Roller Weights Demystified

[Rusty Shackleford]

First things first, this info assumes you know how a CVT works. Allow me to otherwise make some key points to get up and running as quickly as possible...
- Roller weights are part of what controls the RPM the CVT maintains (they have a direct, force/resistance balanced, relationship with the contra spring).
- Higher RPM's per MPH will inherently require a lower gear ratio, just like every other geared vehicle.
- Top speed is ultimately dependent upon RPM, gear ratio, and power, also just like every other geared vehicle.
- Lighter weights' primary function is to maintain higher RPM's while the gear ratio varies. (equivalent to winding the gears out before upshifting)
- Heavier weights' primary function is to maintain lower RPM's while the gear ratio varies. (equivalent to short shifting to reach top gear as quick as possible)
- If you had a low revving bike, the only way to maintain a low RPM all the way to top speed with heavy roller weights would be a higher top gear ratio. (larger variator)
- Once the variator is locked at top gear, it doesn't matter how heavy or light your roller weights are. The ratio is no longer changing so they won't affect top speed.
- To make a CVT analogous to a standard transmission, the weights and spring are roughly equivalent to the gear shifter, and the pulleys are roughly equivalent to the gears.

Here's an animation of roller weights in action...

<iframe width="420" height="315" src="http://www.youtube.com/embed/3yRskb0BYwE" frameborder="0" allowfullscreen></iframe>


Okay, let's start. Look at this dyno below. It's not from a Buddy 125, but the principles still apply. To dyno a scooter is not like a motorcycle. There's no fixed gear to rev through the RPM's. You'd have to lock the variator and driven pulleys in a particular ratio to do so. Therefore, it takes 2 measurements to assess what's going on. The top graph is power over speed; the 2nd is RPM over speed. Both are dependent upon the behavior of the CVT. Notice the 3 different roller weight measurements. Look at the bottom graph. Notice how the lighter the roller weight, the higher the speed it takes to max out the variator, indicated by point A. This effect is just like any other vehicle. The higher the RPM, the more the speed increases by change in gear ratio. Notice the green line representing the heaviest roller weight hits top gear long before the blue one. Now notice that it catches up with the red line, then blue line, and all 3 of them are equal in curve, indicated by point B. This is because the top gear ratio is fixed; the variator is maxed out. As mentioned, the only way to increase the speed of top gear (for any specific RPM) is a higher gear ratio. This is the same effect as a motorcycle with a low redline needing higher gearing to reach higher top speeds.



So, what roller weights should you have in your scooter? That depends on what RPM your engine makes power at and what you want to compromise for a milder ride and more fuel economy. If you want power, put in lighter weights to run a little lower gear ratio and hold RPM's closer to peak power. You'll end up with results more like the red line in the top dyno graph. If you want fuel economy, put in heavier weights to raise the gear ratio and hold cruising RPM's a little lower. As for the blue line, you'd actually lose performance as the engine has exceeded peak horsepower. For details on that subject, here's my write up on gearing.

Finally, the magic number you need in hand to make this assessment is the peak horsepower RPM. I'm in the process of finding a shop that can dyno a scooter, but I've done several web searches in the meantime, and the most consistent average I've seen for the Buddy 125 is 9.5hp @ 7,600rpm.

(I have made corrections to this post as it's been discussed, so if the comments in the thread seem to not make sense, that's my fault and not the members of MB.)

[Howardr]

Oops

[Rusty Shackleford]

If the weights aren't capable of continuously raising the gear ratio in the variator, you'll be limited by engine speed. There's indeed going to be a terminal weight of how light you can go, and how heavy. Ideally, the weights would be used to target a specific RPM range, hence "tuning". Same reason you wouldn't dramatically over-gear or under-gear a motorcycle.

[thatvwbusguy]

Your explanation of roller weights is right on the money. Contra springs and rollers definitely have a direct impact on each other, but the effects are not as linear or simple as a dyno chart might lead you to believe.

A dyno is done in a controlled setting, where all of the results are gathered within a few minutes of testing. This is a prime example of how lessons learned in the lab don't necessarily work outside of the lab.

In the real world of everyday riding, there are many variables that will effect the way your scooter performs. Rider weight, steep hills, temperature, humidity, altitude and a host of other factors will determine how the engine and transmission systems interact with each other.

Rider weight and hills are typically the most important factors that influence the need for experimenting with spring rates other than stock. This being the case, the best setup for your significant other might not be anywhere near ideal for you. If you are both planning to ride the same scoot, you will have to decide whether to tune to one person, or shoot down the middle and both accept a bit of a compromise.

In a nutshell, small displacement engines operating centrifugal/friction based drivetrains will require individual tuning to offer ideal performance for their rider(s).

In your post about the 161cc Big Bore Kit, I linked to an article that explains some of the interaction between the roller weights and the contra spring: http://www.teamcalamari.com/zuma/variator.html

The following text from the article describes some of the reasons that a stiffer contra spring can be an important part of the overall equation:

"The rear pulley has a spring holding it together. This is the contra spring. The inner half of the pulley is torque controlled. There are angled grooves that the pulley travels on. As torque is applied, this limits the belt from traveling in too quickly. Likewise, as you go up a hill and torque is applied to the pulley, it is supposed to force the pulley together giving you a lower gear ratio. If your roller weights are giving you good acceleration, and a good top end, but when you approach a small hill the scooter slows drastically, this may be caused by having too light of a contra spring. The contra spring helps push the rear pulley back together when torque is applied to the rear pulley.

Also if you are running at 50MPH, slow down to say 35MPH, then try to accelerate, and find that you have hardly any acceleration, this can also be caused by having too light of a contra spring. When you slow down, the spring is supposed to push the rear pulley together into a lower gear ratio. If the spring isn't strong enough, the pulley will remain in a high gear, and then when you try to accelerate, there isn't enough power to push you because you are in too high of a gear.

On the other hand, if your spring is too strong, then the roller weights may not be heavy enough to force the belt out all the way and into it's highest gear ratio. If you go with a heavier contra spring, you will most likely need to go to a slightly higher roller weight. Likewise, if you go to a lower spring, you may need to go to a lighter weight. There is a very fine balance that you must achieve between the contra spring and your roller weights. This is only accomplished through trial and error."

[Rusty Shackleford]

That's fair enough. I'm aware that there's a relative relationship between the contra spring and the roller weights. Theoretically, a small portion of the bump in RPM's from lighter roller weights could come from the lower relative weight in proportion to the spring. There would then be a terminal limit in how far they could deviate from each other. Also, the same as there's a terminal limit for how light or heavy roller weights can be given the engine power and variator, there'd be a terminal limit on how stiff or weak the contra spring could be. For example, you couldn't go so light in roller weights that the spring needed to accommodate them wouldn't apply sufficient tension to the belt. This is hypothetical and ignores every other factor but I get the idea. I'm 210lbs and the stock contra spring still allows me to not only maintain, but accelerate up hills (plus my GF is considerably lighter than me), so I'm going to try roller weights 1st. In the event a rider's weight was limiting acceleration and the ability to climb hills, I could see how a stiffer spring would help. So long as the smaller person is not tuning for the bigger person, it should be okay. The effects I feel will be even more pronounced for her (:sigh: I need to get on a treadmill). I'm indeed excited to get the bike back with the tachometer to see what's going on.

[Tocsik]

Excellent technical information from two well-informed members.
I also agree with Howard in that technical specs don't always reflect real-world; even considering rider weight, wind resistance, barometric pressure and ambient temp.

But, my personal real-world riding and tinkering does reflect the interplay between roller weight and contra spring stiffness.

I'm also wondering if slight variations in the belt would have a noticeable effect. Ever since putting in my new belt and sliders, I seem to have shifted the power band and the mid range is lacking. The sliders were a direct Dr. Pulley 12gram replacement but the belt was both new (not worn or stretched) and had a slightly different angle; 243X20X30 (new) vs 243X20X28 (old).

[Rusty Shackleford]

If I look at that tachometer and see the variator is maxing out below top speed and is running below peak power prior to it, I'm getting lighter Dr. Pulley Sliders to give it all she's got. After that? Big bore kit and big valve head. Yes, I'm aware there's some discrepancy in the dyno numbers, but you get the idea.

[thatvwbusguy]

Don't forget that rollers and Dr. Pulley sliders of the same weight will not react the same inside the variator. In general, if 10g rollers are working well for you during test tuning, you will want to use 11g sliders to get similar results.

Going up 1g for each weight when changing from rollers to sliders will usually offer very similar performance results.

[Rusty Shackleford]

By that logic, 12g sliders would still out-accelerate 11.5g rollers. Is this the case? I was thinking 11g sliders.

[thatvwbusguy]

Rollers are usually considerably cheaper than sliders and you can get a set that has 3pcs each of several different weights. I would use these to get your basic tune and then plan to go 1g heavier with each Dr. Pulley slider to get the same results.

My notion was that I was going to use 9g and 11g rollers in my Zuma 125, but I am running 10g & 12g to get the balance I want. Trying different combinations really is the only way to know what will work best.

When my current rollers get flat spots I am going to try out some sliders to see if the difference in variator transition is noticeable.

[jartist]

Great Post! I'm new to the CVT transmission world and transmission tuning seems even more addictive than jetting a carb! I think there's two points where you're a bit off the mark when you apply hp curves to the CVT geared engines which may be why the explanation is not correlating with people's butt dyno.

First off, HP is HP. When you talk about torque you can use torque to your advantage with gearing. When you talk about the maximum "power" that an engine can provide, be it for acceleration rate or maximum speed, it will occur at the peak hp. The red curve WILL out accelerate the blue curve. The reason you shift after the peak HP rpm on a manual transmission is to get the engine closer to the peak HP rpm on the next gear. Since there is a finite number of gears on a manual transmission you compromise between the tail-end of a dropping HP curve and the rising-end of the HP curve at the next gear. With a CVT there are an infinite number of "gears" so you can and should keep the rpm at the max HP output. That's the beauty of a CVT.

Second, the weights alone on the buddy seem to effect top speed because more or less of the variator surface is being used with the spring weight combo.

So it seems to me, to achieve the hottest setup of a CVT you want to keep the rpm at the peak HP while using the correct weight/spring combo to use up the entire variator range.

[Rusty Shackleford]

First off, HP is HP. When you talk about torque you can use torque to your advantage with gearing. When you talk about the maximum "power" that an engine can provide, be it for acceleration rate or maximum speed, it will occur at the peak hp. The red curve WILL out accelerate the blue curve. The reason you shift after the peak HP rpm on a manual transmission is to get the engine closer to the peak HP rpm on the next gear. Since there is a finite number of gears on a manual transmission you compromise between the tail-end of a dropping HP curve and the rising-end of the HP curve at the next gear. With a CVT there are an infinite number of "gears" so you can and should keep the rpm at the max HP output. That's the beauty of a CVT.

1st, thanks for your reply! I can totally relate to your line of thinking, but I must respectfully maintain my position. I know max HP is where the engine makes the most pull, and I mentioned torque (as in engine torque) for the sake of economy, but I have a few comments regarding gearing. As for shifting gears on a manual bike, winding a gear out to redline is more than just a matter of dropping the next gear on to peak power. Peak power is only part of the equation and doesn't translate to maximum rear wheel torque. You could wind every gear out to redline, and each would still have more rear wheel torque than the next gear up at peak horsepower. For a more detailed explanation with examples, see this article. I used to be of the mindset that peak power was always best, but it's only the hardest pull relative to that particular ratio. We also debated this topic heavily over at cbr250.net where I was corrected to this perspective. In the case of a scooter, I bet you a Coke that peak power in a higher ratio is still creating less rear wheel torque than a lower ratio that's passed peak power already. The more you lower the RPM's back toward peak power, the higher the gear ratio, reducing power to the wheel. I also bet a coke that by no coincidence, after redline, it would be beneficial to upshift.

If I'm wrong about this, that's okay, too. I've been involved in this conversation in detail before, just not with scooters until now.

[jartist]

You're absolutely correct about a lower gear ratio creating more wheel torque. Torque, however, is just a rotational unit if force. Lower gears are slower. You can apply a huge amount of force to something and still not move it. (Scooters are for moving us, after all). In order to figure out how much energy an engine makes to move us we look at the unit of work- take that force times distance. But we don't really want to just move, we want to move FAST! so, do that work on a given amount of time and you're measuring HP.

That article you posted is a little bit misleading. Try this one: http://www.largiader.com/articles/torque.html
There is nothing magical about the power output of an engines redline. When you shift a manual transmission you loose time you're putting energy to go and you shift to a gear and have to climb your power curve again. Shifting later puts you higher up on that curve.

[Rusty Shackleford]

And you're right, it would be faster if we were talking a constant speed in a specific gear; but this isn't the same as rate of acceleration.

Either way, yes, if I wanted to optimize my power in a scooter, I definitely concur that I'd run it closer to what the red line on the graph is displaying. My fear would be the cruising RPM would be so high it'd be screaming around town burning considerably more gas than necessary.

[jartist]

You want to run at max hp not max rpm for acceleration too. Part of the problem with making the correlation between max hp and max rpm on a bike like the cbr250 is that the max hp occurs close to the indicated redline on the tachometer. You can safely wind up the motor to the rev limiter which is where the real redline is. But you can feel the hp (and acceleration) drop off after the indicated redline which is a conservative mark on the tachometer that is closer to the max hp. Give it a try on your cbr. Take it to the rev limiter in third gear while riding on a slight incline and watch the revs. Note where the rate of acceleration starts to drop off and compare that to a Dyno chart of your bike. I dare you to try it- calibrate that butt Dyno!

[Rusty Shackleford]

EDIT: ^This post above is correct and I was mistaken. Torque is multiplied by gear ratio. Horsepower is invariant across gearing.

I understand exactly what you're saying, and yes, the rate of acceleration does indeed drop after peak power, but peak power is only the rate of hardest acceleration in that particular gear. Any RPM, all the way up to redline, will yield more acceleration power than upshifting to a higher gear ratio. This is because peak power is not the only variable affecting rear wheel torque. To say peak power is the sole indicator completely disregards gearing, which multiplies power as opposed to a linear measurement like horsepower. The whole purpose of gearing is to trade speed at the rear wheel for power at the rear wheel. This is why every time you upshift, the bike gets faster but has weaker acceleration. Look again at this chart of power applied to the rear wheel and see if it makes sense. The 1st column of data is engine power alone. The stars denote peak torque and peak horsepower as reference points. The other columns are measuring power output from the transmission via gearing. Notice the gear ratios are torque multipliers for the engine. To make things pefectly even, you'd need a gear ratio of 1:1 so the crank and the output shaft would be turning at the same rate (hence why any gear with a ratio of 1:1 or higher is considered "overdrive", where there's no longer any multiplied torque to the wheels). The reason 1st gear has a slower top speed but a heck of a lot more power is because it has so much lower of a gear ratio... from the looks of the chart; 3.54 turns of the crank for every 1 turn of the output shaft. It takes a lot more force to get weight moving than to keep it moving at a constant speed. Low gears have the power to get a vehicle moving from a stop; high gears can afford to sacrifice power to hold a higher cruising speed.

Code: Select all

       Engine     Transmission output torque (ft-lb):
       Torque      1st     2nd     3rd     4th     5th
 RPM  &#40;ft-lb&#41;     3.54    2.13    1.36    1.03    0.72  <- gear ratio
----  -------     ----    ----    ----    ----    ----
1000       50      177     107      68      52      36
1500       65      230     138      88      67      47
2000       80      283     170     109      82      58
2500       92      326     196     125      95      66
3000      104      368     222     141     107      75
3500      114      404     243     155     117      82
4000      120      425     256     163     124      86
4500      125      443     266     170     129      90
5000      130      460     277     177     134      94
5500*     133      471     283     181     137      96
6000      130      460     277     177     134      94
6500*     122      432     260     166     126      88
7000      110      389     234     150     113      79

To translate this into scooter terms, I'd bet the farm that the multiplied effect of a lower gear ratio in the CVT would yield more power at the wheel than the linear decline after peak horsepower up to redline. Kind of like how 3+4=7... while 4+4=8; 3x4=12. Acceleration is an effect of both power and gearing. In any event, with a CVT scooter, there's no way to short shift. Therefore, since lighter roller weights also raise cruising RPM's, I've decided to go the "just a little lighter than stock" route. I can't wait to get it back and take it for a spin!

[ericalm]

Dyno values are, at best, an indication of what might happen in real-world conditions. When it comes to rollers, which some generalizations hold true and the curves shown in the dyno results tend to stay the same shape (if they don't reflect the numbers), there's a lot of variation when you actually get on and ride.

This is why it's so hard to recommend a specific roller setup for other riders. Even trying to find the setup I prefer was a matter of trial and error over a couple years.

A scooterist asked me about this last weekend, and wanted to know whether he should change to different weights. The answer is always, "Ummmm… maybe." But as with all modifications, it's best to start with reasonable expectations and the understanding that results are often not consistent between scooters/riders and that they may not be completely satisfied. This usually leads to more tinkering and tweaking (and expense).

In general, if people are doing their own work, not such a big deal. And swapping rollers is daunting for some but actually very easy, requiring little knowledge beyond using a wrench. If you're relying on your mechanic/dealer, it's often not worth messing with because if you're unhappy with the results you're paying for another trip into the transmission.

[ericalm]

Oh—and great post/thread, BTW!

[Rusty Shackleford]

First off, HP is HP. When you talk about torque you can use torque to your advantage with gearing. When you talk about the maximum "power" that an engine can provide, be it for acceleration rate or maximum speed, it will occur at the peak hp. The red curve WILL out accelerate the blue curve. The reason you shift after the peak HP rpm on a manual transmission is to get the engine closer to the peak HP rpm on the next gear. Since there is a finite number of gears on a manual transmission you compromise between the tail-end of a dropping HP curve and the rising-end of the HP curve at the next gear. With a CVT there are an infinite number of "gears" so you can and should keep the rpm at the max HP output. That's the beauty of a CVT.

@jartist: I know this thread was a while ago, but you are correct about this. I was mistaken. Calculating torque and gearing can be done the way it is in the chart, but, you put it best; "horsepower is horsepower". The measurement itself is inherently invariant through gearing, and I was missing that part of the equation. The above paragraph is exactly what was needed. I have revised the OP. Thanks for your correction! Seriously.

[rick71454]

Hello BuddyLand:

I got a Buddy 125.........

Does anyone know if the two sides of the CVT driving pulley actually contact each other when the variator is maxed out at higher speeds and assuming enough roller weight?

If yes, then my idea is a moot point.

If no, then I have this idea that might incease top speed.

A slightly bigger diameter than stock 18X14MM roller weight would push the two sides of the driving pulley together closer, when variator is maxed out. That would force the drive belt to run closer to the outer edge of the driving pulley. But would this slow the RPMs?

I know there are 20X15MM 12g sized roller weights made by Dr. Pulley and NCY.
I think the Gen Blur uses this size. If the 15MM is to wide to fit in the variartor, then one could grind off 1 one MM of the width to fit in a Buddy 125 variator.

What do you all think?

rick71454

[Rusty Shackleford]

I wouldn't, but that's just me. Given the limited amount of horsepower available, if you're getting a GPS 60mph, you're probably doing about all it's gonna do... and that's a pretty reasonable speed for 125cc's. With a higher ratio, you'd lower the RPM's, yes. The problem is, you'll likely run out of steam to overcome the wind before you reach peak power. This is why larger people often hit higher speeds with lower gearing. Wind resistance increases exponentially, the faster you go.

[thatvwbusguy]

Most rollers are comprised of a metal (usually brass) core wrapped in a thin layer of composite (plastic). If you try to grind away any width, the plastic that wraps around the edge of the metal core will be compromised and the whole roller will most likely delaminate and make a mess of your variator.

Check to see where your belt is riding on your variator by making some marks with a Sharpie on the inner pulley from the center outward to the edge. After riding for a couple days, check to see how much of the Sharpie marks remain. If your marks are 2-3mm from the edge of the pulley, you are pretty much maxed out on your high gearing ratio. If the marks on the variator are much longer than that, try going up 1g each on 1/2 of the weights in the variator and see how that works out.

By playing with the weights, you should be able to get the most out of your variator within a couple tries.

[rick71454]

Hello BuddyLand:

Thanks for your reponses. I discovered that the brand Adige from Italy, makes roller weights size 20X14.7.

I think the extra 2 milimeters height would push the variator such that the drive belt will ride closer to the outer edge of the driving pulley. I noticed my drive belt is at least 4 milimeters from the outer rim of the driving pulley at full throttle with stock 18X14 roller weights.

I think that their is enough slack that the extra .7 milimeter width would fit in the stock Buddy 125 variator. Yes or No? They come in 11.5, 12.5, 13.5, 14.5 grams.

Alternatively........Adige and other companies such as NCY and Dr. Pulley make 20X12 roller weights. I am thinking with the pressure on roller weights the width being 2 miliments shorter than stock 18X14 would not be a bang around problem.

What is your opinion on this?

Rick71454

[skully93]

my understanding is that it is the weight of the roller/slider. if you use too light of weights, it won't go all the way.

[rick71454]

Hello BuddyLand:

I agree that the weight of the roller weights is one factor in getting to the
them to slide out to the end of the variator. Imagine if the diameter of the roller weight were 20mm instead of the stock 18mm. Unless this causes the variator and weights to fall apart, and given the increased power of my installed 161cc kit, the top end speed might increase.

Good idea using 20mm diameter roller weights in a buddy, or foolish?

Rick71454

[JettaKnight]

Hello BuddyLand:

I agree that the weight of the roller weights is one factor in getting to the
them to slide out to the end of the variator. Imagine if the diameter of the roller weight were 20mm instead of the stock 18mm. Unless this causes the variator and weights to fall apart, and given the increased power of my installed 161cc kit, the top end speed might increase.

Good idea using 20mm diameter roller weights in a buddy, or foolish?

Rick71454

Ah, I see you found the post you're looking for.

I'm not even sure this assembly will fit together.

Don't forget there's a counter to the variator - the clutch assembly. No matter how far that pulley wants to increase in diameter, the other pulley can only reduce so far. Meaning, there's a physical limit to the ratio of input and output speeds of the transmission. The only way to overcome this is to change the final gear ratio in the gear box.