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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 3162 times)

Offline JadusMotorcycleParts

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The title says it all.  This is going to be more of a speed and performance focused build than the last.  Less time and energy spent on finishes, more time spent on finessing and fettling.  I am hoping to implement and share a lot of ideas that both myself and other awesome members of this forum came up with through the process of the previous build.  The stock SR will get about 120kph, all Jadus bikes - both the test mules and the black standard get around 135kph.  All make around 20hp at the crank.  So looking to get 30hp with some trick mods to be able to get to 160kph or... the ton!

Something I noticed when I completed the black SR was that it was the nicest bike I have ever build, and I didn't/don't like it.  Not because I don't love it, but because I hate having nice things.  I hate the feeling of being 'precious' with something - not wanting to scratch the paint, not wanting it to get dirty etc.  That is why I will never own a nice car.  I want to use, thrash and trash my toys.  I don't want to spend time cleaning and polishing!  What a waste of time!  So this bike will be more along those lines and I can't wait to get stuck in.

I thought I would be ready to move onto the next model of bike to design parts for by now, but I feel I have one more good SR250 in me before that.  Who knows, maybe even more parts for the SR250 along the way!

The reason I want to document the build here is twofold.  One, I can share what I am doing with a greater audience, and two, it opens things up for a great discussion for ideas and allows me to get feedback from more experienced members of the forum who have 'been around the block' a few times  ;)

You can also follow the hashtag on insta:  #100mphSR250

Let it begin!  ;D

Offline JadusMotorcycleParts

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I have been holding off on starting this thread - waiting to make absolutely sure everything was tied up with the black bike.  But I started the thinking process long ago and have already got stuck in with a couple basics - engine out of the frame plus some angle iron supports in the frame.  This will allow me to run two projects in parallel - an engine project and a chassis improvement project.  The SR uses the engine as a stressed member, so messing with the frame (sitting on it, getting ride heights, foot peg positions etc) without extra supports is not a good idea - it would bend under load.

I have read a lot since I began building bikes as well.  All the books below I have read cover to cover and have many book marks.  Some of the mods I decided to do might sound strange to some, but I have pondered long and hard and have read and re-read about these mods to make sure.  I have also purchased a great chassis design pdf - more on that later.

Some of the number goals might be a stretch but I am absolutely confident I can get 160kph.  Will it be a first for an SR250?  Maybe not - if you know of anyone doing it, please point me in their direction  :D  100kg, possibly, 110kg, definitely.  30hp?  At the crank, yes, at the wheel, maybe 26!

Offline Ryan Stecken

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I'm in man!!!cant wait to see thst thing finished!

Gesendet von meinem SM-A310F mit Tapatalk


Offline JMPUK

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This will be epic!!

Can't wait to see it

Offline zap2504

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Like your other builds, I'm in! I also have had that Smith book for a very long time. Originally got it when interested in modding car engines.

One thing to suggest to your list of engine mods is thinking about pumping losses. There was an article by Dave Searle in Motorcycle Consumer News about this a couple months ago. Thumpers are notorious here because they don't have other cylinders working together to offset the crankcase pumping action. Many builders just vent their crankcases to look "cool" but it really doesn't do anything to decrease pumping loss (just keeps oily junk out of the carb like an oil catch can would); I got a Krank Vent off evilbay to experiment with creating a partial vacuum in the crankcase instead of trying to get all that air in/out of that skinny hose outlet (still planning). Krank Vents are a 1-way check valve and seem to be very popular with the H-D crowd as they reduce crankcase pressure and keep gaskets from blowing. My thoughts are that the engine will pump through the Krank Vent a couple times until the partial vacuum is created, then no more pumping.
« Last Edit: Aug 22, 2017, 12:48:17 by zap2504 »

Offline firebane

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This will be interesting. I had a SR250 that topped out about 110kph but vibrated like hell lol

Offline Hurco550

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I enjoyed watching you last one, so of course im going to watch this one too =)
Keep er' Between The Ditches!!!

1976 BMW R90/6 farkilitious: http://www.dotheton.com/forum/index.php?topic=74349.msg882477#msg882477
1975 RD350 Road Racer: http://www.dotheton.com/forum/index.php?topic=70652.msg833688#msg833688
1973 Ironhead Hardtail: http://www.dotheton.com/forum/index.php?topic=68135.msg788081#msg788081
1976 GT250 Rebuild: http://www.dotheton.com/forum/index.php?topic=64973.new#new
Full Custom Pit Bike: http://www.dotheton.com/forum/index.php?topic=66954.new#new
1971 Yamaha 90cc twin HS1: http://www.dotheton.com/forum/index.php?topic=70498.0
"Two strokes are the motorcycle equivalent to Thors chariot. Full of glory and memories of battles victorious" ~ von yinzer

Offline Sav0r

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I shed around 38kg from my RD350. It has a battery, a stock charging system, a heavily filled stock tank, all turn signals and lights. 30kg should be in reach without too extreme an effort for the SR. The wheels, forks brakes, fenders, seat, and a bunch of the other junk are pretty easily eliminated or replaced with modern bits. 
Sav0r the adventure.

Visit www.chrislivengood.net to get more information on my RD350 dubbed Mia Wallace as well as my other projects and snafus.

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Offline JadusMotorcycleParts

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Thanks for the support!

Like your other builds, I'm in! I also have had that Smith book for a very long time. Originally got it when interested in modding car engines.

One thing to suggest to your list of engine mods is thinking about pumping losses. There was an article by Dave Searle in Motorcycle Consumer News about this a couple months ago. Thumpers are notorious here because they don't have other cylinders working together to offset the crankcase pumping action. Many builders just vent their crankcases to look "cool" but it really doesn't do anything to decrease pumping loss (just keeps oily junk out of the carb like an oil catch can would); I got a Krank Vent off evilbay to experiment with creating a partial vacuum in the crankcase instead of trying to get all that air in/out of that skinny hose outlet (still planning). Krank Vents are a 1-way check valve and seem to be very popular with the H-D crowd as they reduce crankcase pressure and keep gaskets from blowing. My thoughts are that the engine will pump through the Krank Vent a couple times until the partial vacuum is created, then no more pumping.

Yepp, that is for sure on my list.  Someone (maybe even you?) posted some good stuff on the other build thread from the LS650 Savage forum about a system.  I'll see If I can dig that up.  It is also covered in these books :)

Offline JadusMotorcycleParts

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I shed around 38kg from my RD350. It has a battery, a stock charging system, a heavily filled stock tank, all turn signals and lights. 30kg should be in reach without too extreme an effort for the SR. The wheels, forks brakes, fenders, seat, and a bunch of the other junk are pretty easily eliminated or replaced with modern bits.

True!  Thanks for the encouragement.  I'll start with the big obvious things then see what level I need to go into to shed the last few kgs.