Kagama Rear Suspension/Drive shaft induced oscillation

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Stattle

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Has anyone noticed that the Kagama 4S rear suspension allows very little droop before violent oscilation and vibration occurs under throttle when wheels are in the air? I started noticing it when I was back flipping, when the rpm's increase I could see the car shaking violently in the air. If you pick up the car and rev is, it goes wildly out of balance. If you turn the wheels by hand it modulates the suspension about 10mm and binds badly isolating the diff. I put a set of CVD's in the rear because the dogbones were in phase and the front end isn't exhibiting the same problem at max droop, I thought that phase was the main source as my other cars have 90 degree out of phase dogbones and don't do this, I was wrong. I then began to reduce the droop, but to get rid of the suspension oscillation I lost a third of the suspension travel, so the cars going to get pounded on the landings as the suspension has less shock travel to absorb the landing energy. This car does not appear to be designed for reving while airborn. It jumps well and lands well, but its more like a buggy with big wheels than a basher...
 
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ridgehead44 said:
🤔 Isn't the Kagama 4 basically a truggified Snychro?
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It looks near identical looking at the photos, and its certainly more manuvarable on the ground than in the air. I think punters like me have mistaken the Kagama 4S for a basher and gone out and bashed it only to find the drive train geometry and how basic maintenance is difficult don't align to what a basher needs to be. Putting bigger wheels, longer axles and more suspension travel on a buggy doesn't make it a basher, in my opinion. Instead, a cheap way of selling more of the same platform.
 
Stattle said:
Has anyone noticed that the Kagama 4S rear suspension allows very little droop before violent oscilation and vibration occurs under throttle when wheels are in the air? I started noticing it when I was back flipping, when the rpm's increase I could see the car shaking violently in the air. If you pick up the car and rev is, it goes wildly out of balance. If you turn the wheels by hand it modulates the suspension about 10mm and binds badly isolating the diff. I put a set of CVD's in the rear because the dogbones were in phase and the front end isn't exhibiting the same problem at max droop, I thought that phase was the main source as my other cars have 90 degree out of phase dogbones and don't do this, I was wrong. I then began to reduce the droop, but to get rid of the suspension oscillation I lost a third of the suspension travel, so the cars going to get pounded on the landings as the suspension has less shock travel to absorb the landing energy. This car does not appear to be designed for reving while airborn. It jumps well and lands well, but its more like a buggy with big wheels than a basher...
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Balance your wheels. 👍 You might have a stinker.
 
Stattle said:
Na, when the suspension is held flat (parrallel to chassis), there is no vibration, wheels spin perfectly, only vibrates when suspension droops.
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This is often an artifact of driveshaft design - especially dog bones.

But it can also happen with universal joints if they are assembled incorrectly. Sometimes they even come out of the factory with this easy to miss mistake:

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This can result in 'phasing'.

Still, the designs we use on typical 1/8th or 1/10th rigs are not actual constant velocity designs, despite being called CVD's misleadingly.

As the angle increases, this will always result in vibration - it cannot be prevented without a proper CVD design, or unless driveshaft angles stay within a few degrees.

Fact: By far the worst vibration comes from unbalanced wheels. Even on 1/8th wheels, we typically see an imbalance of more than 5g on a wheel. At 20,000 rpms - mid back flip, that's no joke.
 
Horatio said:
This is often an artifact of driveshaft design - especially dog bones.

But it can also happen with universal joints if they are assembled incorrectly. Sometimes they even come out of the factory with this easy to miss mistake:

View attachment 8004

This can result in 'phasing'.

Still, the designs we use on typical 1/8th or 1/10th rigs are not actual constant velocity designs, despite being called CVD's misleadingly.

As the angle increases, this will always result in vibration - it cannot be prevented without a proper CVD design, or unless driveshaft angles stay within a few degrees.

Fact: By far the worst vibration comes from unbalanced wheels. Even on 1/8th wheels, we typically see an imbalance of more than 5g on a wheel. At 20,000 rpms - mid back flip, that's no joke.
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This has been address in the first post in the thread.
 
Stattle said:
This has been address in the first post in the thread.
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Cheers - I brought to the table a diagram of how a universal joint can vibrate worse by incorrect assembly.

Everybody else must have missed the issue. Which leads me to conclude that there's an issue with your specific driveshafts at the rear.

A binding CVD would be the only thing I can think of that would physically lift the suspension up by 10 mm as you claim.

It's not fundamental design flaw of either the synchro or the Kagama 4S - hailed as an excellent rig by all accounts - but something mechanically wrong with your example.

Perhaps contact Corally and explain to them why you believe their product isn't appropriate as a basher and try a warranty claim.

Let us know how you get on solving the problem.
 
Horatio said:
Cheers - I brought to the table a diagram of how a universal joint can vibrate worse by incorrect assembly.

Everybody else must have missed the issue. Which leads me to conclude that there's an issue with your specific driveshafts at the rear.

A binding CVD would be the only thing I can think of that would physically lift the suspension up by 10 mm as you claim.

It's not fundamental design flaw of either the synchro or the Kagama 4S - hailed as an excellent rig by all accounts - but something mechanically wrong with your example.

Perhaps contact Corally and explain to them why you believe their product isn't appropriate as a basher and try a warranty claim.

Let us know how you get on solving the problem.
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Installing CVDs in the rear reduced the problem, in phase dogbones are significantly worse at full droop, but even with CVD's I've lost significant droop/travel. I think the problem is with the geometry of the design, it's designed for shorter shocks and subseqent less travel of the Synchro. Bashers need more travel, more air = harsher landings, the suspension arms need to be longer instead of longer axles and steep droop angles.
 
Stattle said:
Installing CVDs in the rear reduced the problem, in phase dogbones are significantly worse at full droop, but even with CVD's I've lost significant droop/travel. I think the problem is with the geometry of the design, it's designed for shorter shocks and subseqent less travel of the Synchro. Bashers need more travel, more air = harsher landings, the suspension arms need to be longer instead of longer axles and steep droop angles.
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I think most of here are aware of what the benefit of longer suspension travel affords.

What specific aspect of the geometry, in your opinion, would cause 10mm of suspension oscillation and consequent vibration?

Also, what prevents us from adding longer suspension arms and shocks on our rigs? Surely this is every Truggy that ever existed from the very beginning.

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Perhaps share a video of this phenomenon for us to better understand your specific issue. It doesn't seem like anyone else has experienced this issue. 🤔
 
are you sure the pins aren’t getting stuck in the out drives and can slide smoothly? I vaguely recall there might have been a run of 1mm too short or too long rear dog bones, too. I may be imagining it.
 
It's related to the diff drive cups and the dogbone angle at maximum droop. As droop is reduced the magnitude of the modulation reduces proportianaly. I think the reason it doesn't happen on the front considering both front and rear have CVDs installed is because at max droop the front exhibits positive camber due to the slightly longer top suspension arm, the pillow ball setup and unequal length wishbone geometry, thus reducing driveshaft angle at full droop. Many people may not notice the vibration, especially if you don't do back flips or hold the car in the air and squeeze the throttle. This car from what I can tell has had a bit of a mixed response from the basher community, especially around air control, the oscillation might be part of the reason. People are changing tires and pinions to gain air control, perhaps just adjusting the droop is all thats required. To be honest I'm not overly interested in customising the suspension, I have other machines I can use if I get tired of fixing this one.
 
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AFAIK, dog bones have the worst CV, CVAs next, then universals and then dual universal (double Cardan) are the only true constant velocity.

Dog bones definitely will be the worst.
 
razorrc said:
are you sure the pins aren’t getting stuck in the out drives and can slide smoothly? I vaguely recall there might have been a run of 1mm too short or too long rear dog bones, too. I may be imagining it.
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Hard to say, at steep angle it modulates, so there is a geometry problem, could be a ball or pins or cup or all. The cups are a weird shape on this rig and the rear dogbone pin design is an in phase design, which isn't ideal for a basher, so I replaced the rear with CVDs, which improved it a little. I haven't pulled the front or rear diff out yet, I might see more when I do that service, but according to the manual I have like 40 more batteries to go before thats required. One thing I can say is that cup wear is hyper on these rigs, everywhere.....I thought my Kraton 6s EXB was bad, but this is beyond anything I've seen.
 
razorrc said:
AFAIK, dog bones have the worst CV, CVAs next, then universals and then dual universal (double Cardan) are the only true constant velocity.

Dog bones definitely will be the worst.
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My MT10 has double universal joints made of plastic, and that thing is smooth and bullet proof. One of the best bashers on the market in my opinion, a must have in the collection of bashers, the Kagama 4s has a long list of issues out of the box.
 
Stattle said:
I think the reason it doesn't happen on the front considering both front and rear have CVDs installed is because at max droop the front exhibits negative camber due to the slightly longer top suspension arm....
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A longer upper suspension arm would result in less negative camber at full droop though. 🤔

The response to the Kagama 4S has been great.

This guy sending it on 6S.

 
Horatio said:
A longer upper suspension arm would result in less negative camber at full droop though. 🤔

The response to the Kagama 4S has been great.

This guy sending it on 6S.

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Dude, thats a brand new rig, he hasn't really owned it. One you get 10 batteries in, you'll see....Youtube videos often don't illustrate what ownership is really like. Putting 6S in the Kagama 4S will trash the drivetrain, it can't handle 4S on 17T pinion, I know this from experience.....
 
The most common true constant velocity joints are Rzeppa joints. Used on virtually all front wheel drive cars.

RC cars use universal joints (variations of Cardan joints) which are NOT constant velocity. They're smoother than dog bones.

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These ball drives are similar to Rzeppa joints and are about the only true CV driveshafts used in RC at 1/5th scale. These are £134.

In theory, these could be scaled down.
 
Horatio said:
The most common true constant velocity joints are Rzeppa joints. Used on virtually all front wheel drive cars.

RC cars use universal joints (variations of Cardan joints) which are NOT constant velocity. They're smoother than dog bones.

View attachment 8009

These ball drives are similar to Rzeppa joints and are about the only true CV driveshafts used in RC at 1/5th scale. These are £134.

In theory, these could be scaled down.
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They look nice, perhaps they'll make them for the Kagama 4s, I'd say people are going to be looking for something better than the CVDs it comes with.
 
To add: if you haven't yet balanced your wheels, you don't need to be worrying too much about driveshafts.

From what has been described here, it could well be outdrive slots are chewed and/or cups in the axles. If the slots have divots in them, the pins wont move freely and will bind - most notably when there is an angle. This will happen past a certain limit of droop AND suspension travel on any rig using this type of drivetrain.

It's not a geometry issue.
 
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