Kit Roller
Lighter rollers = better acceleration, heavier rollers = higher top speed?
In the shop we are asked on an almost daily basis for lighter than standard rollers for improved acceleration or heavier rollers for higher top speed. There is widespread belief that rollers are a compromise whereby the rider can choose between higher acceleration at the expensive of some loss of top speed, or higher top speed at the expense of some acceleration. This therefore implies that there are no actually 'correct' rollers for a specific bike, and the best choice of rollers is ultimately down to user preference.
But is there actually any truth to this ?
First of all we need to consider powerbands. All engines make their maximum power at a specific rpm. The further you are away from this rpm the less power your engine is developing at that moment. As you tune an engine to develop more power the power increases but the peak power tends to occur at a higher rpm and tends to drop off much more quickly as the engine speed moves above or below this rpm, this is what's known as a narrow powerband.
A 2-stroke motocross bike is a good example of a narrow powerband engine, they normally have 6 close ratio gears to allow you to keep the bike revving highly all the time and to remain as close as possible to the rpm where all the power is developed. Because the powerband is so narrow there is virtually no power at lows revs and you have to work the bike to keep the revs within the narrow powerband all the time.
Similarly if you fit a power pipe to a ped without changing the rollers the bike at first seems slower because although the engine now has a higher power output the cost of this is that it occurs at higher revs and the powerband has narrowed. So whilst your bike might have developed enough power to happily pull away at 6500rpms with the standard pipe, the tuned and now more powerful bike with the pipe fitted actually develops less hp at 6500rpm than it did with the standard pipe and it struggles to pull away.
Because the engine only actually develops peak power at one specific rpm a geared bike or car is virtually never utilising it's peak power, it's only actually achieved for a brief moment as the revs pass through peak power rpm whilst accelerating. The best compromise for maximum *useable* power is to manage the gearing so as the engine can be kept as close as possible around this rpm for as long as possible.
Even a standard moped is actually in quite a high state of tune and already has a narrow powerband. A 1.0 litre car developing 50hp actually only develops half of the power per litre that a 50cc moped does producing 5hp.
To keep the moped within it's narrow powerband it uses a variator system. The idea is that if you know at what rpm the engine develops its maximum power, the variator system can be tuned to keep the engine at that specific rpm all the time meaning that peak power is available immediately and continuously at any time you mash the throttle.
So back on topic...
Ok, we all know that lighter rollers allow your engine to rev harder and vice versa, and that if the variator is working well it should hold the engine at a constant rpm dictated largely by the weight of the rollers.
But how does this effect top speed? It actually doesn't. Roller weight doesn't have any effect on top speed (except in unusual circumstances)
It's important to realise that the variator can only hold the rpm constant until it has adjusted itself all the way out, as soon as the road speed is reached at which the variator can no longer keep changing out the transmission becomes fixed gearing.
The printout below shows a Yamaha Aerox, pretty much standard with a Viper pipe and Polini variator fitted. The only difference between the graphs is that each run up is done with different weight rollers- 3.5g (blue line), 5g (red) and 6.5g (green).
If you look at point 'A' on each of the runs this shows the point at which the variator can no longer change the gearing (to keep the rpm constant) for each roller weight. As soon as this point is reached the gearing becomes 'fixed linear' meaning that the only way the speed can increase is if the engine rpm's are also increased in direct proportion. At line B on the printout the variator has fully changed all the way out for all the different roller weights and from this point on the engine rpm will always be the same relating to the road speed and consequently the power developed will be the same and so will the ultimate top speed.
After point B the engine rpms simply continue to increase for all examples until the power drops away as we move further and further away from peak power rpm until it no longer has enough power to accelerate further - this occurs at 60.5mph for all roller weights on our printout when the power developed by the engine has dropped to 3hp.
Having established that roller weight has no effect on top speed it becomes clear that there is only really one ideal roller weight for a specific bike when tuning for both acceleration and top speed. In a later article we'll discuss finding it and general variator tuning in more depth.
It's worth noting that the Polini variator is a vast improvement on the standard Aerox one which is particularly poor on even a mildly tuned Aerox. It's able to keep to revs constant far more uniformly meaning the engine is utilising it's available power much better and it's able to adjust out to a higher gearing than the standard one often meaning a significantly higher top speed even without a gear kit.
Note: there are special circumstances where the above information can be disputed, but it can be considered reliable for general use and road peds.
Pedparts UK
About the Author
Andy is the writer and webmaster of http://www.pedparts.co.uk a distributor of moped parts & accessories dedicated to offering excellent and reliable customer service.
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