72cc BBK first ride

[phatboy]

Hey

installed a 72cc BBK, finally got a bigger jet on there (drilled out to about 90), everything else is stock. running great, a tad rich but not bad

full throttle take off is very strong, acceleration up to 15-20 mph is great but above that it is a dog and seems like its revving a good deal higher than before. theres this big long hill i go up, with 49cc on there the scoot would go up at 20 mph, too slow. with bbk it goes exactly the same speed, no improvement at all. this very hill is the whole reason i put on the bbk, 20 is too slow and its dangerous

do you think I need heavier weights. I want a weight that works for best accel in the 20+ range to the top end, don't care if it's slower off the line. what's on there now, and what weight would you recommend.

[phatboy]

bump

any one have recommendations for heavier weights

i was thinking a set of 8.5g, if too heavy mix with the stock 6.5g for 7.5g average

as said looking for better up hill acceleration , i just want 5 mph more out of this hill that the 49cc did at 20 mph

[naughty lee]

See my zx50 thread about variatior marking and boss shortening. If the hill is really steep, the scooter is gonna struggle. A well set up scooter should rev briskly going up hill and scream going down a hill. Heavier rollers might make a difference. Get two packs so you can mix n match

[JC]

For the 47mm kit, you don't want to go any higher than 6.5g weights. I have this cylinder and a 90 jet. The 6.5 weights get me to 45-50 in no time...

[LoveMyKymco]

The weight of the rider, the contra spring tension, and the clutch triple springs all dictate your roller weights, whats good for one rider with a 2k torque spring, 1.5k triples, polini drive face, polini variator, 72cc kit and weighing  200 lbs isnt going to be the same for a lighter or heavier rider whos running all stock.
Theres too many variables, but if youre having issues with going up hills Id recommend a stiffer contra spring so it will down shift when you hit hills. Or LIGHTER rollers as youre not having issues on top end. You need more rpms to make it uphill.

[baddi]

The weight of the rider, the contra spring tension, and the clutch triple springs all dictate your roller weights, whats good for one rider with a 2k torque spring, 1.5k triples, polini drive face, polini variator, 72cc kit and weighing  200 lbs isnt going to be the same for a lighter or heavier rider whos running all stock.
Theres too many variables, but if youre having issues with going up hills Id recommend a stiffer contra spring so it will down shift when you hit hills. Or LIGHTER rollers as youre not having issues on top end. You need more rpms to make it uphill.

The force which the roller weights apply to the variator is the centrifugal force, which is dependent on radius, mass and the rpm and will always be equal to the force that the spring applies in the other direction plus the friction. The spring force is dependent on a spring constant and the distance that the spring is compressed. Weight of the rider is not in this equation.

This way, the only variables are the taper of the roller lanes, the roller weight, the length of the belt, the spring stiffness and the friction of the rollers and the belt. Usually the taper of the variator is adapted so the increased friction in higher gear does not have a significant effect.

Anyway, even though the right roller weight can be calculated if you know at which rpm the highest power is, but it is way easier to just test and try with a straight piece of road and a stop watch.
Remember not to mix weights with a greater difference in mass than 1g as the heavier rollers will wear out very quickly and give vibrations which is bad for the bearings.

[LoveMyKymco]

The force which the roller weights apply to the variator is the centrifugal force, which is dependent on radius, mass and the rpm and will always be equal to the force that the spring applies in the other direction plus the friction. The spring force is dependent on a spring constant and the distance that the spring is compressed. Weight of the rider is not in this equation.

This way, the only variables are the taper of the roller lanes, the roller weight, the length of the belt, the spring stiffness and the friction of the rollers and the belt. Usually the taper of the variator is adapted so the increased friction in higher gear does not have a significant effect.

Anyway, even though the right roller weight can be calculated if you know at which rpm the highest power is, but it is way easier to just test and try with a straight piece of road and a stop watch.
Remember not to mix weights with a greater difference in mass than 1g as the heavier rollers will wear out very quickly and give vibrations which is bad for the bearings.

Its a nice theory but it doesnt work, If I ride my 140 lbs friends bike thats set up for him and I try to go up a hill Im going to burn clutch and thats it because the contra isnt set up strong enough to maintain the "1st" gear and wants to shift. If he sets up too strong of a contra when he comes out of a corner or slowing down he will feel it try to down shift. Its funny how its all relative. The rider weight is very important when setting up a cvt. Shifting and running great on the stand isnt the same as running with drag.
When I return from my 3 week vacation Im going to start racing in the tuned moped class with my 2 03' kymco zx50s, 08' yamaha zuma, 88&89' honda elites w/ zx50 transplants, and the '13 yamaha aerox Im getting imported that should be here by the time I return.

[MotoRandy123]

What's the equation for optimum roller weight if you know the RPM for max power?

[baddi]

What's the equation for optimum roller weight if you know the RPM for max power?

It is not a simple equation as much as it is something that you can calculate. The centrifugal force is -mass * angular speed^2 * radius. Angular speed is 2 * pi * revolutions per second (that is what is important as we know what the desired rpm is).
This force has to be slightly higher than the spring force (plus friction) to compress the spring and therefor gear up.
Since the spring force is the spring constant multiplied by distance of compression, we will need to know the spring constant. Test this by placing the spring on a scale weight and compressing it while you note the force it applies and how much you have compressed it to find out what the number that the distance is multiplyed by to find the force. Remember to calculate from kg on the scale weight to newton for use in the calcuations.

So the equation is -m*(2*pi*f)^2=k*s+Ffriction
when m is the total mass of all rollers, f are the frequency (rps), k are the spring constant, s are the distance (should be negative because the force is in the opposite direction than the force that the rollers press against the variator with) and Ffriction are the force of friction.
This means that the mass of each roller is m=-(k*s+Ffriction)/(6*(2*pi*f)^2)

The only thing we do not know is the friction, but since friction is normal force multiplied by the friction coefficient, µ, and the normal force on the rollers are equal to the spring force, the only thing we do not know is the friction coefficient of the rollers against the variator, but this can be measured by dragging a roller of known weight across a surface similar to the variator.

Then the equation for a single roller weight is
m=-(k*s+2*k*s*µ)/(6*(2*pi*f)^2)

And this is without the friction of upgearing the belt or other things in the CVT that changes when upgearing.

As i said, it can be done and calculated, but it is way more effective to just buy a bunch of rollers a little lighter and a little heavier and trying it out on a piece of road going ½g down or up each time and timing the accelleration.

But anyway, the forces in the CVT at the vario are made axial when the rollers try to pry the variator from the back plate and are transfered by the belt to the back pulleys where it is still axial.
The weight of the rider has only 2 impacts on the vehicle: Friction between road and tire and bringing up the total mass of the vehicle, requiring more energy to get up the speed.

LoveMyKymco, i have been told before that weight has an effect on roller weights, but have not found a single reason to why it should affect the axial forces in the CVT when the only force it applies is increased friction between tire and road, but if you can shed light on what effect the weight will have on the CVT, i would love to be educated.

Edit: Found a flaw in the equation, since i forgot that the rollers are being pressed with the same amount of force on both sides, so the friction is double.

[LoveMyKymco]

A cvt always tries to balance itself and torque and drag is a huge part of the equation if there was one. It must be added into the equation. Unfortunately there is no true equation for it. It's the same exact reason the EPA is flawed when giving standards for mpg. It's a known fact hybrids don't get the mpg they claim neither do the small turbo cars. Everything is fine on a dynometer, unfortunately real world issues occur such as hills, passing cars, merging , fast braking, wind, too many fat people in the ride, etc.
If there were a sound equation for roller weight and springs I would be all over it. It's always a pain and no good for the crank to continually service the vario.

[phatboy]

Thanks all

i bought a set of 8g rollers they shipped out today, expect them by tuesday

I know someone above said 6.5g is the most you should get but I'm trying heavier first. it has 6.5g on now and the scoot is real quick to about 15 mph, then revs go up and they seem too high and power drops off a lot. It seems to rev higher than with the 49cc. i read the power band on the 72cc is lower so i think i actually want revs lower

i cant think of any other reason except weights too light, why the scoot would go up the same hill at the same speed, 20 mph, with the 49cc and the 72cc when the 72cc definitely has more power.
 
i'll see how the 8g rollers go and report back

[MotoRandy123]

I tried changing the weights a bit but fooled myself when the tube holding the
contra spring didn't seat all the way. I also found the  rear pulley acts differently
depending on the exact pin/slot it is in. The last time I tried them all to get the
one with the smoothest reaction. I've had it apart so many times it doesn't feel
"new" anymore...

[LoveMyKymco]

I know the feeling randy, after changing so many times you forget what it's suppose to feel like. I've spent hours and days and weeks and months tuning a cvt just to change it a day later. Currently I have 6 rear pulleys set up differently. Around 500$ in clutches, 120 in contras, 220 on performance rear sheaves, 4-500 on clutch housings. And about 75$ on triple springs of all makes. This is just my extra sets of rear pulleys for test and tune.
If you're over revving it's too light of rollers. Honestly it sounds like you need a better stiffer contra. If I were you I'd try 42-48g in vario, 2k contra, and 2k triples. On my 82cc (50mm) kits I run 48g (8g x6) 2k contra, and 1.5k triples, I'd use 2ks on the 72cc so it will get going with more torque.

[blue]

lighter weights faster speed heavy weights slower speed.

[LoveMyKymco]

lighter weights faster speed heavy weights slower speed.

Lighter weights, higher rpm, heavier weights lower rpm.