Bob I think its weight related. So at 77kg like myself, AAAC and LB the resistance will climb up to about 10% then it won't get any harder no matter what the gradient goes up to. For yourself at a lighter weight the resistance will continue to climb to a higher gradient, perhaps 12%.
WARNING: layman struggling to remember A level physics spouting half understood concepts...
The more I look into this, the more factors are involved in the physics of the electro-magnetic brakes used in trainers like the bkool and KICKR, etc. and hence the more variables that are involved in what can be simulated. It's not like in the real world, where the major factors are gravity and aerodynamic drag, which have no practical limits to their predictable effects.
Maximum resistance is not "directly" related to weight, but to simulate a gradient for a given weight you would need a greater resistance for a greater weight, so maximum gradient is "indirectly" related to weight.
You would expect it should also be related to speed, in as much as the faster you can cycle at a given gradient, the sooner you will reach the maximum resistance, since the force you are producing is greater and therefore requires a greater resisting force. However, the physics imply this cannot be an entirely linear relationship...
In an electromagnetic trainer the resistance is created using circular eddy currents induced in a rotating metal disc by magnetic fields and these eddy currents are related to speed, so at slow speeds you get much lower resistance (as pointed out by AAAC and hence the on/off technique). As you increase speed the resistance increases, but at some point the resistance will then then reach a practical maximum and cannot increase any further.
A conductive surface moving past a stationary magnet will have circular
electric currents called
eddy currents induced in it by the
magnetic field, due to
Faraday's law of induction. By
Lenz's law, the circulating currents will create their own magnetic field which opposes the field of the magnet. Thus the moving conductor will experience a drag force from the magnet that opposes its motion, proportional to its velocity. The electrical energy of the eddy currents is dissipated as
heat due to the
electrical resistance of the conductor.
If you cycle very slowly, you get less resistance, but if you cycle very quickly you will reach a point where you can get no more resistance for a given system of magnets and conductor.
But if you are cycling slowly you are by definition getting less resistance, even when the electromagnets are on full, so you will have a lower maximum resistance than if you cycle quicker, until you reach the maximum resistance of the whole system, i.e. the eddy currents generated can produce no more resisting magnetic forces and dissipate no more heat.
So cycling slower will give a lower maximum resistance, but cycling faster can reach the maximum resistance of the whole system.
Weight is a bit easier to predict, as being heavier should always reduce the gradients at which these maximum resistances are met.
There must be a range of gradients for each weight that can be simulated over a range of speeds, but above
or below those speeds the resistance will reach a limit or begin to fall off.
A heavy/powerful rider will reach maximum resistance sooner than a lighter/less powerful rider, but a slower rider will have a lower maximum resistance, therefore the simulation will only be accurate between both a minimum and a maximum speed for each given riders weight and power.
Edit: This implies that a heavy slow rider will have a lower maximum gradient than a light fast rider, which seems to fit reported observations here.
This makes it hard to get a one size fits all rule for how steep a gradient can be simulated by a given trainer, as it depends on weight and speed/power, but also temperature (and humidity), airflow (for cooling), not to mention the usual manufacturing tolerances (and undoubtedly other factors not considered)!
The systems are limited by the maximum strength of the electromagnets, the size and magnetic properties of the metal disc, the maximum speed of the disc that the rider can maintain and the maximum heat the system can dissipate in the given environment. Since the energy is dissipated as heat, beyond a certain level the heat can no longer be dissipated and this will limit the energy and hence the resisting force.
Finally, an electromagnetic trainer with a larger rotating disk and/or stronger electromagnets should be able to produce larger resisting forces and hence simulate greater gradients, all else being equal.
These capabilities always tend to be quoted in Power but since power is force times velocity (when force and velocity are constant) this is misleading as it depends on velocity.
Where these capabilities are quoted as a maximum gradient, these are doubly misleading, as it also depends indirectly on rider weight.
So unless the maximum resistance is quoted in Newtons and the maximum gradient is for a specified weight (and speed) then take them with a pinch of salt.
However, I think it is safe to assume that a trainer with a higher quoted maximum power will be able to simulate a higher gradient for a given rider, unless they employ much more misleading marketing staff...
Geoff
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. Sorry to muck you about Geoff 
. I hope all who take part enjoy it 
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. Excellent work
Lost it after 5 words 
