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Michael Moore · 09-08-2012, 12:47 AM

You'll have difficulty getting the seat height below 18" with a chain final drive. Remember that the chain rises as the rear suspension compresses, and you'll want some extra clearance above the top run for a slack chain to rise without sawing the rider's bottom apart. You could do two chains connecting to a jackshaft betwen the engine and rear wheel and lower things some more with a dropped engine but you have more parasitical losses. I did some Rhino3D drawings for a trackday FF based on a YZF250 thumpette with 58" wheelbase, Hossack FFE and the compound chain:

More details on that project (which is resting like a Monty Python parrot) at

Euro Spares - The YZ250F-FF Project

Here's a photo of a Gurney with a 426/450 Yamaha single engine in it that shows how they have a short intermediate chain running up from the dropped engine's countershaft sprocket:

Here's a photo of Tony Foale riding that bike:

56-58" is likely a minimum wheelbase if you try and keep the feet at or behind the front axle.

Airboxes make a big difference in how the bikes run -- think of all the bikes that had the airbox junked and "pod" filters put on, which never ran as good as stock. But if you've got a side-draft intake the airbox ends up behind the carbs pushing the rider back. The later BMW/Rotax 650 singles have a downdraft intake with the airbox (more) on top of the engine.

Singles and narrow parallel twins are about as far as you want to go with somewhat vertical cylinders. A Kawasaki 650 Ninja doesn't appear to be a "narrow" parallel twin. The problem is that there are very few engines out there that are FFriendly if you are doing a low-seat bike.

My current FF project is an SV325, a Suzuki SV650 less the rear cylinder head and with a faux "balance" piston substituted for the stock one to retain the 90 degree twin balance but eliminating the pumping losses/ring drag of a stock piston. Ditching the head reduces the height of the rear assembly by 6", eliminates running a hot exhaust header past the rider's crotch and lets the rider move forward.

I'd originally thought of using a TL1000 engine but it was easier to buy a running 650 than to get the rest of the parts for some partial TL engines that I bought. The TL has more power, but it is also significantly heavier/larger than the 650. HCS is also in the plan. Here's an early Alibre 3D model/mockup. This PDF is "active" -- if you click on the image and you have a late enough version of Adobe Reader you'll be able to zoom/pan/rotate:

http://www.eurospares.com/graphics/F...OnSV325big.pdf

Here's a CAD (cardboard-aided design) mockup of the SV325 engine/frame from a few days ago. The frame spar matches the top of the rear cylinder with a cap plate:

Here's an active PDF of the YZF250 model:

http://www.eurospares.com/graphics/FF/whole%20bike.pdf

You might find something of interest looking though this FF folder on my website:

Index of /graphics/FF

I will assure you that filling all of that underseat area formerly filled with airboxes, batteries, electrics, water catch tanks etc with the rider and eliminating the conventional fuel tank and looking for a new place to put all of the relocated stuff gets to be a real chore!

FWIW, I'm interested in FFs from a performance aspect, not fuel economy, and so my engine choices are different from what an ecomodder might look at. I want something with enough power and modern 17" sport tires so I can see how it stacks up to a conventional bike. With the SV325 I'm shooting for roughly 1/2 the power of a stock 650, which will put me at a 2-5 more than a stock EX250 makes. If I can end up with something that performs as well (or even better) as a stock EX250 I'll consider it a success.

I had twin-damper EX250 racer project

and the airbox/battery are in the way for a low seat bike (the photo shows CR carbs with open stacks, no airbox).

I don't know how the reliability/economy is of those 180cc Chinese Honda 90 clone engines is, but I'd look at an engine like that for an eco project because the form factor works a lot better for a low seat FF. Perhaps there's a 200cc scooter with a flat-single engine that could be used as a donor. I'm not up on what is available in scooters, but buying a complete and titled/registered bike as a donor has a lot going for it over piecing things together from parts.

I'd recommend avoiding "tiller" bars as you end up with bar motion a lot different from what you'd be used to. Instead, do a remote steering head with a drag link to the front end.

Many of the single-damper rear suspensions are going to intrude into the seat area, and reworking one of those designs to avoid that is not a trivial task. As mentioned above, convential twin rear dampers as used on the Gurney bikes avoids that, works fine (with good dampers) and frees up space under the seat (if you rework the swing arm) for a fuel tank or something else to go there.

The big scooters have a lot going for them as the engines are more in the right shape, but they tend to be low powered, heavy, may have problems getting high performance tires, and the scooter drivetrains are funky/clunky (IMO). But a 400 Burgman FF with a Hossack FFE and lower seat would end up much lower/smaller frontal area and might be easier for a (relatively) quick and easy project (for certain values of quick and easy).

I've never understood why the Gurney bikes didn't ditch the teleforks and steering head. AAR certainly has the ability to build something else. If nothing else I think they could benefit from a remote steering head to bring the bars back towards the rider. The one I sat on at the Chandler museum seemed like a long stretch to the bars that would have gotten old very quickly.

It is possible to do a Q&D "chop" of a bike, but I think you'll find that trying to do a nice/integrated design is going to take a LOT longer than you might think. There are a lot of factors to juggle before you reach a condition of Pareto optimality and then you've got to have the infrastructure/skills and/or money to actually get the thing built.

ETA: in case anyone is interested here's a good side view photo of the SV650 engine with a tape measure in it for scaling:

http://www.eurospares.com/graphics/S...Vmounts054.jpg

cheers,
Michael

09-08-2012, 12:47 AM	#12 (permalink)
Michael Moore EcoModding Apprentice Join Date: Dec 2011 Location: San Francisco, CA USA Posts: 142 Thanks: 6 Thanked 53 Times in 31 Posts	You'll have difficulty getting the seat height below 18" with a chain final drive. Remember that the chain rises as the rear suspension compresses, and you'll want some extra clearance above the top run for a slack chain to rise without sawing the rider's bottom apart. You could do two chains connecting to a jackshaft betwen the engine and rear wheel and lower things some more with a dropped engine but you have more parasitical losses. I did some Rhino3D drawings for a trackday FF based on a YZF250 thumpette with 58" wheelbase, Hossack FFE and the compound chain: More details on that project (which is resting like a Monty Python parrot) at Euro Spares - The YZ250F-FF Project Here's a photo of a Gurney with a 426/450 Yamaha single engine in it that shows how they have a short intermediate chain running up from the dropped engine's countershaft sprocket: Here's a photo of Tony Foale riding that bike: 56-58" is likely a minimum wheelbase if you try and keep the feet at or behind the front axle. Airboxes make a big difference in how the bikes run -- think of all the bikes that had the airbox junked and "pod" filters put on, which never ran as good as stock. But if you've got a side-draft intake the airbox ends up behind the carbs pushing the rider back. The later BMW/Rotax 650 singles have a downdraft intake with the airbox (more) on top of the engine. Singles and narrow parallel twins are about as far as you want to go with somewhat vertical cylinders. A Kawasaki 650 Ninja doesn't appear to be a "narrow" parallel twin. The problem is that there are very few engines out there that are FFriendly if you are doing a low-seat bike. My current FF project is an SV325, a Suzuki SV650 less the rear cylinder head and with a faux "balance" piston substituted for the stock one to retain the 90 degree twin balance but eliminating the pumping losses/ring drag of a stock piston. Ditching the head reduces the height of the rear assembly by 6", eliminates running a hot exhaust header past the rider's crotch and lets the rider move forward. I'd originally thought of using a TL1000 engine but it was easier to buy a running 650 than to get the rest of the parts for some partial TL engines that I bought. The TL has more power, but it is also significantly heavier/larger than the 650. HCS is also in the plan. Here's an early Alibre 3D model/mockup. This PDF is "active" -- if you click on the image and you have a late enough version of Adobe Reader you'll be able to zoom/pan/rotate: http://www.eurospares.com/graphics/F...OnSV325big.pdf Here's a CAD (cardboard-aided design) mockup of the SV325 engine/frame from a few days ago. The frame spar matches the top of the rear cylinder with a cap plate: Here's an active PDF of the YZF250 model: http://www.eurospares.com/graphics/FF/whole%20bike.pdf You might find something of interest looking though this FF folder on my website: Index of /graphics/FF I will assure you that filling all of that underseat area formerly filled with airboxes, batteries, electrics, water catch tanks etc with the rider and eliminating the conventional fuel tank and looking for a new place to put all of the relocated stuff gets to be a real chore! FWIW, I'm interested in FFs from a performance aspect, not fuel economy, and so my engine choices are different from what an ecomodder might look at. I want something with enough power and modern 17" sport tires so I can see how it stacks up to a conventional bike. With the SV325 I'm shooting for roughly 1/2 the power of a stock 650, which will put me at a 2-5 more than a stock EX250 makes. If I can end up with something that performs as well (or even better) as a stock EX250 I'll consider it a success. I had twin-damper EX250 racer project and the airbox/battery are in the way for a low seat bike (the photo shows CR carbs with open stacks, no airbox). I don't know how the reliability/economy is of those 180cc Chinese Honda 90 clone engines is, but I'd look at an engine like that for an eco project because the form factor works a lot better for a low seat FF. Perhaps there's a 200cc scooter with a flat-single engine that could be used as a donor. I'm not up on what is available in scooters, but buying a complete and titled/registered bike as a donor has a lot going for it over piecing things together from parts. I'd recommend avoiding "tiller" bars as you end up with bar motion a lot different from what you'd be used to. Instead, do a remote steering head with a drag link to the front end. Many of the single-damper rear suspensions are going to intrude into the seat area, and reworking one of those designs to avoid that is not a trivial task. As mentioned above, convential twin rear dampers as used on the Gurney bikes avoids that, works fine (with good dampers) and frees up space under the seat (if you rework the swing arm) for a fuel tank or something else to go there. The big scooters have a lot going for them as the engines are more in the right shape, but they tend to be low powered, heavy, may have problems getting high performance tires, and the scooter drivetrains are funky/clunky (IMO). But a 400 Burgman FF with a Hossack FFE and lower seat would end up much lower/smaller frontal area and might be easier for a (relatively) quick and easy project (for certain values of quick and easy). I've never understood why the Gurney bikes didn't ditch the teleforks and steering head. AAR certainly has the ability to build something else. If nothing else I think they could benefit from a remote steering head to bring the bars back towards the rider. The one I sat on at the Chandler museum seemed like a long stretch to the bars that would have gotten old very quickly. It is possible to do a Q&D "chop" of a bike, but I think you'll find that trying to do a nice/integrated design is going to take a LOT longer than you might think. There are a lot of factors to juggle before you reach a condition of Pareto optimality and then you've got to have the infrastructure/skills and/or money to actually get the thing built. ETA: in case anyone is interested here's a good side view photo of the SV650 engine with a tape measure in it for scaling: http://www.eurospares.com/graphics/S...Vmounts054.jpg cheers, Michael __________________ http://www.eurospares.com Last edited by Michael Moore; 09-08-2012 at 01:08 AM..