This is the first in a series of short blogs we're going to be doing which covers some of the basic queries which we're asked on a regular basis.
This blog addresses the question of whether or not it's important to fit a gear-up kit after fitting a 70cc cylinder kit
Firstly it's important to recap how a moped variator works in it's basic form. If working and set up properly a moped variator allows the engine to run at peak power constantly as the road speed increases. It does this by seemlessly and constantly changing the gearing over an 'infinite' range as the road speed increases to keep the engine rpm constant at a set rpm determined by the choice of roller weight. This means that whenever you mash the throttle the revs jump to the rpm at which the engine makes the most power (provided the optimal weight rollers have been fitted) and keeps them there as road speed increases meaning that the full peak power of the engine is always immediately available. This is in contrast to geared bikes where the revs go up and down in direct relation to the speed of the bike and you are virtually never at peak power rpm but only as close to it as you can maintain by constantly changing the gears.
There is an important limitation to the moped variator though in that it can only keeping changing the gearing in this way until you reach a certain road speed at which the variator is fully changed out. As soon as this speed is reached the only way that the road speed can increase further is if the engine speed also increases... in effect the moped then becomes a geared bike and the road speed can only increase in direct proportion to any increase in the engine rpm.
This 'fully changed out speed' is affected by roller weight but only up to a point (which is why roller weight doesn't affect top speed (see Moped Tuning- ROLLERS)) but it is affected predominantly by the final drive gearing in the gearbox (can be changed with gear kits) and other factors such as tyre or wheel size which do affect top speed.
The following graph shows a dyno printout of a standard Aerox...
You can see here that the variator is holding the revs at around 7000rpms (where a standard aerox makes peak power) *until* it reaches 33mph at which point it can no longer continue changing gear and the revs increase on a 'fixed gear slope' until it gets to 9000rpms at 42mph at which point the standard Aerox is no longer making enough power to continue to accelerate.
If you change roller weight the 7000rpms will change *until* it meets up with this 'fixed gear slope' as in the following graph...
This graph is of an unrelated moped but shows clearly how this fixed gear slope comes into play with different weight rollers. For all roller weights point 'A' shows the point at which the variator is fully changed out and the ped can only accelerate further if the rpm's increase. Although these points are at different rpm's they all meet on the same slope and this slope cannot be moved without changing the final gearing (e.g. gear kit, different size wheel etc.)
Ok, so now let's fit a Polini Sport 70cc kit and a Leo Vince ZX pipe to our Aerox.
The following graph shows how much power the most common 70cc sports kits make at different engine speeds with a ZX pipe fitted.
We can see that whilst our standard Aerox was making peak power at around 7000rpm's our 70cc Polini Aerox with ZX pipe is now making much more power, but it now makes this peak power at 9,300rpm instead of 7000rpm. This means we have to fit lighter rollers so as the variator runs at 9,300rpm.
If you look back to the first graph you can see a red dotted line graph added at 9,300rpm which simulates where the engine revs will be set when optimised for our new cylinder and exhaust for maximum performance.
However we've already discussed that it can only hold these revs until the variator is fully changed out at which point it hits our 'fixed gear slope line'. We can see on the first graph that this happens at 45mph with standard Aerox gears. We can therefore see it is impossible to exceed 45mph on a standard Aerox under any circumstances without the engine revs exceeding 9,300rpm.
If we refer back to the 3rd graph showing the power curve for our modified engine we can see the power developed by our Polini/ ZX engine very quickly drops off as the revs start to exceed 9,300rpm....
This means that it is therefore impossible to make full use of the power available from a Polini Sport/ ZX Aerox at any speed over 45mph without a gear kit fitted.
Without a gearkit you are very quickly completely out of the powerband over 45mph. At 50mph the engine is doing almost 11,000rpm and making around 6hp which is way down on peak power and although high enough to maintain 50mph, it isn't enough to significantly increase the speed much further. This means that the bike will spend most of its life over-revving at 11,000 just over 50mph. Because these sports kits aren't designed to run so high they are easily damaged by continual over-revving and the crank is put under more stress running at 11,000rpm under 6hp than it would have been running 9hp@ 9,300rpm.
For these reasons it's imperative that a gear-up kit is used with Malossi or Polini Sport 70cc kits.!!
The addition of a gear kit (up to 20% for this level of tuning) would shift the fixed gear slope significantly to the right allowing approx an additional 8 or 9mph of being held at 9,300rpm. This means we now have the full 9hp available right up to almost 55mph whereas without the gear kit we're down to 6hp (and falling very fast) at just 45mph.
Because once you get up around 55mph you need *significantly* more power to continue to accelerate than you do at 45mph the sports kits tend to limit themselves nicely as the power drops off when the revs finally do increase over 55mph with a gear kit fit and it's therfore not really possible to over-rev them
It's interesting to note that the 'mildest' sports kits i.e. malossi sport and polini sport are the ones where it is most important of all to have a gear kit fitted
The reason for this is that high power kits do not drop off in power at such low revs - e.g. if you look again at the power curve graph for the sports cylinder kits (our 3rd graph) you can see the Stage 6 one continues to make peak power all the way from 9000rpm to almost 11,000rpm. For this reason these sorts of kits will always destroy the Malossi and Polini sports 70cc kits on final top speed although at the price of running at higher revs (they should not however be run on a standard crankshaft as opposed to the Polini and Malossi kits which will happily run on a standard crank).
There is a widespread belief that you will loose acceleration when fitting a gear kit which is a trade off for the higher top speed but this isn't entirely true. This would be true on a single geared bike, if you had to choose only one gear to use on a geared bike then obviously 1st geared would accelerate best but have the lowest top speed... most people think therefore that fitting a gear kit is like selecting 2nd gear instead of 1st gear where you have slightly worse pull off but higher top speed.
Because of the variator mopeds don't work quite like this... the revs are still held at peak power, so in theory if your bike is making 9hp peak power and the bike is set up correctly then when you will still have 9hp available at 10mph when you mash the throttle regardless of what your final gearing is because the final gearing is being compensated for by the variator to have the same overall ratio as is necessary to keep the engine speed at our 9,300rpms. A bit like fitting a smaller gear on the back of a BMX only to compensate it with a smaller gear on the front also - net result is the same only you have the advantage of the higher overall top gearing.
In reality though the variator works less efficiently on initial pull away and struggles to be efficient until the moped is up and moving. Because of the higher final gearing the moped has to initially accelerate through a higher difference in speed until it is fully engaged and working efficiently which can affect pull away initially especially if the engine is not particularly powerful or well matched to the final gearing.
This however means that as soon as you exceed approx 15mph there isn't really any perceivable acceleration difference between a bike with or without a gear kit fitted. So unless you're frequently stopping and starting and in contrast to popular belief you can even quite happily use a gear kit on a standard 50cc bike with a decent powerpipe fitted.