GS frame to take advantage of the S / ST / GT rear engine/swingarm mountings?
  • Front and rear suspension geometry? (For a start, the front suspension needs lengthening by 2" to accomodate the 17" front wheel.)
  • The top rear shock absorber mounting? (Since I haven't yet taken on the conversion, I don't yet know if the shock absorber strut, or the strut mountings, or both, will need modification.)

As usual, looking forward to hearing feedback from interested parties!
 
Greets, everyone. :bounce1

The idea is to take the GS (which is already a better-than-average road bike) and maximize it's strengths on tarmac. I have no interest in taking an F800GS off-road.

I'm a big fan of your boxer projects but that sounds like a whole world of pain for limited benefits. I'd be interested in following your progress but if it was me, I'd be looking at taking a Nuda and adding some touring comfort. A lot less hassle and easy to return to standard and sell on at a later date. Keep us informed though and good luck:thumb
 
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I must admit I would agree, but for the fact that a very high-mileage F800GS and a side-dropped F800S have recently come onto the radar - at a combined price which works out to roughly half what I'd pay for a low-mileage 2011 F800GS.

I've actually owned a 2008 F800GS in matt metallic grey before. As a bike to ride, I loved it to bits.
After my '13 LC R1200GS, the F800GS comes a very close second on my list of all-time favourite bikes. But there were three things about it that drove me crazy:
  • Chain drive (Why?, I ask... Why? In the 21st century, this clunky, messy, fragile deal has no place on a road-going motorcycle.)
  • 21-inch front wheel (Why?, I ask... Why? It's great for rolling over obstacles off-road, but I rarely if ever took my GS off-road. I leave that job to bikes 450cc and under, weighing 135 Kg or less.)
  • Tubed tyres (Why?, I ask... Why? If I have a flat, why do I have to sit levering a tyre off a rim by the side of the road?)

The final, and most important factor, is that outside of work work, I'm bored stiff.
I need a new project to keep me busy, and a conversion like this has piqued my curiosity for quite some time. Some time ago, I asked myself if the F800GS's shortcomings could realistically be fixed.
My answer so far is Yes - if I'm willing to spend some time.
I'm not too fussed about selling it again - if I decide I don't like it, I can get more for it in parts than what I spent on it. :)
 
I agree with most of your comments especially tubeless tyers but I think you are taking on a lot for little benefit. If you think about chains and how versatile they are as in being able to change gearing and cost plus reliability. Don't get me wrong I've had a ST and I loved it just found it too small I now have a F800GSA which I love. To overcome some of the drawbacks of a chain I use a scottoiler and a good chain (DID). By all means carry on with your project and keep us posted. JJH
 
Wouldn't it be easier to engineer a fully enclosed chain?

Why do you need to extend the front forks two inches. Granted the 17" wheel with bigger tyre is a smaller diameter but you don't need all the ground clearance of the GS on a dedicated road bike.
 
Wouldn't it be easier to engineer a fully enclosed chain?

It probably would be - but all I'd be doing is delaying the inevitable.
Given that I ride my bikes every day of the year in all weathers, and my weekday commute is 86 Km one-way, I'd still be looking at chain lubing and adjustment at least every week, and chain life of 20 000 Km, tops.
The S / ST / GT belt has a replacement interval of 40 000 Km with near-zero maintenance. So by the time that goes, I've gone through two sets of chains and sprockets, plus uncounted cans of 'clean' lube and man-hours spent grovelling on the workshop floor.
I really have no inclination for that.

Why do you need to extend the front forks two inches. Granted the 17" wheel with bigger tyre is a smaller diameter but you don't need all the ground clearance of the GS on a dedicated road bike.

It's not about ground clearance - it's about keeping the steering-head angle correct so you don't end up with a bike that shakes its head too badly.
You can't do much about the reduction in trail, but keeping the bike in more-or-less its original level attitude is a good start. :)

It's not a conversion I can rationalize purely with my head, even though Dog knows I'm trying as hard as I can.
But the sums I've done so far are in agreement. ;)

There are other factors in the mix: I've also owned an '07 F800S and an early '08 F800ST (both were picked up for bargain prices). I loved them, too (the ST more than the S), but at the end of the day, their... er... 'compact' dimensions are sadly not suited to my 1.83m of inconspicuousness.

Put it this way: I may not end up owning the best motorcycle in the world. But everything I've seen so far tells me I have a very high likelihood of ending up with best motorcycle in the world for me.
 
As far as I can recall, the cylinders on the belt drive variants of the F800 lean forward at a greater angle than on the GS - apparently, there's 15 degrees difference.

Link

Could make the engine swap tricky...
 
It's not about ground clearance - it's about keeping the steering-head angle correct so you don't end up with a bike that shakes its head too badly.
You can't do much about the reduction in trail, but keeping the bike in more-or-less its original level attitude is a good start. :)

Have you considered then, that by lengthening the front end you will be introducing more flex?
 
A Triumph Tiger (road version) would answer 2 out of your 3 bugbears...and with Rox risers and lower footpegs I fit very comfortably at 6'4". Unfortunately it still has a chain but the triple engine is an absolute joy over the BMW 800...only in my opinion of course. :D
 
Given that I ride my bikes every day of the year in all weathers, and my weekday commute is 86 Km one-way, I'd still be looking at chain lubing and adjustment at least every week, and chain life of 20 000 Km, tops.
The S / ST / GT belt has a replacement interval of 40 000 Km with near-zero maintenance. So by the time that goes, I've gone through two sets of chains and sprockets, plus uncounted cans of 'clean' lube and man-hours spent grovelling on the workshop floor.
I really have no inclination for that.

The chain on my F650GS twin is approaching 60,000km, and is still in very good condition. It's lubricated with a Pro-oiler, which gets filled up now and then. Probably worn out 4 sets of nozzles. Chain gets adjusted when the rear tyre gets changed, or if the workshop does it during a service. Otherwise it's never touched. Sprockets show very little wear, and I'm expecting at least another 10 or 20,000km from them. Of course YMMV.
 
Nick, good luck with your project, it sounds that you have given it a lot of thought, most problems can be solved with the right knowledge base. I just wish I had the time, space and spare cash to even think about having a mechanical project to play with. :thumb
 
You could always lower the back. JJH

It's entirely possible, but by doing that, you introduce new problems by putting the rear suspension into a completely new part of it's arc of movement, and by changing the amount of leverage exerted by the rear shock on the swingarm.

As far as I can recall, the cylinders on the belt drive variants of the F800 lean forward at a greater angle than on the GS - apparently, there's 15 degrees difference.

You're entirely right.
Luckily, the four top engine mountings on the parallel-twin engine are identical between belt-drive and chain-drive variants. New rear engine mountings will be fabricated to mate a belt-drive engine to a GS frame.
The only difference is the position of the engine. The engine's top castings are identical in all iterations.
The other niggle is that the belt-drive engine will need a GS left-hand (clutch) engine cover in order to ensure accurate engine oil-level readings.
Not a problem, since I'll have a GS cover handy. :thumb2

Have you considered then, that by lengthening the front end you will be introducing more flex?
Yep. I'll be having thicker lower extensions added to the existing stanchions, or have all-new front stanchions made up in slightly thicker steel.

A Triumph Tiger (road version) would answer 2 out of your 3 bugbears...and with Rox risers and lower footpegs I fit very comfortably at 6'4". Unfortunately it still has a chain but the triple engine is an absolute joy over the BMW 800...only in my opinion of course. :D

I'm in full agreement, and Triumph triples hold a special place in my heart, too. :bounce1
But second-hand parts for Triumphs are not available in SA in anywhere near the quantity that second-hand BMW parts are, and high-mileage 800XCs for low prices are pure unobtainium.

Nick, good luck with your project, it sounds that you have given it a lot of thought, most problems can be solved with the right knowledge base. I just wish I had the time, space and spare cash to even think about having a mechanical project to play with. :thumb

Thank you kindly! :thumb2
There's always time. Anything can be done using resourcefulness and determination. Most times, reading these forums, I wish I lived in the UK, where second-hand bike parts are very nearly cheap...
I'm a big fan of the NASA ethic that says: There is no such thing as an insoluble engineering problem. There are only time-consuming ones.
 
Thought about using the wheels of a 650/700 variant? Just a thought

Hi Earwig! :)

Those wheels never really entered the equation. Compared to the wheels from belt-driven bikes, they're relatively skinny, and the front is a 19".
Smaller front wheels don't just make the bike quicker-steering by reducing the trail. Assuming that their overall mass is the same as, say, a 19" or 21" front, they also have less rotational inertia ('flywheel effect') for the same overall mass. This was Honda's rationale for equipping stuff like the early Fireblades and RC45s with 16" front wheels.

What I'm after with this experiment is to combine the best of all worlds from all twin-cylinder F-series bikes. And another benefit of using the belt-drive setup is that it comes pre-equipped with a 5.5x17 alloy rear wheel which can accept 180mm-wide tyres. That clears the way for me to use stuff like Dunlop D208s and Pirelli Dragon Corsa Pros, if I choose.

If the wheels were purely what I was after, I'd already be aiming to bring in a set of F800R wheels.
(Sadly, we never got the F800R here, as BMW SA never saw fit to import it. 'Streetfighter'-style bikes have a dedicated following over here, but that following is quite small.)
 
Wow!
A more limited version of this has been playing on my mind - R wheels on a GS - sort of a Nuda but not quite.
Terrific ideas and something to keep you busy over the Summer Months - good luck and keep us posted with progress.
 
Have no technical wisdom to offer, but his project sounds very interesting and I look forward to seeing the progress! :beerjug:
 
Have you thought of doing it the other way turn the f800s into a F800GS as all the belt setup is in place its just the front end and bodywork to change over and make it fit this might be another option
 
Wow!
A more limited version of this has been playing on my mind - R wheels on a GS - sort of a Nuda but not quite.
Terrific ideas and something to keep you busy over the Summer Months - good luck and keep us posted with progress.

I'd say go for it. :thumb If wider 17" wheels are all that's needed, it's probably the most sensible course of action - certainly doesn't require custom-building new wheels from the existing hubs!
If chain-drive F-series bikes follow the same paradigm as shaft-driven R-series, then it's very likely the F800R wheels will be a simple 'bolt-on'.

The only issue I can think of is the front brake discs. F800GS discs are 300mm, F800R discs are 320mm. If you want to use the R discs, you'll have to space out the front calipers. If you don't want to use adaptor plates, the best option might be to use R1200GS front discs on the F800R front wheel.
Have no technical wisdom to offer, but his project sounds very interesting and I look forward to seeing the progress! :beerjug:

Thanks! :D
I haven't started yet, but I'll be sure to post progress updates as and when they happen.

Have you thought of doing it the other way turn the f800s into a F800GS as all the belt setup is in place its just the front end and bodywork to change over and make it fit this might be another option

It is an interesting option, and I did briefly consider it. :)
The problem with it is, I didn't want to fundamentally change the overall look and riding position of the F800GS. It'll actually take less work to modify the GS frame to accept the S/ST/GT engine, than to modify an S/ST/GT frame to accept the GS bodywork and front suspension.
 
Could you not bolt the 300mm discs to the new wheel and save yourself the bother of spacing the calipers?
 


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