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Jonathon Gillham

Double cardan joints

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I am about to make an order for parts and hovering over the dcj's for a TA07. They are really expensive which both makes me think they are a big improvement over the universals and also overrated as we all know about hopups for hopups sake.

I don't run a spool, I have 1m oil on the front diff. I haven't noticed any chattering or adverse effects from the universals but then don't really know what I'm looking for. This is for 21.5T blinky, which I assume is the same for 17.5T (ie mod cars are crazy fast, 21.5T or 17.5T should be similar)

Whats the deal with dcj's?

Also, for context, for 20USD more I could get the TRF420 suspension upgrade instead which would appear to make more difference?

PS I realise I havent been TC racing forever but you know, hopups, and I always intend to go onroad racing again

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(disclaimer - i don't have dcjs so this is hand waving) if you aren't getting chatter with universals and don't plan on running a spool i think they are probably unnecessary. I have run regular dogbones on my TA06 early on and when i stiffened the diff you can really hear the chatter. It's dead obvious. More so than a barking ball diff. I got unis because they were cheap and i was only going to run stiff oil and it's been quiet ever since. 

Seems like the suspension upgrade would be better value/more fun to fit and tinker with. 

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as above, steering chatter happens on touring cars, when on full lock and applying power, its an apparent chatter / wobble from the front wheels. to be honest if your not planning on racing it and have no issues with steering they arent worth buying, even with a bit of chatter, if your just using it for bashing they would be a waste of money. 

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There are some old posts floating around (can't find them now) that discuss the effect of angles and joints on axle velocity.  If you imagine the main long axle shaft spinning at the same continuous speed as the differential, then as soon as the shaft terminates in a joint like a dogbone end or CVD joint, then the axle stub actually doesn't spin at the same continuous speed.  To the naked eye it looks continuous, but through the course of one full rotation the axle stub actually moves a little faster and a little slower than the axle itself.  There's math to explain this.  So, as you increase steering angle then the amount of variation in the axle stub speed increases and it shows up as chatter.

The point of the DCJ is to create a second joint in addition to the first joint, and the intermediate segment between the two joints sees the speed variation but also splits the steering angle evenly so the axle stub's speed doesn't have that variation on every rotation.  Again, there's math to explain this.

So if you're bashing casually, regular old dogbones or CVDs are just fine as you'll not really notice or care about minute levels of cyclic speed variation in the axle stubs.  If you're competition drifting or competition racing, you might think about DCJs as they eliminate that effect and will provide a little more consistent speed (and therefore tire grip) relative to the surface.

It really is a progression of improvement -- dogbones are the lowest with a lot of side play and looseness in their joint cups, as well as the axle stub speed chatter.  CVDs are a little better in that they can eliminate side play and at least one point of looseness; there's only one dogbone end and one CVD joint.  There can still be some axle stub speed variation, but the tightness of the subassembly hides the chatter a little better.  Finally the DCJs are like the CVDs but improve on the joint by having two joints.  So the side play and single dogbone end are still there, but the double joint setup splits the steering angle and eliminates the axle stub cyclic speed variation.  So, they cost more but they are definitely an improvement.

This Wikipedia article explains it more:  https://en.wikipedia.org/wiki/Universal_joint

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I'm using DCJ on one of my FF-03 and universal on the other FF-03. The DCJ is smoother and no play. 

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Thanks @speedy_w_beans, great explanation. Since I'm not noticing the chatter atm I can probably get away without them for now (and it will be a race to see what happens first - a TA08 or me racing onroad again)

I've never seen them on an offroad racer either, since I race on dirt they will be unnecessary (whats this grip thing I keep seeing mentioned?!).

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I recently put DCJ's in my FF-03. Allowed me to increase the steering lock and removed all the chatter. It's a night and day difference, standard dogbones rattled and shook the outdrives at about 30 degrees of lock. The DCJ's are as smooth as butter at nearly 50 degrees of steering lock. Not cheap (I got mine from RcMart, took about a month to arrive) but have transformed how the car reacts to direction changes. 

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20 hours ago, speedy_w_beans said:

There are some old posts floating around (can't find them now) that discuss the effect of angles and joints on axle velocity.  If you imagine the main long axle shaft spinning at the same continuous speed as the differential, then as soon as the shaft terminates in a joint like a dogbone end or CVD joint, then the axle stub actually doesn't spin at the same continuous speed.  To the naked eye it looks continuous, but through the course of one full rotation the axle stub actually moves a little faster and a little slower than the axle itself.  There's math to explain this.  So, as you increase steering angle then the amount of variation in the axle stub speed increases and it shows up as chatter.

 

I think you (like a lot of people) are confusing CVDs with universal joints. 

A CVD - by definition - spins it's output at the same speed as the differential outdrive (it's input). CVD literally means 'constand velocity drive'. 

What you are describing is a universal joint, and your link to the wiki page for 'universal joint' says as much. 

A universal joint, which is the standard upgrade to dogbones, is not a CVD.

A double cardan joint however is a constant velocity drive, but only when the angles are kept equal between the intermediate shaft and out shafts. 

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3 hours ago, rich_f said:

I think you (like a lot of people) are confusing CVDs with universal joints. 

A CVD - by definition - spins it's output at the same speed as the differential outdrive (it's input). CVD literally means 'constand velocity drive'. 

What you are describing is a universal joint, and your link to the wiki page for 'universal joint' says as much. 

A universal joint, which is the standard upgrade to dogbones, is not a CVD.

A double cardan joint however is a constant velocity drive, but only when the angles are kept equal between the intermediate shaft and out shafts. 

I agree with you 100% from a technical perspective.  At the same time, there is a universal joint marketed as a "CVD," and generally people have a mental picture of what that looks like.  If you search on the web for "3Racing CVD," "Yeah Racing CVD," "Xtra Speed CVD," etc. you'll see something like this (which is a universal joint):

s-l400.jpg

Search results also show something like this (which is a DCJ):

s-l640.jpg

Then there's the universal joint most people think of and is functionally similar to the "CVD" shown above:

tamiya-14135028-1-800x800_0.jpg

So, yes, I agree that a "CVD" is really a "universal joint" technically speaking.  At the same time, the manufacturers have chosen marketing terminology to distinguish the appearance of two universal joints.

It's still the case the dogbone is the sloppiest solution, with universal joints (either true universal joints or "CVDs") reducing some slop, and finally DCJs being the best.

Thanks for pointing this out; hopefully this clarifies things for everyone.

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Thanks everyone, it sounds like they are worth the investment and given the shipping costs now these are looking like a better bet. Tried to order a bunch of little things and shipping costs as much as the items right now!

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