Thread: Simson S51
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Old 04-18-2021, 08:07 AM   #1 (permalink)
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Join Date: Apr 2021
Location: Germany
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Aerospyder - '00 Toyota MR2 Spyder
Team Toyota
90 day: 41.98 mpg (US)

Simme - '83 IFA Simson Suhl S51
90 day: 76.59 mpg (US)
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Simson S51

As I've not only got a car, but also a light motorcycle, I'll give it a thread as well.
Many of you may never have heared of that particular model as it was never sold in the US for political reasons.
It was made in the GDR and is still rather popular here in germany as it has a few nice advantages over its modern counterparts:
1. it is legaly a 50ccm "scooter", but is allowed to go 60 km/h (38 mph) and is a foot shifted motorcycle
2. It only goes 60 km/h uphill with a passenger, usualy they go at least 70 km/h (44 mph)
3. The aftermarket and spare parts availability and prices are insane.
Every part is still in production, often by several different manufacturers.
4. Despite beeing an aircooled 2-stroke, it's very fuel efficient.
5. No TÜV

I'd like to improve on point 4 as I'll have to overhaul the engine soon anyway.
As of now it has taller gearing (17:31 instead of 15:34) than stock, electronic ignition and a lightweight rider, leading to higher topspeed as well as lower fuel consumption.

Possible modifications I'm considering:
-raising the compression ratio
The M541 engine was designed to run on VK78, wich is 78 RON.
As the lowest octane fuel I can find is 95 RON, I can increase compression with no issues and likely still have room to adjust the timing.
I will need to tune it regardless, so I'll do my best to make it run efficient without getting lean.
I am considering to swap the 4-speed for a 5-speed.
I'm allready at the limit with the sprockets, I've fit the largest posible front sprocket and a smaller rear one from a different model, wich I had to modify.
Plus I'd prefer a little more spread, 1st gear is now about as tall as 2nd gear used to be...
It is currently running tube type bias ply tires, not exactly known for their low rolling resistance.
When they are worn out I will likely mount radials.
I'd realy like to optimise that aspect.
However it will likely get complicated to do so and require complicated fiberglass fabrication.

My end goal is to break the magical 1L/100 km (235 mpg) sound barrier as well as the 100 km/h (62 mph) barrier.
While this may sound overly ambitious, keep in mind the manual mentions a fuel economy of 1,8L/100 km at about 40 km/h and my topspeed allready exceeds 80 km/h.

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