What determines "aggressiveness"?

What is it that makes a razor more aggressive than another? Is it the amount of blade exposed from under the top plate or silo doors or is it the size of the gap between the safety bar and the blade edge? Or maybe a combination of both? I have a 40s style SS and a New Parker and the Parker is WAY more aggressive but I'm trying to figure out why.

On another note, do you find that a more aggressive razor requires a certain type of blade and a more mild one needs something different? I have found that Astras shave me much better in the Parker than the old Gillette.

gasdoc said:

What is it that makes a razor more aggressive than another? Is it the amount of blade exposed from under the top plate or silo doors or is it the size of the gap between the safety bar and the blade edge? Or maybe a combination of both?

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Yes...both blade exposure and blade gap contribute to the aggressiveness of the razor.

http://wiki.badgerandblade.com/Double-Edged_Safety_Razors_Ranked_by_Aggressiveness

I think that blade exposure and blade gap are biggest factors to the aggressiveness of the razor.
blade exposure, blade gap > blade angle >>> Razor weight and other factors

And the angle made between the edge of the blade and the safety guard when the razor is held vertically. The closer that angle is to 90 degrees, the more aggressive the razor. If you ever find one that is >90, you effectively have a straight razor.

I would argue that it's not so much gap as exposure. You can have a lot of exposure with almost no gap. Think of the Gillette Old Type for example. That's a very aggressive razor with almost no gap. A Progress set very low has much more gap than an Old Type but is very mild. While it is true that many aggressive razors have both a wide gap and a lot of blade exposure, it is not necessary for an aggressive razor to have a wide gap. Gap is not essential to aggressiveness. Exposure is. Remember, exposure is the amount the blade protrudes beyond the tangent between the head cap and the safety bar not the amount of gap between the safety bar and the blade.

Dead on. Gap is one way to push the blade past that tangent line (by moving the line). I will add however that, assuming you keep the safety bar on your face, a widening gap increases the blade angle against your skin. This also contributes to the aggressive feel of a razor. Unfortunately increasing blade angle also increases the chance for irritation, and generally reduces blade life.

Foyle said:

I would argue that it's not so much gap as exposure. You can have a lot of exposure with almost no gap. Think of the Gillette Old Type for example. That's a very aggressive razor with almost no gap. A Progress set very low has much more gap than an Old Type but is very mild. While it is true that many aggressive razors have both a wide gap and a lot of blade exposure, it is not necessary for an aggressive razor to have a wide gap. Gap is not essential to aggressiveness. Exposure is. Remember, exposure is the amount the blade protrudes beyond the tangent between the head cap and the safety bar not the amount of gap between the safety bar and the blade.

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I am going to have to give another vote for exposure. The reason, at least for me, why a razor is a aggressive is because the safety bar is not there to guide you. Essentially, a blade with a lot of exposure is like shaving with a straight because you have to rely on your ability to keep the angle against your face consistent. When you have little to no exposure then you have a guide that you can rest on your face without having to worry about the blade nicking you.

I also think blade gap does play a role but in the same sense as it exposes more of the blade. Not necessarily by making the blade protrude further out but by making you have to be more aware of the blade angle. But, I also believe that any razor can be shaved with by figuring out the angle of the blade due to the exposure. Essentially all a safety razor does is allow you to not have to worry so much about the angle you hold the razor at because you have a guard from the blade.