Need some help understanding diffs
06-12-2007, 02:39 PM
#1
Need some help understanding diffs
As I was trying to answer a question about diff oil yesterday, I started to realize that my knowledge in this particular area is quite lacking. Although I understand the basic principles of what increasing and decreasing the diff oil weight does (see afm's post here), I really don’t fully understand the mechanics and physics behind what is happening with the car itself. I hope you guys can help me get it straight and help educate everyone else as well.
I found some very helpful information that explains how an open diff works, but it didn’t cover everything:
http://www.rctek.com/general/differe...they_work.html
As I understand it, the diff helps deal with the fact that when the car turns, the outside wheel has a farther distance to go than the inside wheel and therefore the outside wheel rotates more than the inside wheel while turning. The diff also controls how fast or slow the inside and outside wheels rotate relative to each other and how much power is applied to each wheel. It is also important to understand the weight transfer to the outside wheel and how much traction is at each wheel as a result of this weight transfer.
Selecting the proper diff oil has to do with traction of the track, tire shore, track layout (sharp turns and 180s vs flowing), body roll, being on-throttle and off-throttle and probably a few other things that I've left out. Diff oil selection also impacts how the car performs during corner entry, mid-corner and corner exit – and corning speed.
Lighter diff oil provides less internal resistance within the diff, so the gears inside the diff spin faster. Heavier diff oil provides more internal resistance in the diff, so the gears inside the diff spin slower. In a turn, using a lighter oil causes the outside wheel to rotate much faster than the inside wheel. As you increase the weight of the oil, you slow down how fast the outside wheel rotates relative to the inside wheel. When you use very heavy oil, the diff starts to act like a solid, so there is less "differential action" between the outside and inside wheel and they rotate closer to the same speed. As the diff oil gets heavier, you also start to lose some corning speed. With lighter oil, if one wheel loses traction, you get less power to the remaining wheels than if you use a heavier oil.
Fill in the blanks…
BTW… If you use the term “power to the ground”, please explain what that means relative to the rotational speed of the tire and the level traction.
On throttle, on a track with medium traction, when the rear diff oil is too heavy, the rear of the car may break loose and come around because (fill in the blank). Another reason the car may break loose is because the rear tire shore is too high. If the diff oil is too heavy, you want to use lighter diff oil because (fill in the blank).
On throttle, on a track with high traction, when the rear diff oil is too heavy, the car will start to push because (fill in the blank). In this case, you want to use lighter diff oil because (fill in the blank). Another reason the car may push, is because the rear tire shore is too low. If the diff oil is too light, (what happens?) because (fill in the blank).
The reason why you start to lose cornering speed with heavier diff oil is because (fill in the blank).
Questions:
Thanks for the help guys!
I found some very helpful information that explains how an open diff works, but it didn’t cover everything:
http://www.rctek.com/general/differe...they_work.html
As I understand it, the diff helps deal with the fact that when the car turns, the outside wheel has a farther distance to go than the inside wheel and therefore the outside wheel rotates more than the inside wheel while turning. The diff also controls how fast or slow the inside and outside wheels rotate relative to each other and how much power is applied to each wheel. It is also important to understand the weight transfer to the outside wheel and how much traction is at each wheel as a result of this weight transfer.
Selecting the proper diff oil has to do with traction of the track, tire shore, track layout (sharp turns and 180s vs flowing), body roll, being on-throttle and off-throttle and probably a few other things that I've left out. Diff oil selection also impacts how the car performs during corner entry, mid-corner and corner exit – and corning speed.
Lighter diff oil provides less internal resistance within the diff, so the gears inside the diff spin faster. Heavier diff oil provides more internal resistance in the diff, so the gears inside the diff spin slower. In a turn, using a lighter oil causes the outside wheel to rotate much faster than the inside wheel. As you increase the weight of the oil, you slow down how fast the outside wheel rotates relative to the inside wheel. When you use very heavy oil, the diff starts to act like a solid, so there is less "differential action" between the outside and inside wheel and they rotate closer to the same speed. As the diff oil gets heavier, you also start to lose some corning speed. With lighter oil, if one wheel loses traction, you get less power to the remaining wheels than if you use a heavier oil.
Fill in the blanks…
BTW… If you use the term “power to the ground”, please explain what that means relative to the rotational speed of the tire and the level traction.
On throttle, on a track with medium traction, when the rear diff oil is too heavy, the rear of the car may break loose and come around because (fill in the blank). Another reason the car may break loose is because the rear tire shore is too high. If the diff oil is too heavy, you want to use lighter diff oil because (fill in the blank).
On throttle, on a track with high traction, when the rear diff oil is too heavy, the car will start to push because (fill in the blank). In this case, you want to use lighter diff oil because (fill in the blank). Another reason the car may push, is because the rear tire shore is too low. If the diff oil is too light, (what happens?) because (fill in the blank).
The reason why you start to lose cornering speed with heavier diff oil is because (fill in the blank).
Questions:
- Would you ever want the front and rear diff oil to be the same? Why or why not?
- Would you ever want the front diff oil to be lighter than the rear diff oil? Why or why not?
- Why would you want to use a front diff with very heavy oil instead of using a solid? Why not just use the solid?
- When the track temperature reaches a point that the tires start to lose traction, can changing the diff oil improve traction?
- Should you ever change your diff oil to deal with a traction-rolling problem?
Thanks for the help guys!
06-13-2007, 01:10 AM
#2
i think it is not easy question..
and.. frequent changing diff oil test is not also easy.. (bothersome ^^)
2. Why would you want to use a front diff with very heavy oil instead of using a solid? Why not just use the solid?
-> most of RC user know as Front hard diff is more steering than
front spool.
and front spool is used in very low grip.
But japanese driver who like anomaly enjoy to use front spool.
they know how to go well with front spool as like one-way.
front-spool had merit like full-braking, body check,
and very easy driving.
3. When the track temperature reaches a point that the tires start to lose traction, can changing the diff oil improve traction?
-> i changed diff setup in FEMCA 2007 (over ground temperature 60C) But i got a little effect..
4.Should you ever change your diff oil to deal with a traction-rolling problem?
-> i quess... maybe not..
5. Would you ever want the front and rear diff oil to be the same?
Why or why not?
-> i tested front 20000/rear 20000, front 10000/10000 in 3-4 years ago.
i remember my car go well in that setup..
But i was novice driver 3-4 years ago.
so i can't say that setup was great!!
and.. frequent changing diff oil test is not also easy.. (bothersome ^^)
- Would you ever want the front diff oil to be lighter than the rear diff oil? Why or why not?
2. Why would you want to use a front diff with very heavy oil instead of using a solid? Why not just use the solid?
-> most of RC user know as Front hard diff is more steering than
front spool.
and front spool is used in very low grip.
But japanese driver who like anomaly enjoy to use front spool.
they know how to go well with front spool as like one-way.
front-spool had merit like full-braking, body check,
and very easy driving.
3. When the track temperature reaches a point that the tires start to lose traction, can changing the diff oil improve traction?
-> i changed diff setup in FEMCA 2007 (over ground temperature 60C) But i got a little effect..
4.Should you ever change your diff oil to deal with a traction-rolling problem?
-> i quess... maybe not..
5. Would you ever want the front and rear diff oil to be the same?
Why or why not?
-> i tested front 20000/rear 20000, front 10000/10000 in 3-4 years ago.
i remember my car go well in that setup..
But i was novice driver 3-4 years ago.
so i can't say that setup was great!!
Last edited by gansei; 06-13-2007 at 01:11 AM. Reason: miss typing
06-13-2007, 02:51 AM
#3
For my preference I prefer front diff slightly harder then rear. The reason is depend on your driving style. For me most of the time if the corner is not very tight and enought to have half on trottle to get throught especially big sweeper will be advantage on the steering. Front diff slighly harder will give a bit extra steering at on trottle mode.
For the rear its depend on the track's traction and also the driving style wheater or not the driver like to on trottle hard out of the corner.
I don't really prefer hard front diff because most of the track they will be a small corner that you need to turn with off trottle and here a lighter front diff will gives you better turning-in response. If you again set the front diff too light then you might have lesser acceleration or on trottle steering out of the corner. The reason here is because too light front diff or rear diff there will be an over power distributed to either side of the wheel instead both side equally. ie LSD effect. When over power goes to one side you will have a lot of wheel spin but it only spin on one side of the wheel, loosing power.
sometimes it may not be any different of handling between using very hard front diff and solid. Its depends on the traction that you have on the track. If the traction that is good enought too make your very hard diff working then it maybe a different in handling otherwise it is not different the runing solid.
Again also there is a different between gear diff and ball diff. The effect will be different.
hope that helps
For the rear its depend on the track's traction and also the driving style wheater or not the driver like to on trottle hard out of the corner.
I don't really prefer hard front diff because most of the track they will be a small corner that you need to turn with off trottle and here a lighter front diff will gives you better turning-in response. If you again set the front diff too light then you might have lesser acceleration or on trottle steering out of the corner. The reason here is because too light front diff or rear diff there will be an over power distributed to either side of the wheel instead both side equally. ie LSD effect. When over power goes to one side you will have a lot of wheel spin but it only spin on one side of the wheel, loosing power.
sometimes it may not be any different of handling between using very hard front diff and solid. Its depends on the traction that you have on the track. If the traction that is good enought too make your very hard diff working then it maybe a different in handling otherwise it is not different the runing solid.
Again also there is a different between gear diff and ball diff. The effect will be different.
hope that helps
06-13-2007, 07:32 AM
#4
Originally Posted by gansei;3394143
4.Should you ever change your diff oil to deal with a traction-rolling problem?
[B
4.Should you ever change your diff oil to deal with a traction-rolling problem?
[B
-> i quess... maybe not..[/B]
Your first fix should be sway bars either add them or stiffen them or switch to firmer tires, but I always prefer to keep as much tire grip as possible so that the car will handle more throttle on exit.
Next you can fine tune it ride height and roll center changes. Lowering the ride height or reducing roll center will take some chassis roll out and effectively reduce the traction rolling issue. If you were to change the diff oil to help any, I would sway toward the thinner direction. It will help free up the drag on the outside wheel thus slowing the transfer a bit, but probably not enough to eliminate the problem. However, stiffer diff oil is very hard on the internals if the car is unloading in the corners. Every time the inside lifts and spins the tire, the shock to the internals, when it hits the ground again, is much harder with thicker oil. I suspect this is what snapped my gear in my MTX-3 last summer.
06-13-2007, 03:48 PM
#5
to me it all depends on how you want your car to handle, driving style, the traction available to you, power, and how tight the track is. apply all those variables to off power and on power, then decide by trial and error to get it just right for how you want it.
heavier diff = less turning, less power lost to unloaded tire
lighter diff = more turning, more power lost to unloaded tire
in four wheel drive you want a heavier diff in front so that the front wheels can pull the car through a turn.
heavier diff = less turning, less power lost to unloaded tire
lighter diff = more turning, more power lost to unloaded tire
in four wheel drive you want a heavier diff in front so that the front wheels can pull the car through a turn.
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