Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp

JadusMotorcycleParts

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I got these back from the machinist this week too. They look great and I sure hope they fit! To avoid needing the stock end seals (which go for 35Euro each!) I increased their flange thickness a little (so they mate up to the inside of the frame) and added an oring groove on the inside. This is to keep grease in and dirt and water out. This works really well with other products I work on (water treatment industry) so hoping it works here too.
 

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lunacy

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I’m so humbled by your comments about my old bike. I was surprised at how much interest it received at the time, and there seems to be a lot more SR250’s being modified now.

You’re right I never got to test the rear brake but I’m told it worked okay. I am very much a “cardboard aided design” (TM) engineer and hand made all the customised parts on that bike with hand tools, electric drill, grinder, bench vice .. MIG welder, heat and a hammer. When it came to the rearsets and linkages I consulted my friend (an actual engineer) to check I had the basic ideas right, and looked at lots of photos of old racing bikes. If I’m honest I was quite pleased when it all worked :)
 

JadusMotorcycleParts

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lunacy said:
I’m so humbled by your comments about my old bike. I was surprised at how much interest it received at the time, and there seems to be a lot more SR250’s being modified now.
True, I think around that time was the beginning of a boom of interest in the SR250. It was somewhat of a natural progression from the SR400/500 though as they became harder to find and more expensive. Either way, I think your build was highly influential to many DIYers :D
 

JadusMotorcycleParts

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An update will be coming soon, but I have been working on a couple of other projects recently. One is my daily rider - see images below. It is getting closer and closer to something I really like and it is super fun to ride. Pretty tricked out now too with all these prototype parts on it! The fork adjusters are awesome!

The other project I have kept pretty quite about but to drop a hint, it involves a turbo. Not a turbocharged vehicle, but the turbo itself (so others can turbo charge motorcycles easier).
 

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JadusMotorcycleParts

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More work on the weight reduction program. This is turned into more of an obsession rather than something that is going to effect the performance of the bike. These kind of weight reductions will be inconsequential. However my thoughts have been that if I can reduce part weight by 100g on 10 parts, thats a kg! Haha.

I really like how the top triple clamp came out. There is probably about 5hours of work in that all up. I used a hacksaw, auto file, dremel with sanding wheels and then sandpaper. I removed a few big clumps of aluminium that were no longer needed and removed all casting marks and split lines. I did not remove any material that I felt contributed to its structural integrity. I will bead blast it to - to try and get a nice even finish.
 

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teazer

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That all adds up. Does that bike have an aluminum steering stem or steel? There's almost a kilo right there.

Then add hollow axles, aluminum wheel and bearing spacers, aluminum and Titanium hardware and your wallet will be lighter than the bike. It's addictive.

I wouldn't put Ti axles on a street bike, but Ti nuts and washers, for sure. And alloy sprocket nuts and shock nuts. Keep adding lightness until wallet is empty. Replenish wallet and repeat.
 

crazypj

Split personality, I fake being smart
shocks I made some alloy lock-nuts. 4 alloy nuts were lighter than one original steel nut. Also made bearing spacers and wheel spacers from 6061 to shed a few more pounds. I bought some 7075 to make new steering stem as it's fair bit stronger than 6061 but lathe is too small to be able to machine it. It willget used for something though (eventually ;D )
 

zap2504

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Interested in performance/operation comparison of front disk on "daily rider" vs front drum brake. I assume its from a Virago (or maybe just a newer SR250 that did not come to the USA)?
 

JadusMotorcycleParts

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crazypj said:
shocks I made some alloy lock-nuts. 4 alloy nuts were lighter than one original steel nut. Also made bearing spacers and wheel spacers from 6061 to shed a few more pounds. I bought some 7075 to make new steering stem as it's fair bit stronger than 6061 but lathe is too small to be able to machine it. It will get used for something though (eventually ;D )
Great tips! Imma open up these wheels and make aluminium spacers :)
 

JadusMotorcycleParts

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zap2504 said:
Interested in performance/operation comparison of front disk on "daily rider" vs front drum brake. I assume its from a Virago (or maybe just a newer SR250 that did not come to the USA)?
The front end is standard on the later model SR's - which were only manufactured in the Spanish Yamaha factory. This fork and disc brake is on models from '95 onward.

I find the disc marginally better than a well dialed in drum. That is because I hardly ride hard enough on twisty enough roads to experience fade, which is where the disc would out perform the drum. Bite is of coarse a little better with the disc as well. The lackluster performance is also because it is a single pot set up. This could probably be improved a lot with better pads, a better disc or upgrade to a twin pot set up.
 

crazypj

Split personality, I fake being smart
Drilling axles is hard work because they are tough and long. The originals are massively overbuilt. Very long time ago( 1974 maybe?) I asked an engineer what kind of shear load they would take. Anyway, he did all the calculations and 5/8" axle in my Royal Enfield 'Continental GT' 250 was over 100 tons (Britain's first 100mph 250cc four-stroke) 100g is about 4 ounces?
 

JadusMotorcycleParts

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Exactly. The machine shop actually had to EDM them, which is very time consuming. But yepp, 100g is about 3,5 ounces. These are still well over dimensioned for the job!
 

sav0r

Member
That's awesome! When I center drilled my axles I started with a standard jobber bit and then moved onto a very small boring bar. It was tedious work, especially considering I had already machined the axles by hand, so screwing up was not something I was too keen on. I didn't have the tooling to get all the way through the axle, so the threaded end is still solid, which is fine because it's the small OD. EDM is amazing technology, I wish I had thought of it when I doing my axles. I considered buying some tube and then threading both ends, not sure how that would have worked exactly but it may have been easier than doing what I did in the long run. Then again, paying for some EDM would have been easier too.
 

doc_rot

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would it be more cost effective to find a axle from a sport bike and retro fit it to your application? I have done it a couple times with axles and pivots now. Works well and is cheap.
 

crazypj

Split personality, I fake being smart
Majority of sport bike axles are much larger diameter. I'm guessing the SR250 has 17mm front axle and 20mm rear? Could probably find a swing arm pivot bolt to fit but it would need cutting and threading. I think Suzuki used 17mm 'gun drilled' on early GSX-R? Not too familiar with other brands so couldn't make a recommendation
 

JadusMotorcycleParts

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I think if you were to machine these yourself, you would be best to start with chromoly tube first and then thread the ends - like Sav0r suggested. Drilling these would not really be possible with home equipment. A bit like the internal engine component I took to a machine shop to 'turn down'. They couldn't because of it's hardness and had to grind it down instead. Can you grind drill? I dunno but I am very familiar with EDM - I have visited many tool making shops and it is standard affair there. My day job involves designing a lot of injection molded and dies-cast parts - where they machine up an electrode male/plug and 'plunge' that into the metal of the tooling to make the cavities. Plus they can do EDM drilling in all sorts of materials down to 100th's of a millimeter, amazing stuff.

I think if you have the equipment and the skills (I don't have either), it wouldn't be so hard to adapt sports bike axles - there is plenty of meat in the forks to bore up to a bigger axle. Same with the swingarm. Then you would need new bearings and wheel spacers to suit but they must exist.
 

JadusMotorcycleParts

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I finally got the dent pulling tool in the post this week. The bracket that holds the support pads was not quite wide enough for the large dent I needed to pull, so I modified it a little to be able to spread them right to the edge.

Long story short, it worked surprisingly well! I would highly recommend trying something like this if you yourself have a dent in your tank of this nature. The first pull did the most work, then things were a little uneven - see red circles I marked out. So I did a few more pulls with different sized pads in different positions. It only got marginally better and I think this process is limited in terms of how even you can get sheet metal. I imagine with modern plastic panels in cars for example the panels would just 'pop' back out. Anyway, I think it is the best it'll get and the most important thing is... I am satisfied and I get to keep the cool paint job without destroying it's charm 8)

The hardest thing was getting the hot melt glue off afterwards without damaging the paint. I used a plastic 'spudger' from Ifixit (from my Mac repairing days). It was brilliant - a hard and stiff enough plastic that you can scrape with, but soft enough not to gouge the paint.
 

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