Hotrodding the GS

[def]

Originally Posted by johnjen

And did you notice that those cams are horsepower cams, not mid range torque cams? So while this may provide a top end speed boost you will loose low end and some mid range power. Which some may desire but it wouldn't be the best choice for going slow in the dirt.JJ

Boy, did I ever! After reducing the flywheel weight, blueprinting the engine, balancing, cleaning up the intake ports, installing Witham valve springs and adding RCMs, that 650 CC Triumph engine would turn ~10,000 RPM and made lots of HP above 4200 RPM.

The sounds of a vertical twin on song at 6000RPM and above gave me goosebumps!

[Dan Cãta]

Originally Posted by def

Boy, did I ever! After reducing the flywheel weight, blueprinting the engine, balancing, cleaning up the intake ports, installing Witham valve springs and adding RCMs, that 650 CC Triumph engine would turn ~10,000 RPM and made lots of HP above 4200 RPM.

The sounds of a vertical twin on song at 6000RPM and above gave me goosebumps!

What's the point in reducing the flywheel's weight? I knew that this only helps if you rew the engine while standing, and that while riding, the whole clutch+gearbox+shaft+rear wheel weight would not influence the flywheel, since it's so small compared to the other parts.

Dan.

[johnjen]

Originally Posted by Dan Cãta

What's the point in reducing the flywheel's weight? I knew that this only helps if you rew the engine while standing, and that while riding, the whole clutch+gearbox+shaft+rear wheel weight would not influence the flywheel, since it's so small compared to the other parts.

Dan.

What I've noticed is it lets the motor rev quicker. Which can change the way the bike shifts, due to rev matching being a bit trickier sometimes. And sometimes the motor has more vibration, but mostly it allows the engine to pick up revs faster, which for a hp engine can be a good thing.

JJ

[def]

Originally Posted by Dan Cãta

What's the point in reducing the flywheel's weight?

Dan.

The flywheel and any other moving parts powered by the engine creates parasitic loss. It takes energy to increase the RPM of the flywheel, energy that could be used to accelerate.

I also laced mag rims onto both front and rear wheels, further reducing loss.

And while I never dinoed the engine, the bike was the fastest accelerating street motorcycle (pump gas) for several years in my neck of the woods. I estimate 65 hp at the crankshaft (1960 stock bore and stroke 650 CC Triumph with a splayed head, which I fabricated myself and twin Amal carbs).

[Dan Cãta]

I agree that a flywheel takes power from the engine but I was thinking that since the most power would be directed to the rear wheel, the flywheel would not consume that much power.

And, why didn't they design a smaller diameter flywheel and a more powerfull starter engine ? That would have been a big plus in flywheel bottleneck ;)

Dan.

[Disco Dean]

Originally Posted by Dan Cãta

I agree that a flywheel takes power from the engine but I was thinking that since the most power would be directed to the rear wheel, the flywheel would not consume that much power.

And, why didn't they design a smaller diameter flywheel and a more powerfull starter engine ? That would have been a big plus in flywheel bottleneck ;)

Dan.

I don't understand the starter connection but as pointed out above - the flywheel weight directly relates to parasitic losses and also most importantly drive ability. Most lower revving higher torque motors incorporate a larger heavier fly wheel to allow for that bottom end grunt as the intertial mass helps that - but the higher revving you get the less that is needed - ie. light 2 stroke motors that rely on super high revs have little or no fly wheel. A modern 2 stroke trials bike that is designed for huge bottom end has a fly wheel that is huge in comparison to a moto crosser. It all translates to the rear wheel. One of the reasons that crank hp and rear wheel hp are so different - ie. a chain uses approx the same amount of hp to move around as an inline straight cut gear set - but the gear set takes up tons more room. If you factor in the ring and pinion of the GS drive shaft and spinning the drive shaft you lose more power in that system. But you gain in other areas. We run a vintage BSA Gold Star and the fly wheel mass is one of the key determining factors of how the bike comes off the turns - when we wanted to sacrifice drive ability for a few more top end hp - among other things - we reduced the fly wheel weight a bit... A big GS that needs to lug around the dirt and be drivable around town - and even keeping momentum up on the highway helping mpg - a heavier fly wheel does it.

It's not a bottleneck of sorts but a basic design element/parameter that contributes to how the bike runs. And you can manipulate that by adding or reducing the weight of it.

[hasenwerk]

Originally Posted by Dan Cãta

I agree that a flywheel takes power from the engine but I was thinking that since the most power would be directed to the rear wheel, the flywheel would not consume that much power.

And, why didn't they design a smaller diameter flywheel and a more powerfull starter engine ? That would have been a big plus in flywheel bottleneck ;)

Dan.

The flywheel doesn't really *take* power, think of it as a power storage device. Newton wrote some laws... his second law states: The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector. So, the more mass an abject has the more force is necessary to get it moving, so a lighter flywheel will require less input energy to get it accelerate faster. The issue is, say you made your flywheel, clutch, wheels etc weigh in at 1kg - acceleration would be fantastic, but every external force would easily stop that assembly from spinning. You wouldn't be able to ride your bike as you would stall it at every intersection when you release the clutch. As the flywheel assembly is on the other side of a gearbox that acts as a power multiplier it as a two way effect. This is why a taking 1kg off the flywheel has more "acceleration" affect than taking 1kg off the wheel. But, say you add 10kg to the flywheel, acceleration rate goes down but you ability to sustain a given speed goes up... with a heavier flywheel you can go off roading and crawl along at idle in 1st, hit a big rock and you bike is less likely to stall as all that energy is stored in that larger mass that will want to keep moving. So same principal starting at the intersection - some flywheel mass is necessary so you don't stall the engine if you take off at an engine speed slightly above idle.

So, for a BMW S1000RR, a lighter flywheel is going to help as you arn't loading it up for a transcontinental off-road journey. Put that same flywheel on a R1200GSA and you'll be cursing at it as you will have to rev your engine to 4000 rpm just to start moving - the GS is more of a tractor than you think!

[Dan Cãta]

Thanks all for the enlightening posts ;)

Dan.

[def]

Originally Posted by Disco Dean

We run a vintage BSA Gold Star and the fly wheel mass is one of the key determining factors of how the bike comes off the .

I really respected the BSA single...I used to race a fellow with a highly modified Clubman...it made lots of power and really sounded menacing...beautiful engine...all polished alloy! Photo available?

Somebody mentioned starter motor?

Yes, your right leg!

[def]

Originally Posted by hasenwerk

- the GS is more of a tractor than you think!

And tractors have huge flywheels...also, the GS sorta looks like a farm implement...in a high tech kinda way.

[def]

Originally Posted by Dan Cãta

What's the point in reducing the flywheel's weight? I knew that this only helps if you rew the engine while standing, and that while riding, the whole clutch+gearbox+shaft+rear wheel weight would not influence the flywheel, since it's so small compared to the other parts.

Dan.

Dan, you should try some alloy connecting rods, light wrist pins and a titanium crankhshaft...quit wasting all that energy in moving the engine parts around and up and down.

Also, to improve acceleration, loose 20 lbs of body weight....those racers are usually little guys...loosing weight is the same as hotrodding...same net effect...more acceleration.

[Dan Cãta]

There's plenty acceleration for me on my RT. I rode a friend's GS after doing meintanance on the bike and it was weird... The GS seems so much lighter... It accelerates faster than the RT, way faster, maybe because of FD ratio and less weight on the GS.

Anyway... I can't wait for this product to be released on the market!

When is that going to happen??

Dan.

[scooteraug02]

This flywheel talk reminded me of the Stealthy flywheel weight I put on a YZ250 two stroke. Here is their explanation:

"About Steahly Offroad

When we started making flywheel weights in 1987 very few riders understood the benefits of adding flywheel weights. Since then the popularity of adding flywheel weights has increased dramatically. More and more riders discover that they can go faster for longer with the smooth power delivery, better traction, and reduced stalling that Steahly Flywheel Weights provide. Adding a flywheel weight will increase the rotating mass, or inertia, of the motor. The result is an engine that is a little slower to rev, has more controllable power delivery and less hard hitting power, a more usable low to mid-range, and is less likely to stall at slow speeds or with the rear brake. What you will feel is better traction when it is slippery or hard pack, a torquier and broader powerband, and the top end power on many bikes will be improved with less tendency to lose RPM’s when climbing or hitting obstacles. The easier to control power will help you conserve your strength and energy, plus many bikes start easier with the added weight."

http://www.steahlyoffroad.com/about-steahly-offroad

[barlytreat]

Originally Posted by supershaft

Well, I guess we have different definitions of "hot rodding". That's fine but I do think tweaking the injection and calling it "hot rodding" is a bit misleading. The results of fine tuning the mapping are going to be a smoother engine and maybe a bit more power via a better mixture. IMO, hot rodding is about increasing the engine's volumetric efficiency through weight reduction, friction reduction, porting, cam timing, compression, combustion chamber shape and the like. Unless the injection mapping is WAY off (which I don't think it is), the increases in performance through changing the mapping won't be that great. Having said that, I CAN appreciate subtle improvements! They all add up! Personally, I don't see how some deal with how their bikes surge. Completely eleminating that is more than a subtle difference but I still don't call that "hot rodding".

I Agree

[Disco Dean]

Originally Posted by def

I really respected the BSA single...I used to race a fellow with a highly modified Clubman...it made lots of power and really sounded menacing...beautiful engine...all polished alloy! Photo available?

Some in action... too bad I don't have a close up of the motor for you. It is a period correct Geoff Monty Special frame and GoldStar engine. The bike is producing high 50's hp. All hand made and built - frame up by my friend Ken Rosevear. All polished alu and gobs of magnesium... very very nice bike for me to ride.

Sorry for the photos they are a bit corny as they were used for sponsors...