Moped tyre pressures vs performance.

This is a short blog covering the often overlooked effect on performance of tyre pressures.
Mopeds and scooters are affected more significantly than geared bikes by low tyre pressures and consequently it's common for a moped to be booked into our workshop for poor running where low tyre pressure is a significant factor of poor setup. Sometimes we even see them where there is nothing wrong at all other than the tyres pressures have been neglected!

It's fairly obvious that the more a tyre is under inflated the more power will be lost and the less efficiently the moped will run. A significant part of this is simply that where a tyre sits on the ground, that area of the tyre is deformed. As the tyre turns it consequently has to reform and deform the tyre. This requires energy which in turn heats and wears the tyre, so the flatter the tyre the more energy is required to turn it which is also exacerbated by the weight of the bike and rider.
This can be demonstrated by the much larger effort required when pushing a moped with a flat tyre.

This drivetrain loss is fairly uniform and can be seen in the graph below when you look speeds from 25mph upwards. Each change in tyre pressure results in a fairly predictable change in the power developed at the rear wheel. There is a significant loss in power in this area once the tyre pressure gets down to 20psi but nothing too serious at higher pressures.
This region above 25mph is basically what you would expect to see on a geared bike except that on a geared bike this loss would extend fairly uniformly across the entire rev range. It isn't quite uniform as centrifugal forces tend to 'round' out the wheel as speed increases but it's not far off in these speed ranges.

We mentioned already that mopeds can behave disproportionately adversely to low tyre pressures compared to geared bikes and we can see this happening in the graph below at speeds below 25mph.
At 35mph the 25psi results in only a 3.5% loss in power compared to the tyre being at 35psi (6.75hp vs 7hp)
However at 15mph the 25psi results in a massive 37.5% loss in power compared to the tyre being at 35psi (3.25hp vs 5.2hp)
3.25hp @ 15mph is less than a standard derestricted Aerox would make at this speed, so for this particular bike (with a Leo Vince ZX pipe fitted) just by allowing the rear tyre to drop to 25psi would result in being out accelerated by a standard Aerox !
In fact the resulting losses can be even worse than this because the graph only takes into account the rear wheel being low.

click to enlarge...

So what's happening below 25mph?
Well basically the variator is being delayed in the time it takes to allow the engine to reach peak operating power rpm at these speeds... so the engine is revving lower than it should be in these sections of the graph at an rpm where there is much less power developed until eventually it makes it through to peak rpm and the bike is finally away.
Although ideally the variator should allow the engine to rev straight to peak power whenever you mash the throttle, moped variators (and particularly standard ones) tends to always engage progressively meaning that it requires a fair amount of power to initially get it there.
On a sports pipe and particularly the higher performing ones (and especially in combination with a worn/ standard variator/) this progressive application means that due to the pipe developing lower power than a standard pipe at low revs it struggles badly to pull through to get to the higher revs.
It can therefore be noted that the effect would be lessened if there was a standard exhaust fitted or a quality aftermarket variator kit (e.g. Stage6, Malossi) fitted which is designed to work well with performance exhausts as they can pull through the lower revs much more efficently.

Or in other words, keep your tyres pumped up !

*Please note this blog does not consider the affect of tyre pressures on handling performance*