Project RX-8 w/ GS450h Drivetrain

[steveknox]

Hi All,

I'll get around to documenting my setup at some point, but i'm currently in need of some help.
I'm testing my setup with the custom driveshaft/propsharg i've had made, and it's very unstable. Like a wet noodle. See the videos below.
This is the 2nd propshaft -- the first one was a modified rx8 shaft. I replaced it thinking it must be the shaft, but the brand new shaft has exactly the same issue.
Can anyone offer advice on how to diagnose this issue, i'd appreciate it.

I've measured with a dial indicator that the shaft varies by ~0.5mm on both ends when moved by hand,
I know rx8s are quite particular about transmission - diff mounting, but surely this is excessive for that.

https://photos.app.goo.gl/B8yHfMTdsHgxL4Aj8

[tom91]

The issue here is you are neglecting these "simple mechanical facts"
1. the RX8 drivetrain couples the transmission and diff.
2. The RX8 prop slips in and out of the diff.

It looks like have done nothing to properly mount the diff and simply tried bolting the brace to the car. This wont work.

You now have no length variation compensation thus over stressing the rubber donut causing it to run out of round. This is even very apparent at stand still the rubber donut looks pre stressed.

[steveknox]

Thanks tom. Back to the drawing board I guess.

[tom91]

Back to the drawing board I guess.

Yes you need to solve two problems.

1.Diff mounting so it is done right
2. a single piece prop that has a slip joint

[steveknox]

I don't understands why my diff mount is not acceptable.
Can you please explain this.

Yes you need to solve two problems.

1.Diff mounting so it is done right
2. a single piece prop that has a slip joint

[tom91]

I don't understands why my diff mount is not acceptable.

Your diff bounces all over the place. You just bolted the PPF frame to some random undertrays under the car it looks like. So for the diff to move that much a whole lot of things are flexing. Now add the face the diff is seeing VERY minimal load as tires in the air this will break first time you even put any sort of power down.

Note how the diff only has those two small bushings that are transverse bolted. Thus the diff can rotate around those and the also offer very little side to side stiffness to the diff location. You have drastically altered the way the drivetrain is mounted/suspended in the vehicle without making sure to properly accommodate the forces and required movements.

The RX8 , RX7 and MX5 share the same setup so have a look what other people do when they remove the PPF frame/brace

Example video

[jrbe]

Help us understand what we're looking at.

Does the transmission to rear diff frame C shaped channel connect to the GS450h transmission on multiple planes? To me it looks like you have the frame up along the driveshaft connected to the diff stock but have something not tied in well at the transmission connection because
the wobble looks to pivot at the transmission.

Can you post pictures of the diff to frame and frame to GS450h transmission connections? Those should help a ton.
And if you post them on Google photos it can help to add a comment to each explaining what we're looking at.

Is that the stock driveshaft? If it was modified is it concentric and balanced?

Don't give up. If you have ideas your unsure of, post them here as ideas and we can help you work through them and help course correct.

[steveknox]

Thanks. In addition to triple checking alignment, I've decided to try to source a gs450h shaft, and have it modified to fit the rx8 diff. As it has a centre bearing, it should help to reduce vibrations and offset the front-back movement issue. I chose not to go this route to begin with as there is nowhere obvious to mount the centre bearing, but it seems like i've got little other choice.

I had my rx-8 driveshaft modified to couple to the rx8 transmission. When I got the vibrations, my first thought was that the shaft wasn't modified correctly, so I had a brand new one made. This made the problem worse (likely because it's heavier).

My first port of call will be to solve the driveshaft issue by using a gs450h. I will then re-assess. Similar diff mounting solutions have been used on multiple other rx8 conversions. I even tried the solution used in ls1 conversions, which can take up to 400hp, but decided it wasn't stiff enough, so I used this approach instead. One problem at a time though. My time and budget (and motivation) are limited, so I'll sort the prop and then we'll see.

[jrbe]

It might be that the heavier modified driveshaft is more rigid and kind of amplifies the problem more.

Having another driveshaft made likely will not solve the issue that I think I'm seeing in your videos. It looks to me there's a plane that's not locked down between the gs450h transmission and the rear diff.
It looks to me like:
- the tail of the transmission is allowed to move / flex left to right too much. Missing diff bolt to the torque frame? The diff doesn't look like it's tied to the torque frame well at the back.
- looks like some transmission to driveshaft to diff angles might not be right. These aren't always straight forward as things move and twist when loaded up.
- your transmission mounts might be too soft. I've seen shot motor mounts (ice) allow this amount of vibration in a drivetrain before.
- The u joint with the rubber mount up front may allow a wobble to start then throw the whole thing off balance. Is there something keeping the u joint collinear to the transmission output shaft? Can you flex the u joint to the side? I may see why you think a solid mount under there would help the situation. I'd focus here first.

What flex is there at the rear of the driveshaft / diff flange? CV? U joint?

It might be worth tieing the torque frame to the transmission instead of the floor? You have rubber mounting on the diff then solid? Mounted the torque frame to the floor. That turns the floor into a speaker.

[rstevens81]

It might be worth tieing the torque frame to the transmission instead of the floor? You have rubber mounting on the diff then solid? Mounted the torque frame to the floor. That turns the floor into a speaker.

tieing the PPF (torque frame) into tranmissin is quite significant job ... just googling mx5 PPF delete will get you lots of pictures what people have done to remove it and also to retain it (rx8,rx7,mx5 have similar setup)....ive got a is300h as an upgrade for my mx5 project and ill be amaised if i can do it in less than a year and ive brought stuff to jig it outside of the car. the ppf is about 5ft long and its one hell of an effective lever.
with the 450h propshaft, instead of having one < 5ft propshaft you get 2 that are just over 2 feet, so as long as its all aligned it should be better.. remeber in oe application without the ppf there is always alot of flex of the diff (as they are normally rubber mounted). at the very least i would expect tghe vibrations with a 450h gearbox to happen at 2-3x the current speed.

[jrbe]

tieing the PPF (torque frame) into tranmissin is quite significant job ...

I used to do spec Miata prep, so pretty familiar with this, just not all the names its got.
I believe you could do it with 1 bent piece of sheet metal that bolts the stock PPF to the gs450h. But I'm spoiled with Solidworks, a 3d scanner and a bunch of sheet metal experience.
For clarity I don't think this is the root of op's issue, I think it's just amplifying the sound - and I'm making assumptions because I haven't seen pictures of most of his setup yet. It's hard to see what's what in the videos.

[Ev8]

Ok so UJs are not a constant velocity joint and if the gearbox flange and diff flange aren’t square to each other, so that the angle of each uj cancels the other out you can create a vibration, also can’t see your entire prop shaft there but is it correctly phased? the yokes at each end of the prop should align, being 90degrees out causes issues

[Ev8]

I think if I were doing this swap I’d build a solid aluminium doughnut and include a slip joint between UJs something like

[steveknox]

I've got a gs450h prop shaft, and an mx-5 prop shaft.
I will chop the end off the gs450h shaft and put on the diff-end of the mx-5 shaft.
The gs450h has the slip joint in the middle which will solve one issue. I've no idea where i'll mount the middle section...but i'll figure it out.
The only difference compared to a stock gs450h shaft will be that on the diff-end there will be a U-joint instead of a donut.
I can't upload an mp4 file so here's a link to the video explaining my idea
https://photos.app.goo.gl/Na9mM86KXQq73LB67

I do have a concern about the extra u-joint but I think it's my best option. I will very likely make/buy a solid coupler at some point

[jrbe]

An odd number of u joints isn't a great idea as they are not constant velocity when angled.
Worth a watch before you decide,

[steveknox]

I understand the issue but there are a few mitigating circumstances, the main one being that the u-joint will almost never be at a different angle from its set angle as the diff angle doesn't change (much) in relation to the transmission owing to the PPF holding it in place.

Plus there will only be 2 u-joints in the setup. One in the middle section of the prop and the other at the diff end.

The illustration show in the video above assumes the diff angle will change drastically in relation to the transmission which in an rx-8 doesn't happen.

[steveknox]

My only other alternative would be to shorten the lexus shaft, but keeping the lexus end, and then have a coupler of some description which connects the rear of the shaft to the diff.

[jrbe]

If you're making an adapter at the diff end anyways you could make it fit a guibo and shorten the gs450h driveshaft. The center support bearing could likely rotated 90° and be picked up by the ppf.

[steveknox]

That's a fair point. I'll do some design work and get back when i've figured something out.

[Kelju]

Probably your issues are originating from these (which have already been pointed out):
1. Improper fixing of the diff (this however is propably not the source of vibration)
2. Universal joint fixed onto a rubber bushing (U-joints should always be used in pairs and the alignment of the input and output angle is critical. Some misalingment along with one of the U-joints connected to the rubber bushing might cause the vibration)
3. When swapping in the GS450h tranny, did the length of the shaft prohaps increase? You should check the critical speed of your shaft with your dimensions:
https://spicerparts.com/calculators/cri ... calculator

I would higly recommend using CV-joints in conversion projects. As the name implies, the joint provides the output speed according to the input speed, regardless of the angle. This has the benefit that perfect angular alignment is not needed. Also, you should try to keep the input and output angle as close as possibly to each other but still have the input and output shaft on adjacent planes to have some angle in the joints. This way the bearings of the joint will wear more evenly.

[Ev8]

I think before I made another set up I’d try you current one with an aluminium donut you don’t need the movement the rubber provides

[steveknox]

I tried it with a solid coupling, and the vibration was still there (albeit reduced)