The BMW R100 LPG Conversion

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MY QUEST: To have the cheapest useable transport possible.

THE PATH: Is that of taking a reliable and economical motorcycle, attaching a sidecar, fitting it with car tyres and then converting it to run on LPG.


This is a short description of my current project of converting a BMW R100 to Run on LPG, a blog if you like.

For almost a year now I've had a Dnepr MT11 running on LPG and after being offered a BMW R100RS for what can only be described as silly money I'm going to attach a sidecar to that and then convert it to LPG.

 

Converting the Dnepr to gas has taught me a lot.

Design points and considerations.

Tank: The tank has been specially chosen to be small but still give reasonable range. As there aren't nearly enough LPG service stations on the motorways this has to be as large as reasonably possible. The Saluki sidecar is a fiberglass monocoque construction and because of this has built in wheel arches. Being of a universal design i.e. for right / left hand roads has two wheel arches meaning that the ne closest to the bike will be redundant and is normally wasted space. However the the tank I am going to use is thin, 244 mm diameter, and because of this

will fit within this unused wheel arch space. A donut tank was considered because obviously its tyre shaped and designed to fit in a tyre shaped hole and the wheel arch is quite naturally tyre shaped. However I opted not to use a donut tank because apart from them being un proportionally heavy for the extra fuel it would carry very few of them are built to be mounted vertically and because of this they are not available on the surplus market. The other reason for not using a donut tank is that the wheel arch space is designed to have a portion of the wheel and hence tank protruding out of the bottom and even if it was tucked right up at the top of the wheel arch the risk of this protruding part coming into violent contact with the road or a rock is not worth it. The chassis for mounting the sidecar to the bike also uses some of this unused wheel arch space that a donut tank would occupy.

Safety: Now this is a big consideration and I have taken it seriously. I am taking care of all a whole host of problems by using parts especially designed for use in automotive LPG conversions. I am also taking special care to not have any gas part in a position that it may be protected in the event of an accident. The tank is also mounted at the center of the vehicle again to provide maximum protection in the event of an accident.

 

Vapouriser:

A Car vapouriser will be used for the conversion, I see no reason why a bike specific one should be sourced. There are however companies around the world that sell bike specific LPG equipment but these are almost all for small capicity engines and therefore can be much more compact.

The vapouriser needs to be heated due to the latent heat of the liquid propane turning into a gas, this is normally done using the hot water in the cooling system of the engine. However the BMW R100 is air cooled so an alternative system needs to be arranged.

I've read that the vapouriser needs to be kept at about 50°C and that heating it to engine temperature is a bad idea for longevity so some ingenuity will be needed. The vapouriser will also have to be kept warm (above -10°C or so) to ensure vapourisation.

The options as I see them are:

Use the hot engine oil: The oil system in ght bike is at several bar and it is unknown if the vapouriser will be capabable of surviving at these pressures, there are also rubber seals and plastic fittings on the vapouriser heating circuit and it is also unknon how long these plastic parts will survive in hot oil.

Heat water from the exhaust: Totally uncontrolable heat transfer and high posiability of boiling the water.

Heat water from the engine: More controlable however a seperate water pump will be needed, a thermal syphon could be use but without experimentation it is difficult to say if there will firstly be enough flow and hence heat transfer to work and secondly wether the temperature gradient when the engine was cold would be enough to start circulation. A small pump is therefore preferable.

Circulation Fluid: Water freezes at 0°C and boiling at 100°C and is within the expected temperature it will go through so its use is uacceptable. However Ethylene Glycol antifreeze (Not to be confused with Methonol based antifreese) has a melting point -12.9°C and a boiling point of 197°C  and although evaporating propane can chill below this it is expected that the working system will prevent this situtation from occuring. However if Ethylene Glycol freezes in winter conditions during starting and before the engine has warmed up then Propylene glycol will be considered due to its much lower melting point of  -59°C whilst still maintaining a relativly high boilling point of 188°C.

Mixer:

There are no specific kits available for the BMW, or for that mater any motorbike that I know about so a silimar mioxing stage to that of the Dnepr conversion will be constructed.

PROJECT SO FAR

So far the project is in the collecting bits stage, remember that this is a low budget project and I have to source quality components second hand or find new surplus that are being sold off cheap. So far I have.

A BMW R100RS

A Saluki Sidecar

A new 35l LPG tank

A new LPG multi valve

A vapouriser

Some pipe work.

And lots of tea.

 

PICTURES

In no particular order.

The Saluki Sidecar on the day I bought it.

The Saluki Sidecar on the day I bought it.

10mm steel tube brazed into place (Dnepr conversion)


Showing orientation of cutoff to encourage gas flow from the mixer inlet (Dnepr Conversion)


UPDATES

UPDATE  (Month Year)

None yet as its not finished

LINKS:

Here are some of the more interesting links to bikes running on LPG that I have found.


CAR TYRES

Car tyres have many benifits over bike tyres and if fitted to a combination will have the following mile enhancing benifits.

They are approx 4 times as wide as bike tyres and so will, hopefully, last 4 times as long

They cost about a quarter that of bike tyres.

A car tyre has much deaper tread when new than a bike, about twice.

A bike and sidecar is much lighter than a car so the contact pressure to the road is a lot lower causing the tyre wear slower

The lower and to also run cooler due to the lower contact poressure.

So the benefit of these two factors multiplies. The result is that a bike, with sidecar, will have a running costs that is about one sixteenth that of a bike.

 

(When this conversion takes place it will have its own webpage.)


CONTACT: aarrgghh @ aarrgghh dot co dot uk

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