Ton up SR250: 100mph, 100kg, 30hp

JerryAssburger

Been Around the Block
For your high-speed run ONLY, I'd go with the narrowest tires that can go on those rims. Both for rolling resistance, wind resistance and weight. Tires are pretty heavy so that's an opportunity to trim some KGs, right?
 

stroker crazy

crazy as a fox
"For your high-speed run ONLY, I'd go with the narrowest tires that can go on those rims. Both for rolling resistance, wind resistance and weight. Tires are pretty heavy so that's an opportunity to trim some KGs, right?"

... and you could inflate them with helium!
 

sav0r

Coast to Coast
10kg is a long way to go, that's for sure. Composite fenders would help. The kick starter, kick stand, sub frame, and tank could all be replaced with light weight units. That might get it close. Tubeless would help, maybe. A composite headlight bucket would help, as would composite ears. Maybe a plastic lens if it is glass. Those chrome pipes can't be light.

Also, bathroom scales rely on human feet being there, they usually have two load cells, one for each foot. So when you load the scale in the middle they don't always work well. We went through this in our karting days before we were willing to pony up for proper scales. You can use a piece of wood that bridges two runners to simulate feet and that helps keep the scales more reliable.

My RD350 is like 120kg without fuel. I will lose a few pounds when I go to the new ignition. So maybe 119kg. Without building a bespoke frame or going nuts on high end wheels and a new tank I don't see how it could get much lighter. Maybe 115kg in street trim, that's going pretty bonkers for a bike that gets ridden a 1000 miles a year.
 

JadusMotorcycleParts

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"For your high-speed run ONLY, I'd go with the narrowest tires that can go on those rims. Both for rolling resistance, wind resistance and weight. Tires are pretty heavy so that's an opportunity to trim some KGs, right?"

... and you could inflate them with helium!
Hehe maybe not. But I will for sure be 'over inflating' them for less rolling resistance :)

@JerryAssburger smaller tyres is actually a really good idea. I built a bike in Australia with a 2.5'' wide on the front and a 2.75'' on the rear. That thing did not have much rolling resistance at all, but it felt pretty sketchy!
 
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JadusMotorcycleParts

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10kg is a long way to go, that's for sure. Composite fenders would help. The kick starter, kick stand, sub frame, and tank could all be replaced with light weight units. That might get it close. Tubeless would help, maybe. A composite headlight bucket would help, as would composite ears. Maybe a plastic lens if it is glass. Those chrome pipes can't be light.

Also, bathroom scales rely on human feet being there, they usually have two load cells, one for each foot. So when you load the scale in the middle they don't always work well. We went through this in our karting days before we were willing to pony up for proper scales. You can use a piece of wood that bridges two runners to simulate feet and that helps keep the scales more reliable.

My RD350 is like 120kg without fuel. I will lose a few pounds when I go to the new ignition. So maybe 119kg. Without building a bespoke frame or going nuts on high end wheels and a new tank I don't see how it could get much lighter. Maybe 115kg in street trim, that's going pretty bonkers for a bike that gets ridden a 1000 miles a year.
Thanks for your response savor. Tubeless would help for sure help. And composite parts everywhere. Many stock items made from steel have been swapped out for aluminium and that has saved weight but carbon would be the next step. You are right, the silencer is not light so that could be swapped out for something else. I am still thinking of design considerations though too, so I am not sure how much a sport bike silencer or carbon parts would suit the bike.

One easy thing I could do, which seems to have been a fad for a while, is remove all the road going things - guards, blinkers, mirrors etc. That would save a couple of kgs.

I didn't know about the load cells in bathroom scales! I'll see if I can get to a proper weigh station at some stage to double check - like the ones at the tip must be pretty accurate - because you pay per kg of scrap dumped.

An RD350 at 120kg is pretty impressive. 2 strokes have a'slighty' :p simpler head than 4 strokes so I guess there is a fair bit of weight not needed there.
 

JadusMotorcycleParts

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How much does the seat weigh? And those grips look pretty hefty.
1.68kg, so not too bad. The grips are pretty thin and only weigh a couple hundred grams together. Or do you mean the switches?
 

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JadusMotorcycleParts

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Here comes a bit of a post bomb... I'll try get this up to date (missed a couple of posts from a couple of weeks ago) so I can post about the dyno testing that just happened on the weekend! Because that is a bit of a doozy.

Anyway, here are some shots of the bike being worked on in the sun - my happy place. Then the O2 sensor set up, that unfortunately didn't work this time for unknown and un-diagnosed (yet) reasons. I gave up this time because I knew we could do better work on a dyno.
 

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JadusMotorcycleParts

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I also splashed out and bought myself a dial indicator, finally. I set up the bike on a depo stand to be able to tweak the wheels a little more and got them within +/-0.5mm radially and axially.
 

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JadusMotorcycleParts

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And here are a couple of shots of the vacuum line delete to run the non vacuum petcock. Plus the labyrinth fuel line system I have. It has a quick connect adapter which I love - because that tank comes off quite often to work on the bike, and a 90 degree fuel filter, plus some tubing support springs. I could probably come up with a better looking solution but I like that it is practical and displays that.
 

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JadusMotorcycleParts

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Also, actually took some time to enjoy it a bit and went for about an hours ride. It handles nicely!
 

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John Murray

Been Around the Block
I'm really enjoying this thread - lightweight, skinny bikes have always appealed to me. I've been land speed racing at Lake Gairdner (Australia) for the last few years - class record holder - and suspect you'll have no trouble cracking the ton. I think you'll need more than a half mile runway though; you need room to slow down. Light weight isn't an advantage in speed work provided you have room to accelerate; more weight helps with traction on salt and also with stability in crosswinds. Many LSR racers add ballast for these reasons. Drag is the most critical factor and always trumps weight, though there isn't a lot you can do with a naked bike without spoiling its looks. But if you find yourself just a couple of mph short try a 'Busa style front fender that covers as much of the front wheel as possible. Wheels - especially spoked wheels - contribute a large proportion of the drag. Anyhow, it's a lovely bike and I love what you've done with it.
 
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JerryAssburger

Been Around the Block
And here are a couple of shots of the vacuum line delete to run the non vacuum petcock. Plus the labyrinth fuel line system I have. It has a quick connect adapter which I love - because that tank comes off quite often to work on the bike, and a 90 degree fuel filter, plus some tubing support springs. I could probably come up with a better looking solution but I like that it is practical and displays that.
Looks good! Dumb question.... can a person bypass the vacuum switch on an original petcock by using the "prime" position, or do I need to go shopping for manual petcocks?
 

JadusMotorcycleParts

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I'm really enjoying this thread - lightweight, skinny bikes have always appealed to me. I've been land speed racing at Lake Gairdner (Australia) for the last few years - class record holder - and suspect you'll have no trouble cracking the ton. I think you'll need more than a half mile runway though; you need room to slow down. Light weight isn't an advantage in speed work provided you have room to accelerate; more weight helps with traction on salt and also with stability in crosswinds. Many LSR racers add ballast for these reasons. Drag is the most critical factor and always trumps weight, though there isn't a lot you can do with a naked bike without spoiling its looks. But if you find yourself just a couple of mph short try a 'Busa style front fender that covers as much of the front wheel as possible. Wheels - especially spoked wheels - contribute a large proportion of the drag. Anyhow, it's a lovely bike and I love what you've done with it.
Thanks John :) Awesome you have been land speed racing. It was a dream of mine to get to an event at Lake Gairdner while I was living in Melbourne but I never got there! What class are you the record holder in?

All good advice, I think if I was so intent on having a road going bike I could do a lot more!
 

John Murray

Been Around the Block
Thanks John :) Awesome you have been land speed racing. It was a dream of mine to get to an event at Lake Gairdner while I was living in Melbourne but I never got there! What class are you the record holder in?

All good advice, I think if I was so intent on having a road going bike I could do a lot more!

APSF-350. It's a 350cc single. Record is at only 128mph, but best speed unofficially is 155mph. Hope to get at least into the 160s.
 

JadusMotorcycleParts

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Looks good! Dumb question.... can a person bypass the vacuum switch on an original petcock by using the "prime" position, or do I need to go shopping for manual petcocks?
Yes you can do that. The only problem is, if the petcocks membranes have failed, there is no real way to turn off the fuel delivery. Then you really have to hope that the float bowl valve is sealing well and not leaking fuel into the carb throat and then into the engine ;)
 

JadusMotorcycleParts

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Here we go...

As usual, dyno testing for me brought about excitement and nerves! Finally, I get to see what this engine concoction is capable of, but, was it all worth it?! What if it is a total flop?!

Thankfully, the first pulls (4 runs per test) showed an ok power curve right away - 21hp peak with a nice curve - meaning jetting was pretty close from my guess work. I thought ok, well, it’s more than the stock 18hp (same engine - albeit stock, same dyno, 6-7 years ago) but it’s less than that engine made with the Jadus header, intake kit and advance brackets - that made 22hp. After the test the dyno operator noticed that the rear tyre smelt a bit and after inspection was really warm and sticky - huge friction losses. This was because the air pressure had gotten down to 18psi! For consistency's sake, all tests are done with 35psi rear tyre pressure.

Once we had filled the tyre with air, we ran another test, with the fourth test being a drivetrain loss test - the operator cuts throttle and pulls in the clutch instantaneously, waits for the dyno drum to slow some, then releases the clutch. The program then can measure engine power at the crank. The results were much better! This time, 24hp peak, again with a nice curve, and around 28-29hp at the crank.

The next 8 tests (32 pulls total) consisted of:

-With and without foam air filter
-With long intake set up (similar to Jadus intake kit, just shorter for higher revs for this engine)
-With short bellmouth/velocity stack
-With and without the PCV system hooked up
-With a 115 main jet and a 120 main jet
-With the jet needle in positions 3 and 2.5 (dropped to next position down but shimmed with a washer)
-With and without the Jadus ignition advance kit (stock timing vs 5 degrees advance)
-With screwdriver held over exhaust opening (covering 10-15% of it)
-A few tweaks of the air/fuel screw

The shorter bellmouth/velocity stack lost power, as did running no filter. The explanation being that yes, more air may flow at high rpm, but there is less of a vacuum signal at the carbs venturi to lift the right amount of fuel from the emulsion tube. Plus, the shorter bellmouth might not allow the airflow to be fully laminar when it hits the venturi. Plus the length (if calculated correctly) of the longer intake should give the 3rd harmonic effect.

The PCV system didn’t do anything, so that is going in the bin, but hey, worth testing the idea. Once we disconnected it again, I put my finger over the crankcase ventilation opening on the engine throughout an entire dyno pull just to feel what happens there and the vacuum I felt was marginal, so very little to be gained there. These kinds of systems must have more effect on larger displacement and or multi-cylinder engines.

Changing back to stock ignition timing lost power everywhere as well - about 1hp up top but across the whole rpm register and more down below 6000rpm. The operator thought that the cam I chose might want less timing but that theory didn’t hold. Worth testing though to put that one to bed!

The bigger main jet (120) smoothed the curve slightly above 7000rpm but worsened it slightly below, so that is why we adjusted the needle position slightly and that made the curve better everywhere.

The screwdriver over the exhaust opening is a classic test. Holding it there blocks about 10-15% of the opening and this allows you to see if it makes more power like that - meaning you have got too little ‘back pressure’ in your system for what the engine wants. Fortunately in this case, we lost power everywhere, meaning the system is correctly dimensioned (could probably be even better)?

Then we did a pull through all of the gears. What we saw was surprising and awesome. The bike was making over 26hp in third, dropping off to around 24hp in 5th. Meaning the gearing I calculated is way too tall. So the remaining few tests we did all in 3rd gear and managed to get a peak power figure of 26.8hp (DIN) at around 9000rpm and peak torque of 25.3Nm at 6500rpm, with over 22Nm of torque from 4500 to 8500, a pretty nice spread of power! Also, in the same final runs we did the drivetrain loss test again and it was consistent with the first test - 29.5hp at the crank.

With the gearing I chose, the bike could spin the dyno drum up to 190-200kph (118-124mph) but in real life, with wind resistance, it won’t have the power to get there. With the dyno software, you can lay over a graph of wind resistance and see how much power you will need for what speed with what type of bike and the prediction was that I would need around 24hp. So with over 26hp it should be achievable, but I need to do something about the gearing. The fact that it can reach 130kph in third is just wrong. I have checked a few calculators again and I will be ordering a 14, 15 and 16 tooth front sprocket to test. That should put peak power rpm closer to 160kph instead of 190kph.

Are these good results? One thought was, ‘all that work for almost 27hp’?! Then again, looking at rules of thumb, 100hp/l is still considered good for NA engines, so 29.5hp (at the crank) for an air cooled, 263cc 2-valver is not that bad! I also wonder which modifications gave the most effect… Or would it have always been the combination of all of the mods together? 26.8hp at the wheel is around 46% more power than the stock 18.2hp (again, same dyno) and around 20% more power than the same engine with the Jadus bolt on mods at 22.7hp. All of these numbers are pretty close to my predictions as well, so with that said, I am pretty happy. This together with the weight reductions makes the bike pretty fun to ride and it handles very well. Now just to see if it will do the ton...
 

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