What is the difference between the unbranded-cranks found on the Motobecane- USA- Mirage Sport vs the FSA Vero cranks(sometimes LSC) on the Motobecane-Record or Grand Record? Are they basiclly the same thing quality wise?
In general, there are several factors that go into a crankset:
- Mounting: What kind of bottom bracket do they go on? Square Taper, Octalink, etc. Are they cottered? cotter-less? one piece?
- Single/Double/Triple: How many chainrings do they have?
- Chainring Shape: Some chainrings like the Biopace are somewhere between a rounded rectangle and a circle rather than circular. Biopace is supposed to be more comfortable than the usual circular rings, for example. You can also design different tooth shapes to improve shifting performance. Other chainrings seem to have slightly different vertical profiles (SRAM X-glide for example).
- Chainring Type: How are the chainrings mounted? Are they fixed into the crankset? Are they 110 BCD and replacable?
- Chainring Material: Usually Aluminum for the outer two, Steel for the inner for a MTB triple, but other options are possible. This affects durability and weight- just because you're using the same materials doesn't mean the quality is the same.
- Crank length: 170 mm is probably most common.
- Chainring size: A compact road double is usually 50t/34t, for example - different chainring sizes give you different gearings.
- Q-factor/Tread: Width of the crankset. This will affect comfort and is usually narrow to match how we walk.
- Crank Profile: The angle of the crank arms leaving the bottom bracket. If you ride with your toes outward, this prevents interference and affects Q-factor.
- Crank arm material: Affects durability, strength, weight.
- Power Meter: Some high end cranksets have a power meter built into them for measuring riding power which can be sent to your cycling computer.
- "Speeds": Often these days, cranksets are marketed with a different number of speeds in the back in mind. Mostly marketing.
- Pedal type: Pedals come in 1/2" or 9/16". You need to have the right sized holes for the pedals to screw into, for a given set of pedals.
Between manufacturers of compatible cranksets (i.e. they can be mounted to the same bike and have essentially the same geometry (Q-factor, crank length, number of chainrings and chainring sizes), there are likely to be different durabilities of chainrings, weights of the crankset assembly and aesthetics.
In your particular case, I'd guess the non-name brand one is probably a bit heavier and has softer material for the chainrings so they don't necessarily last as long. The geometry may also be slightly different. Its hard to tell with Motobecane when the parts aren't exactly specified since something just shows up on the bike depending on whats in the factory that day which meets the specs stated online.