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Old 09-27-2016, 11:54 PM   #1021
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The rear wheels maintain their spinning momentum, when throttle is applied, less energy is required to bring all 4 wheels up to speed
I don't agree with this. When coasting, the rear wheels are rotating at the same rate they would have been without the one way, since the rate of rotation is dependent on the velocity of the car when the wheels are touching the ground (since it's just coasting and not wheelspinning/skidding). There should be no difference in momentum.
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Old 09-28-2016, 12:48 AM   #1022
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I don't agree with this. When coasting, the rear wheels are rotating at the same rate they would have been without the one way, since the rate of rotation is dependent on the velocity of the car when the wheels are touching the ground (since it's just coasting and not wheelspinning/skidding). There should be no difference in momentum.
You do have a point. I do agree with what you are saying. But we need to take into account more than just the wheels in this situation. Yes, I did say wheels in my previous post. But I ment that to be the end point of the rear drivetrain.

In both setups staring from the front, we have a motor, 2 internal (reduction?) gears. A front diff, front pulley, belt, rear pulley connected to a, internal (counter) gear, rear diff, dog bones and axles (or CVDs), then the rear wheels.


When coasting:
In the regular setup, the negative force of the motor is acting on the entire drivetrain.

In the freewheel setup, the negative force is acting only on ~50% of the drivetrain.

For all intensive purposes, the rear end is neutral, not being acting on by a positive force nor negative (other than friction, which we shall negate since both setups will have this, thus we can cancel it out.)

When accelerating:
When the power comes on, what happens? The front end spins up, then transfers to the rear end (through the drivetrain) to the rear wheels.

In the standard 4wd, when the power comes on, the whole drivetrain (acting on by a negative force) needs now to accelerate to a positive force.

In the freewheel setup, the front end needs to change from negative to neutral to positive, but the rear end needs only to change from neutral to positive.


I am not saying that the car is so much faster. But I am observing an increase in acceleration from a vehcile that is already in motion compared to before. Maybe it's the confined space that I am in? Maybe I just charged my battery? Maybe I am mental 🤣

The truth is, that it surprised me to even notice a difference. The car seems zippier.

I can really only confirm this with my eye dyno. But I will try this out more on the big street later this week.

I reckon this to be similar to how accelerating less (rotational) mass, makes for quicker acceleration.

Last edited by 4roller; 09-28-2016 at 01:03 AM.
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Old 09-28-2016, 01:14 AM   #1023
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Regarding your braking effect of the motor in a coasting situation, the motor will still have the same net braking force. It's just now distributed entirely to the front wheels, but unless the front wheels break traction, the mild braking effect will provide the same level of deceleration. Since the rear tires are still in contact with the ground, the rear drivetrain momentum will decrease at the same rate as the front.

The only difference is that there's no force applied at the pulley, but that doesn't affect the momentum of any of the rotational parts. If you're noticing any acceleration benefits, I'd suggest it's a placebo effect, since on power, the one way pulley behaves just like a 4WD, and during coasting, all the drivetrain components should be rotating at the same rate.
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Old 09-28-2016, 09:08 AM   #1024
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If the momentum of the front and rear drivetrains are equal while coasting (in both setups), then how can it be seen that off power coasting is affected? I agree with you that the velocity if the car, and the wheels are spinning at the same rate, but energy in the rear drivetrain is greater due to the freewheeling.

It can be seen in the last picture posted (granted that no wheel slip is happening when coasting) but the energy present in the rear wheels when power is off, remains.
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Old 09-28-2016, 09:18 AM   #1025
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I got one for sale if anybody is interested: Tamiya XV-01 Red Bully Rally Car Roller
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Old 09-28-2016, 11:45 AM   #1026
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If the momentum of the front and rear drivetrains are equal while coasting (in both setups), then how can it be seen that off power coasting is affected? I agree with you that the velocity if the car, and the wheels are spinning at the same rate, but energy in the rear drivetrain is greater due to the freewheeling.

It can be seen in the last picture posted (granted that no wheel slip is happening when coasting) but the energy present in the rear wheels when power is off, remains.
You mean the effect when rear wheel are still rotating for some time after the power is cut off, when they are not touching the ground? Well, they keep their angular momentum (because they are disconnected from both the braking motor and most importantly from the ground as well), and they would spin indefinitely, wouldn't there be some friction in the system (bearings, but more importantly the rear diff). Since the friction is there, they will stop rotating after some time.

However, the case when the wheels are off the ground doesn't say anything about what happens when they are touching the ground. As gigaplex said, in that case the rear wheels (and the rear diff as it is directly connected to them) are slowing (losing momentum) with exactly the same rate as front wheels, because their angular momentum is directly related to momentum of the whole car, which is slowing down. What connects momentum of the car and angular momentum of the rear wheels is the ground.

Basically you have only altered braking and reversing, nothing else.

I hope that this makes sense, I typically don't discuss physics in english :-).

BTW - on my other car, I have one-way drive shaft, and for off-road, it doesn't make much sense, IMO. The penalty of losing front brakes and having 2 wheel reverse is just too much. I'm going to rip it off and throw away during next rebuild. Your situation is of course different, because you have kept front brakes, which I believe do most of the work while braking.

What I really like is fyrstormer's SH-AWD build. Full time 4WD with 50/50 distribution can be pretty annoying under full throttle on gravel. The car constantly wants to travel somewhere else then where it should go. AVC/TSM helps, but the problem is that they don't really work on my kind of track (eventually they get fooled and shoot the car straight through the turn, or they make you too wide). FWD car works great under acceleration, but it understeers. So I'm really tempted to try this SH-AWD drivetrain. I'm just afraid that it will chew the belt for dinner, because most of the fellow racers say that the XV-01 pulley is already too small. So I'm afraid to switch one of them for an even smaller one... Perhaps I'll build one more car, and try it with that...
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Old 09-28-2016, 12:09 PM   #1027
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I usually do CVDs all around, because they work better than dogbones. On my Tamiya tourinc cars (including my XV-01s), I put CVDs on the rear and DCDs on the front. I tried CVDs on the front briefly, but the amount of suspension articulation caused the tiny touring-car wheels to vibrate like crazy in sharp corners. DCDs solved that.

The only place I still use dogbones by choice is on the rear axle of my DF-03Ra. The rear axle is swept forward to achieve a proper touring-car wheelbase, because the underlying chassis comes from a buggy, and having mismatched axle joints (dogbones on the inboard ends and CVDs on the outboard ends) caused severe vibrations, so I switched back to dogbones on the rear axle. The DF-03Ra is the only vehicle I've ever run that actually needed dogbones to work right, though; the rest I upgrade to CVDs (or sometimes universal-joints on the rear axle only) as soon as I can.
I have a DF-03ra and switched to all CVDs because I was constantly having dogbones fall out. Can't remember if that was mainly the front or rears though.
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Old 09-28-2016, 07:31 PM   #1028
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So what haplm and gigaplex are saying is, the effect of the one way diff is negligible, when it comes to acceleration when coasting.

Perhaps then, I am just experiencing less motor braking, while coasting. So I am carrying speed through the corner.

I guess this would be similar to a person who runs 20% drag brake all the time, then removing the drag brake, experiencing a higher average speed?

I don't know, maybe you guys are right and its all placebo.

I should really go drive the car to see if this is truely happening or if I was in some sort of lunacy from the heat.
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Old 09-28-2016, 08:28 PM   #1029
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I think the tires I am using (USGT) are making me feel like acceleration out of corners is better because they just grip corners better. You guys are probably right.

I switched back to the original tires after. I can't notice a significant difference, except the momentum carry.
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Old 09-29-2016, 03:12 AM   #1030
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I have a DF-03ra and switched to all CVDs because I was constantly having dogbones fall out. Can't remember if that was mainly the front or rears though.
CVDs on the rear of my DF-03Ra caused a horrible chattering that vibrated the entire chassis. Switching back to dogbones fixed it. The dogbones are secured with O-rings in the stub-axle cups and foam dots in the diff outdrives. The front can use CVDs no problem. But that's all detailed in my build thread.
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Old 09-29-2016, 03:17 AM   #1031
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There *shouldn't* be any noticeable difference in the car's acceleration with the one-way pulley. As was pointed out, the rear axle is kept spinning at the correct speed by virtue of being attached to the same chassis as the front axle, and thus being pulled along with it.

One of the side benefits of the smaller front pulley I installed is the rear axle freewheels all the time, even when cornering when the front wheels are rotating faster than the rear wheels, because the rear axle is underdriven by the differing pulley sizes. I did the math and the faster rotation of the front wheels when cornering should only overwhelm the rear axle's underdrive ratio when the steering is very near full lock, at which point you're probably not accelerating hard anyway. The rest of the time, the underdrive ratio allows the one-way pulley in the rear to do its thing and allow the rear axle to freewheel without any power input from the drive belt.

Getting a tensioner pulley big enough to keep the belt taut is a problem, though. It might have been easier if I'd thought to install an idler pulley somewhere, just to take up a little more slack.
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Old 10-01-2016, 12:29 AM   #1032
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Rally in .cz

https://youtu.be/ww3bfLUKKnU

Haplm, which one are you 😁
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Old 10-01-2016, 01:13 PM   #1033
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Default OMG that rear sping pre-load - best adjustment ever for counter acting over steer

So yeah, I am pretty happy now.

I am finally figuring out how to cancel out uneven on-power over steer, which equates to loss of rear end traction. It is most easily achieved with rear pre-load adjustments. No longer am I obsessed with both shock collars being an even number of turns. Once I realized that the car will never be perfectly balanced, I was able to move past this mental barrier of needing to have "visually symmetrical suspension." Now, one shock collar may be turned 5 times, while the other one is turned 8.5 times. They may look off especially left and right sides, but driving wise, the car is neutral, which makes for a more sane and enjoyable driving experience.

To get a really good sense of if you are over steering more one way than the other, I usually test this with slick tires (where you can wash out pretty easy) and coming out of a turns at full throttle. With grippy radials the over steer is not as evident. You might notice your car just turn wider one side verses the other. Anyway. To correct over steer, I have found if the car over steers too much to the left then add a half or full turn to the rear right shock collar. Still over steering? Add another turn, and so on. What's interesting is that you can tune it to have no over steer (boring but efficient), or with a tiny bit of over steer, which makes for more exciting driving. This is where the rear end swings wide (just past the limit) then grips up and goes! I love that!

I am pretty confident in this adjustment now, such much so, that I've increased my punch rate to 20/30 instead of 10/30 to get that extra hit of acceleration. The slipper doesn't really come into affect unless all four tires have traction.

So yeah, I am loving this setting now, learning more and more about suspension and how it affects driving.

One way Pulley
BTW, the rear one way pulley is a blast to drive. Coming in hot! You definitely need to change your braking habits. Straight line braking (stabbing), temporarily moves the weight forward, then go through your turn. Before, I could brake mid corner, and it would have little effect on the position of the car. Now if I brake mid corner, the rear end may slide out.

I must say though, that this new way of braking actually makes more sense to me. I come into and exit a corner much faster, assuming my tires don't slide out On power (not full power) usually helps drive the car deeper into a turn. Without power (or brake stabbing) the front end pushes, especially with slicks. Experience is the best teacher.

All's good in the hood.
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Old 10-01-2016, 07:40 PM   #1034
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Old 10-01-2016, 10:22 PM   #1035
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Rally in .cz

https://youtu.be/ww3bfLUKKnU

Haplm, which one are you 😁
None :-). Same track as I have posted, but a different race and different cup week earlier. I didn't have the car ready at that time. I will post a video once there's one available, most likely from the race that happened yesterday - which to make it more complicated is the same cup as you have posted ;-).

I also made bunch of videos myself, I'll PM you (not to hijack this thread too much) with location once I edit slow motion sections...
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