This is a basic introduction to dynos and moped tuning. This blog just covers the basics of what a dyno is and the difference and limitations between different dynos. It is only really basic, but the next blog will cover some actual basic moped tuning and moped setting up using a dyno.

A dyno is something that is normally out of reach for most scooter or moped tuning workshops within the UK. The cost simply isn't financially viable for the returns. The market for moped tuning doesn't exist commercially in the UK, the biggest moped racing class in the UK has hardly more than 10 entrants per year meaning that except for perhaps a small specialist shop the main commercial market for moped tuning would be 16yr old school kids with an Aerox or Piaggio Zip which they want to go faster than their mates' bikes!

Given the large investment and running costs of a dyno, dyno moped tuning is clearly not a golden goose commercially and reserved solely for the pure enthusiasts

We are in the lucky position that because we also setup superbikes which involves a lot of fuel injection work and requires the use of a high end eddy brake dyno, we have a dyno which normally wouldn't otherwise be accessible to mopeds. This is because the hourly rate these dynos command can quickly exceed the value of a moped, and normally the sorts of workshops or race teams owning them are superbike specialists with no interest or experience in mopeds

Whilst a few moped tuning shops can afford a basic dyno they tend to be budget intertia models. An intertia dyno works by measuring the time taken to accelerate a load (i.e. how long it takes for the moped wheel to accelerate the dyno drum) and then mathematically calculates a hp reading over the speed increase. So basically you start it recording and record a single run with wide open throttle (WOT) as it accelerates to top speed.

Basic inertia dynos are certainly useful but have a number of limitations for moped tuning. Firstly the actual hp readings vary widely between them and need to be taken with a pinch of salt, but for the tuner the accuracy of the readings actually aren't too important, you just need a comparison so as you can compare changes you make to the moped or bike to see the effect. So if you tweak your timing or change your carb all you really need to see is a comparison of before and after with the actual figures not being too important.
This introduces the first problem, engines develop different power depending on atmospheric conditions. For example in cold weather the same volume of air contains more oxygen than it does in hot weather, so if you dyno your moped on a warm day and then dyno it again the next day with a new exhaust when it's cold and wet how do you know if any changes are due to atmospheric changes or to the exhaust?

Because of this it's necessary to have some sort of climate monitor when moped tuning. Our dyno has an external weather station which will measure humidity, pressure, temperature and relative air density and from this data will calculate 'corrected' compensated hp figures. With this system you can get virtually identical readings when moped tuning whether in the snow, in the summer or on top of a mountain (!)

Another absolute basic requirement of a dyno is to have an air/fuel ratio monitor (AFR). This is how you tell if the bike is running lean or rich. The AFR is the ratio of the mass of air compared to the mass of fuel. With petrol the ratio of air required to completely burn all the fuel is approx 14:1. With moped tuning we find that normally the moped will make maximum power at around 13:1. The AFR (mixture) is measured by a probe which goes into the exhaust. When moped tuning we normally are only really interested in the mixture when the bike is running at full throttle and under load (as if it was riding on the road with a rider) so in basic terms it's just a matter of changing the main jet until we get a consistent reading of 12.5 - 13. Also at this slightly lower ratio the mixture is richer which helps cool the cylinder and piston to prevent seizures.

The next problem with budget end dynos is that mass of the dyno drum is often very low. This means that it's easy for the drum to be spun up and the moped will accelerate faster than it would in real life. Because of this the moped engine doesn't experience the same load or strain as it would in real life which can adversely affect the relevance of any readings and tuning results which may not be duplicated in real life and allows the moped to run at unrealistic speeds. Imagine tuning your moped and testing it just by doing downhill runs- if you set everything up like ignition timing at this lighter load when you finally decide to go uphill and put some real load on it you may find things very different or even disastrous !

The drum mass of our dyno is approx 500kgs (almost the same as a small car !) This ensures realistic loads are placed on the tested bike even when running in inertia mode.

Besides this our dyno has a load cell eddy brake. This is an extremely useful (but very expensive!) piece of kit which can be fitted to high end dynos. The load cell allows the speed of the dyno drum to be computer controlled by extremely powerful electromagnetics. This adds a multitude of options to the dyno tuner. For example you can program it until it exactly replicates the performance, acceleration and top speed of the bike in the real world. In the real world the air resistance against the bike increases by 4 times each time the speed of the bike doubles. This can be accurately replicated by the load cell. Perhaps you're tuning for a hill climbing event, again you can realistically replicate the real world higher loadings to get realistic tuning results

Say for example when moped tuning you want to see the effect of changing the ignition timing on power or torque at different engine speeds. An inertia dyno can only measure power whilst the drum is accelerating so you would do a full run which sweeps through the rpm you're interested in and read the graph to get a reading and keep repeating with different settings. With the load cell you just set the drum to hold the engine at say 7000rpms. No matter how hard you throttle or how powerful the bike, the magnets will hold the drum at the speed which will keep the engine at 7000rpms. It will continuously read a 'live' hp reading at this steady rpm calculated by how big the load the magnets have to apply to hold the revs steady. You can then hold the throttle fully open and adjust the timing whilst observing the effect on power, or then repeat at different throttle positions or different rpms and see instant live comparison readings.

It's almost impossible to setup fuel injection or race bikes without a load cell or eddy brake dyno. With moped tuning, when setting up a carb at a basic (but perfectly acceptable for road use) level you only really need to pick the correct main jet so as the mixture is correct at full throttle under full load. Because the variator holds the revs fairly constant this is fairly easy. Eventually the moped will reach a speed where the variator has finished changing all the way out the moped revs will inevitably increase which can then affect the mixture. Normally we make sure that at this point the mixture becomes richer as the revs increase for engine safety reasons, but otherwise moped carbs are very forgiving and it's normally just a case of finding the best main jet for normal road use.
Motorbike carbs can be trickier, because of the higher power of motorbike engines they are often ridden at different throttle positions (a moped throttle normally spends its life fully open) and across a wider range of rpm- the jet that works perfectly at full throttle at 7000rpms also has to be made to supply the correct mixture as well as possible also at half throttle at 5000rpms for example.

With fuel injection you can have much more control (not including fuel injected mopeds!). With an injected superbike we can plug a laptop into the bike fuel injection system and adjust the mixture to make it leaner or richer at any rpm and at any throttle position. With our load cell dyno we can then program the eddy brake to allow the engine to increase in steps of 500rpms and hold it there for a few seconds before stepping up the next 500rpms. So we might start at 10% throttle and set the 'jetting' (mixture) correction with the laptop using the dyno air/fuel analyser at every rpms so as it is perfect and then repeat the whole process over and over at other throttle positions. In total there are around 250 different rpm and throttle positions combinations all of which you can precisely set the mixture at. The whole process can take up to a couple of hours but at the end of it you know that whatever throttle position and whatever revs the mixture is always absolutely perfect. The result is an exceptional smooth ride with maximum performance when accelerating from any throttle position at any revs.

There is actually a fairly new breed of low intertia dyno which does in fact use a very low weight drum. It does however bear little resemblance to the budget low drum weight dynos as it uses an eddy brake module to replicate the effects of a much heavier drum and all the other benefits of a load cell dyno and certainly doesn't fall into the budget category !

Next time we'll show some basic dyno moped tuning examples on customer mopeds complete, to demonstrate how quickly and easily the performance of a moped can often be transformed simply by setting it up correctly !