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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 37481 times)

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #190 on: Nov 09, 2018, 03:26:53 »
Question im going to be re gearing my exciter. Have you messed with different set ups?

I have done in the past, I think I ended up with 17-40.  But I will look more into it when the time comes.  I know there are online calculators that help you work out rpm at certain speeds with different gearing ratios.  I will be looking for max power output at around 170kph so I have the tip of the torque curve around the 100mph mark so this gearing will be very different to stock bikes.  I will also be hoping to have power to 10,000rpm as well.

While on this topic, I have studied the parts diagrams for both the SR250 and the XS250 gearbox's - the latter because it is a 6 speed!  From the diagrams I could tell they weren't compatible but I found a good deal on a whole set up on ebay so I bought it anyway, in the hopes of learning something or maybe even swapping some ratios.  I learned something for sure, but this idea is a dead end - just look at the differences in the photo - SR250 gearbox at top (most of it), XS250 gearbox at bottom (all of it).  Very different beasts!


Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #192 on: Nov 09, 2018, 17:52:49 »
As a turbo-bike-builder... which s/c are you thinking about? AMR300 or have you found something even smaller?
Real freedom starts with the freedom of thought.

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Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #193 on: Nov 10, 2018, 10:53:36 »
As a turbo-bike-builder... which s/c are you thinking about? AMR300 or have you found something even smaller?

Nope, you got it!  I managed to source a refurbished AMR300 from alibaba.  It seems in pretty good shape.  I will draw up custom billet intake and outlet ports for it to bolt up to the SR carb and then the engine on the other side (with a plenum-ish thing).

Offline der_nanno

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #194 on: Nov 10, 2018, 15:05:45 »
If you go draw through, you want as little plenum between engine and supercharger as possible... AMR300s are nice. In your case run it at roughly 1:1 and you should see about 3psi + a bit for the inefficiencies of the engine, so just about enough to make you lust for more and then go completely bananas on the boost level.  8)
Real freedom starts with the freedom of thought.

My Blog:
http://greasygreg.blogspot.co.at

My Mule TR1.1 build here on DTT:
http://www.dotheton.com/forum/index.php?topic=75458.0

My Turbo TR1.1 (XV1000) build here on DTT:
http://www.dotheton.com/forum/index.php?topic=72571

My XS Triple Sidecar build here on DTT:
http://www.dotheton.com/forum/index.php?topic=74211

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #195 on: Nov 14, 2018, 16:00:22 »
After being at the motorcycle workshop FOREVER (seems like vintage bikes here get very low prio) the tyres have finally been mounted on the wheels - and they look great.  These Avons can be mounted front or back and have a mounting/direction arrow to indicate which direction for which wheel (front or back).  Somehow, despite me writing in large text in permanent marker on the rim, the workshop managed to fudge this as well.  They were embarrassed and fixed it onsite for me while I waited.  So they should.

Anyway, next stop, rolling chassis! 

Offline Tim

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #196 on: Nov 14, 2018, 20:11:33 »
Awesome tires - nothing like a set of fresh wheels all ready and waiting.
"Quality . . . you know what it is, yet you don't know what it is. But that's self-contradictory. But some things are better than others, that is, they have more quality. But when you try to say what the quality is, apart from the things that have it, it all goes poof! There's nothing to talk about. But if you can't say what Quality is, how do you know what it is, or how do you know that it even exists? If no one knows what it is, then for all practical purposes it doesn't exist at all. But for all practical purposes it really does exist."

Offline hooligan998

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #197 on: Nov 15, 2018, 00:47:12 »
Ssssseeeexxxaaayyyyy wheels!

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #198 on: Nov 18, 2018, 09:03:51 »
Before I put the wheels back on I really want to sort out the suspension.  The rear will be relatively straight forward after the front is sorted.

For the front I have been investigating what can be done with the stock springs and cutting them.  I managed to work out what the stock spring rate is with the dual windings, it is a really low 3.3N/mm, which according to some literature, is suitable for riders weighing less than 50kg!  Haha.  Now I don't know if I am exactly right with all this stuff but it makes pretty good sense to me - see that attached equation plus the weight/spring rate charts from various manufacturers.  Many replacement springs for classic bikes are however much higher ratings - up towards 7-10N/mm for riders ranging from 75-105kg.  I'll do some of my own experimenting.  The second rate on the spring is a lot more in the ball park at 4.5N/mm. 

I started out investigating cutting the spring in three locations, one to separate the dual rate, then another two cuts to shorten each spring based on a percentage increase in spring rate - see attached how I thought a spacer could be made to connect the two and keep their compression separate, plus a chart of some calculations I made.  This way, the dual rate could be retained.

Once I did the calculations and did a test cut of the spring, I realised this is not a good idea.  The tightly wound part of the spring becomes coil bound quite quickly (did a test compression) and becomes useless (this is the purpose/intended behavior), plus cutting in a few places is a pain.  I figured after reading a lot, especially racetechs literature, that straight rate springs with correct rate, preload and damping are much easier to control, adjust and ultimately will behave as intended = predictability and hopefully a well performing fork.

One good thing that came of the exercise though is the spacer I designed to be able to receive the cut end of the spring and provide a flat landing at the other side of it.  The spacer matches the angle of the spring in its relaxed state and collects the end of the cut spring perpendicular to its winding.  This gives the whole first coil and the end support.  I will use this moving forward.

Now I will start experimenting with cutting out the tight windings completely and work out some suitable spring rates by cutting out a certain amount of coils (and calculate for a suitable amount of initial preload as well).

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #199 on: Nov 18, 2018, 09:08:55 »
And a prototype of this spacer.  Works well.