i need some help. im always arguing with my friend about which type of drive losses more power to the wheels. does rwd or fwd loose more power before it reaches the ground.

 

It largely depends on the drivetrain, but generally FWD loses less power, as it does not have to change angles, excluding longitudinally mounted FWD powertrains.

 

also, fwd engines don't always have to spin the fan that cools the radiator

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beanbag &raquo;</TD></TR><TR><TD CLASS="quote">also, fwd engines don't always have to spin the fan that cools the radiator</TD></TR></TABLE>

Well, the fan is not really part of the drivetrain, and the losses from a viscous clutch are so close to the losses of an alternator to turn a high-amp fan motor, that they fall close to even in this respect.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 91DA9 &raquo;</TD></TR><TR><TD CLASS="quote">It largely depends on the drivetrain, but generally FWD loses less power, as it does not have to change angles, excluding longitudinally mounted FWD powertrains.</TD></TR></TABLE>

It's not that there isn't a change in 'angle' through which power has to pass, but that FWD doesn't need 'hypoid' gears in the differential. Hypoid crown wheel and pinion gears allow the pinion to mesh with the crown wheel lower on the crown wheel and thus lower on the differential casing, which in turn allows a significantly smaller transmission tunnel running through the car. If it weren't for hypoid gears the pinion gear would need mesh with the crown wheel and be mounted to the casing at half shaft height, requiring a much larger transmission tunnel.

However, hypoid gears have a a substantial degree of 'sliding' action between the gear teeth inherant in their operation, and this sliding action involves more frictional losses (than non hypoid gears) that consume some slight power (energy dissipated as heat), and also slightly increase fuel consumption.

This sliding action is why differential oils for RWD cars typically have 'hypoid' marked on the bottle (or an 'H' in the oil description), and this means they have special additives designed to deal with the high pressure sliding action involved with the hypoid gear operation, these additives being responsible for the distictive smell of differential oils.

FWD cars tend to be a bit lighter than RWD casrs (all else being equal), so this is somewhat like losing less power, i.e. better power to weight with FWD, though the difference isn't all that much.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by johnlear &raquo;</TD></TR><TR><TD CLASS="quote">

It's not that there isn't a change in 'angle' through which power has to pass, but that FWD doesn't need 'hypoid' gears in the differential. Hypoid crown wheel and pinion gears allow the pinion to mesh with the crown wheel lower on the crown wheel and thus lower on the differential casing, which in turn allows a significantly smaller transmission tunnel running through the car. If it weren't for hypoid gears the pinion gear would need mesh with the crown wheel and be mounted to the casing at half shaft height, requiring a much larger transmission tunnel.

However, hypoid gears have a a substantial degree of 'sliding' action between the gear teeth inherant in their operation, and this sliding action involves more frictional losses (than non hypoid gears) that consume some slight power (energy dissipated as heat), and also slightly increase fuel consumption.

This sliding action is why differential oils for RWD cars typically have 'hypoid' marked on the bottle (or an 'H' in the oil description), and this means they have special additives designed to deal with the high pressure sliding action involved with the hypoid gear operation, these additives being responsible for the distictive smell of differential oils.

FWD cars tend to be a bit lighter than RWD casrs (all else being equal), so this is somewhat like losing less power, i.e. better power to weight with FWD, though the difference isn't all that much. </TD></TR></TABLE>

You went into extensive detail as to the exact changing of direction I was talking about. The "EP" additives are mostly sulfur compounds and have a distinctive smell.

I am failing to see how using heavyweight oil and having to move the power flow 90° will not increase friction. You even said yourself hypoid gears are inherent to have more frictional losses.

While you are correct in the chassis weight generality, that has nothing to do with parasitic losses of a powertrain.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 91DA9 &raquo;</TD></TR><TR><TD CLASS="quote">You went into extensive detail as to the exact changing of direction I was talking about.
I am failing to see how using heavyweight oil and having to move the power flow 90° will not increase friction. You even said yourself hypoid gears are inherent to have more frictional losses.
</TD></TR></TABLE>

I'm not disagreeing with what you said, but you weren't "exact" about it, but rather vague and as a result your statement was a little misleading. You said "but generally FWD loses less power, as it does not have to change angles", which implies that the reason a RWD drivetrain does is due to the directional change that torque is obliged to take, which isn't actually correct.

If with a RWD car, instead of a hypoid gearset we had a 'normal' gearset mounted higher in the differential that also changed the direction of forces through 90° there would be no inherant power loss any greater than you'd get with a FWD differential, which does have a 'normal' gearset. A FWD differential doesn't change the direction of force, but this doesn't matter because it's not the change in direction that is responsible for the power loss in RWD differential, but the sliding action in the hypoid gears (i.e. sliding = friction).

Despite lubrication, a hypoid gear is frictional in the same way but to a lesser degree than a 'worm gear' is. A worm gear works by sliding the spinning 'worm' shaft 'teeth' through the teeth of a rotating gear at an acute angle, but since there is so much sliding and thus friction they aren't usually used for higher speed / loading applications. From memory I think Peugot used to use worm gears in differentials at least for a while, but abandoned them due to poor effeciency. A hypoid gear is an acceptable compromise used purely to create a less obtrusuve transmission tunnel.

The sliding effect at the hypoid gear 'interface' is inherantly 'frictional' and is what saps power, and would do so regardless of the oil viscosity, though thicker oil certainly won't help matters (though thinner oil may cause even more friction if it can't maintain an adequate oil film).

If you still think that it's the change in direction itself that causes a power loss, can you explain the physics behind the phenomenon?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 91DA9 &raquo;</TD></TR><TR><TD CLASS="quote">While you are correct in the chassis weight generality, that has nothing to do with parasitic losses of a powertrain. </TD></TR></TABLE>

I know. I merely mentioned it because it had some bearing on the slight 'power' benefits of FWD vs RWD, even though it has nothing to do with parasitic losses but rather with power to weight.