Diffs and "spider bite"
Posted: Thu Jul 16, 2015 8:59 am
I know this is completely out of left field, but I just had a brainstorm and thought I'd share it for discussion.
I've seen a lot of talk about open vs. torsen vs. locked diffs and "spider bite" on Audi forums, and this morning a little light bulb went off above my head re what's going on.
For simplicity's sake, let's consider a RWD vehicle that's going around an oval racetrack. And consider how each diff reacts to differing speeds on its outputs - specifically that locked and torsen diffs try to make both outputs spin at the same speed, and open diffs don't care.
Going down the straightaway, not much is happening. Both rear wheels, and thus both diff outputs, are turning the same speed and receiving the same amount of torque, and all is well regardless of what kind of diff is installed.
Now the car enters a turn, and suppose it has a torsen differential. Relative to the car's average speed (represented by the input to the diff), the inside wheel is turning more slowly and the outside wheel is turning faster.
The torsen diff does not like this, as it wants both of its outputs to rotate at the same speed. So initially it's going to respond by sending more torque to the inside wheel (which is the one with less traction due to cornering forces causing weight shift to the outside wheels) and less to the outside wheel. In other words, to a degree the diff is trying to make the inside wheel break loose. Of course it's trying to make the outside wheel break loose at the same time, but when a wheel actually breaks loose, it won't be the outside because that one has more traction.
Now because our hypothetical car is an Audi with one million horsepower (one of Hank's 07K engines), the inside wheel does in fact break loose. At that exact moment, it's still turning slower than the outside wheel, the diff is still unhappy about that, and thus is still sending the inside wheel more torque. This situation will not reverse until the inside wheel is spinning faster than the outside wheel. This means the torque shift will be a fairly violent event and will not happen until after the inside wheel has completely broken loose. I think that effect is what some people describe as "spider bite" in torsen-equipped cars.
I would think all the above would apply to a locked diff too, as it shares the torsen's characteristic of always trying to make both outputs spin at the same speed... it just tries harder.
So in my clueless opinion, it seems like right until the moment a tire breaks loose, an open diff is actually the best because it isn't actually *contributing* to the tires breaking loose (in a corner) like a torsen or a locked diff. Of course the moment a tire does lose traction, that all changes.
Re the location of the diff, the above would pretty much directly apply to a front diff also. With a center diff it's a little more complicated, but the basic principles and effects are the same.
I've seen a lot of talk about open vs. torsen vs. locked diffs and "spider bite" on Audi forums, and this morning a little light bulb went off above my head re what's going on.
For simplicity's sake, let's consider a RWD vehicle that's going around an oval racetrack. And consider how each diff reacts to differing speeds on its outputs - specifically that locked and torsen diffs try to make both outputs spin at the same speed, and open diffs don't care.
Going down the straightaway, not much is happening. Both rear wheels, and thus both diff outputs, are turning the same speed and receiving the same amount of torque, and all is well regardless of what kind of diff is installed.
Now the car enters a turn, and suppose it has a torsen differential. Relative to the car's average speed (represented by the input to the diff), the inside wheel is turning more slowly and the outside wheel is turning faster.
The torsen diff does not like this, as it wants both of its outputs to rotate at the same speed. So initially it's going to respond by sending more torque to the inside wheel (which is the one with less traction due to cornering forces causing weight shift to the outside wheels) and less to the outside wheel. In other words, to a degree the diff is trying to make the inside wheel break loose. Of course it's trying to make the outside wheel break loose at the same time, but when a wheel actually breaks loose, it won't be the outside because that one has more traction.
Now because our hypothetical car is an Audi with one million horsepower (one of Hank's 07K engines), the inside wheel does in fact break loose. At that exact moment, it's still turning slower than the outside wheel, the diff is still unhappy about that, and thus is still sending the inside wheel more torque. This situation will not reverse until the inside wheel is spinning faster than the outside wheel. This means the torque shift will be a fairly violent event and will not happen until after the inside wheel has completely broken loose. I think that effect is what some people describe as "spider bite" in torsen-equipped cars.
I would think all the above would apply to a locked diff too, as it shares the torsen's characteristic of always trying to make both outputs spin at the same speed... it just tries harder.
So in my clueless opinion, it seems like right until the moment a tire breaks loose, an open diff is actually the best because it isn't actually *contributing* to the tires breaking loose (in a corner) like a torsen or a locked diff. Of course the moment a tire does lose traction, that all changes.
Re the location of the diff, the above would pretty much directly apply to a front diff also. With a center diff it's a little more complicated, but the basic principles and effects are the same.
