***Disclaimer***
This is the property of me (Yoshi Jeffery). Please don't copy and paste this elsewhere. Quote sections are ok but please link back to this thread or e30performance.info to give me credit
I have seen many threads and posts in the past about the pros and cons of welded differentials. The main pro is that it is cheap. The main "con" I see is initial understeer and that is about it. While both of these are valid points, neither are definitive and neither side seems to ever have proof. The majority of this article is based on an SAE article I got from http://www.sae.org/ on handling characteristics of welded differentials on track. This does not cover drifting but the same principals apply.
Differences between tightly packed LSDs and welded diffs:
Lsds will break tires loose at the same time. Welded differentials will break the inside wheel loose first. For a full explanation of this, see my previous article on LSDs. Below a specific torque, both wheels are powered. Once the wheels break loose, they are driven independently. Welded differentials will always spin each wheel at the same speed. As a result, one wheel will break loose first in a corner.
Tightly packed LSDs help corner exit while welded diffs cause slower exit speeds. This is due to scrubbed tires throughout the corner "fighting" and thus slowing down the car. BUT as a result of this effect, while oversteering, it is easier to manage. IE throttle inputs have less effect on oversteer.
Welded differentials have both tires fight traction. IE the inner wheel spins faster and propels the car forward while the outside tires spin slower and actually push the car backward. This tries to "right" the car straight again. This causes the alleged understeer.
The understeer and oversteer effects are less on larger radius corners. This is fairly simple. The closer to straight the corner is, the less the welded will react differently from an open diff or lsd.
Cons of welded diffs:
Welded differentials are more likely to snap oversteer on throttle lift. This effect is more noticeable on decel but any quick changes input will quickly change the speed of the tires, shifting weight instantly. As a result, any change in inputs need to be gradual and deliberate.
More likely to break axles. The high torque difference between sides makes IRS type suspensions, at least than ideal axle angles, break with less abuse than LSDs.
Welded diffs give less total available grip. Therefore, their drifts and traction movements are slower overall. Due to un-ideal movement of tires, the rear tires will slip sooner and unevenly. This means starting a drift will be slower using strictly power/decel and over all drift speeds will be slower. More grip means a faster drift.
Welded differentials create outward lateral acceleration. This is a hard one to explain. The image way below will help. The outside tire, on turn in, will decel and pull the car to the outside of the corner in the rear. The inside tire will have the most influence on the direction of the car but also overpower the front tires.
Welded differentials increase weight transfer due to the outside tire. This decreases the side force generation possible on that axle. IE slower drifts and the outward accel previously mentioned.
Due to this effect, the inside tire has less load on it and slips much easier. This slightly increases understeer.
Welded differentials are only 100% ideal in a slim section of corners. Mainly gentle large radius corners.
Welded differentials are much more sensitive to sudden inputs. As mentioned above, abrupt input changes will make a car snap oversteer or spin out altogether.
WDs with a decently high roll couples will undresteer at first, then oversteer around 1G, then understeer again. the rear tires will still have some grip but the G forces will overpower the front tires.
Initial Understeer (fact).
Below is a picture of the understeer moment on turn in with a welded differential. The size of the circles are directly proportional to the load on the tire. The arrows are pointing in the direction of movement of the tire.
the front tires are creating a slight braking force but pointing the car in the appropriate direction for the corner. The outside rear tire is actually creating a significant braking force due to rolling resistance. The inside rear is giving us the forward appropriate momentum. Again, the inside rear tire has less load on it and can break loose easier. This is the "fighting" force that tries to right the car to go straight that causes understeer.
Biblio
SAE article: Oversteer/Understeer Characteristics of a Locked Differential (942485)
This is the property of me (Yoshi Jeffery). Please don't copy and paste this elsewhere. Quote sections are ok but please link back to this thread or e30performance.info to give me credit
I have seen many threads and posts in the past about the pros and cons of welded differentials. The main pro is that it is cheap. The main "con" I see is initial understeer and that is about it. While both of these are valid points, neither are definitive and neither side seems to ever have proof. The majority of this article is based on an SAE article I got from http://www.sae.org/ on handling characteristics of welded differentials on track. This does not cover drifting but the same principals apply.
Differences between tightly packed LSDs and welded diffs:
Lsds will break tires loose at the same time. Welded differentials will break the inside wheel loose first. For a full explanation of this, see my previous article on LSDs. Below a specific torque, both wheels are powered. Once the wheels break loose, they are driven independently. Welded differentials will always spin each wheel at the same speed. As a result, one wheel will break loose first in a corner.
Tightly packed LSDs help corner exit while welded diffs cause slower exit speeds. This is due to scrubbed tires throughout the corner "fighting" and thus slowing down the car. BUT as a result of this effect, while oversteering, it is easier to manage. IE throttle inputs have less effect on oversteer.
Welded differentials have both tires fight traction. IE the inner wheel spins faster and propels the car forward while the outside tires spin slower and actually push the car backward. This tries to "right" the car straight again. This causes the alleged understeer.
The understeer and oversteer effects are less on larger radius corners. This is fairly simple. The closer to straight the corner is, the less the welded will react differently from an open diff or lsd.
Cons of welded diffs:
Welded differentials are more likely to snap oversteer on throttle lift. This effect is more noticeable on decel but any quick changes input will quickly change the speed of the tires, shifting weight instantly. As a result, any change in inputs need to be gradual and deliberate.
More likely to break axles. The high torque difference between sides makes IRS type suspensions, at least than ideal axle angles, break with less abuse than LSDs.
Welded diffs give less total available grip. Therefore, their drifts and traction movements are slower overall. Due to un-ideal movement of tires, the rear tires will slip sooner and unevenly. This means starting a drift will be slower using strictly power/decel and over all drift speeds will be slower. More grip means a faster drift.
Welded differentials create outward lateral acceleration. This is a hard one to explain. The image way below will help. The outside tire, on turn in, will decel and pull the car to the outside of the corner in the rear. The inside tire will have the most influence on the direction of the car but also overpower the front tires.
Welded differentials increase weight transfer due to the outside tire. This decreases the side force generation possible on that axle. IE slower drifts and the outward accel previously mentioned.
Due to this effect, the inside tire has less load on it and slips much easier. This slightly increases understeer.
Welded differentials are only 100% ideal in a slim section of corners. Mainly gentle large radius corners.
Welded differentials are much more sensitive to sudden inputs. As mentioned above, abrupt input changes will make a car snap oversteer or spin out altogether.
WDs with a decently high roll couples will undresteer at first, then oversteer around 1G, then understeer again. the rear tires will still have some grip but the G forces will overpower the front tires.
Initial Understeer (fact).
Below is a picture of the understeer moment on turn in with a welded differential. The size of the circles are directly proportional to the load on the tire. The arrows are pointing in the direction of movement of the tire.
the front tires are creating a slight braking force but pointing the car in the appropriate direction for the corner. The outside rear tire is actually creating a significant braking force due to rolling resistance. The inside rear is giving us the forward appropriate momentum. Again, the inside rear tire has less load on it and can break loose easier. This is the "fighting" force that tries to right the car to go straight that causes understeer.
Biblio
SAE article: Oversteer/Understeer Characteristics of a Locked Differential (942485)
Comment