Electric Bikes - One motor that will accept different voltages?

I have seen some ebike kits that advertise that you can later change the voltage of their motor by changing the controller. Can someone tell me what about a motor will allow you to do this? Is it a certain kind of motor? Brushed? Brushless? some special technology? or just a well built, hefty motor?

Nothing special it is just a motor that can take the higher voltage then the controller they sell in the less expensive kit.
Electric motors work up to a rated voltage well and can even be run at a higher voltage with care depending on the quality. The limit mostly has to do with the insulation and parts used. If it is a permanent magnet motor which usually gives a specific number of RPMs per a volt the max voltage could be influenced by the max rpm rating of some parts.

In most electric vehicles I have ever had if you are running a 48volt rated motor with a 48volt nominal max controller at half throttle you are sort of only putting 24 volts to the motor from the batteries. So if you started off with a 36volt controller it would run just fine. And if the only thing different was the voltage on the controller in the whole system and both were 10 amp controllers you'd have a roughly 500watt kit at 48 volts and a 360watt kit at 36 volts

Nothing special it is just a motor that can take the higher voltage then the controller they sell in the less expensive kit.
Electric motors work up to a rated voltage well and can even be run at a higher voltage with care depending on the quality. The limit mostly has to do with the insulation and parts used. If it is a permanent magnet motor which usually gives a specific number of RPMs per a volt the max voltage could be influenced by the max rpm rating of some parts.

In most electric vehicles I have ever had if you are running a 48volt rated motor with a 48volt nominal max controller at half throttle you are sort of only putting 24 volts to the motor from the batteries. So if you started off with a 36volt controller it would run just fine. And if the only thing different was the voltage on the controller in the whole system and both were 10 amp controllers you'd have a roughly 500watt kit at 48 volts and a 360watt kit at 36 volts

Dead on, really. For a lot of hub motors, you can increase your voltage simply with a controller upgrade. Crystalyte is a very good mention here, as their 4-series kits are very reliable at 48+v and the 5-series motors have been pushed as far as 72v with no problems. A motor at that voltage would be overkill for any practical application though, I find :D

So far as controllers, the 407 and 408 hub motors will also take 48v with the standard 36v/20a controller included in your kit. I can attest to this personally. You can always go online to a store that will sell the parts individually and buy the higher capacity controller you need without ending up with the stock 36/20 if you know you'll want more right away, and save money. Ebikes.ca is definetly a good source.

Of course, you'll also need a higher voltage battery to match your desired output. If you have a 3 battery 36v power pack already you can always add a 4th battery, given that its the same chemistry and capacity. However, the weight of these packs can become astronomical, I have a 48v/10ah SLA pack and its weight is very cumbersome for packing around. Lighter chemistry or lower capacity is a good idea.

A few things to remember about increasing voltage:

Maximum speed increases considerably, as does torque at increased throttle. Efficiency decreases however, simply due to increased wind resistance.

Remember too, that a 48v/8ah battery contains much more power than a 36v/8ah battery. Multiply volts times amps to find watt-hours, which is how much power the batteries will have. Thus if you use the bike to move at the same speed, a 48v battery will go much farther than a 36v will.

Guess it should also be mentionned that brushed motors used with more power wear their brushes faster, and they'll really eat them up if you push them too far.