Broken drive shaft

[x808drifter]

Originally Posted by blsfrs

Wikipedia reference, so take it for what it's worth:


All rotating shafts, even in the absence of external load, will deflect during rotation. The unbalanced mass of the rotating object causes deflection that will create resonant vibration at certain speeds, known as the critical speeds.


And


Many practical applications suggest as good practice that the maximum operating speed should not exceed 75% of the critical speed.

That alone with their stated speed would make me never buy their product.

[blsfrs]

Originally Posted by Sapphireho

WTF? Sounds like they want to work with you for a solution? What more so you want?

I say good job DSS.

I responded to DDS that, while I do appreciate their offer of another driveshaft, I have read about vibration issues with CF driveshafts and I'm not sure that a 3.5" shaft would work in our cars. Do these options also have critical speed issues? I asked that they reimburse me for the parts and labor for a used OEM driveshaft ($290).


My main problem is that the aluminum driveshaft is advertised as, and assumed to be, a high performance product. There were absolutely no warnings about critical speed. If so, to stay within safety margins, you should never drive your car over 86.3 mph.


Hopefully, because of this issue, DDS will do more R and D on their product and come up with something better. And, anyone who has an aluminum DS in their car will be aware that it can fail without warning.

[ZDan]

So Vivid is selling DSS 3" aluminum driveshafts for the FT86 that are only rated to 110-115mph?

Seems like an extremely bad idea on both their parts...

Originally Posted by blsfrs

I did find out that there is a 1 year warranty on DDS aluminum shafts. My receipt is dated 12/20/2020. It broke on 01/14/2022. I've emailed Vivid racing and DDS but have not yet heard back.

Originally Posted by blsfrs

Here is the response from DDS. It leaves a lot to be desired.
Good afternoon. We calculated the critical speed rating to be around 110-115mph. There are a couple of different options for moving forward. If you're interested in upgrading to the carbon fiber we can do that, Also, we can make a 3.5 inch aluminum shaft, but we'd just need to make sure that it clears everything under the car. let me know what you would like to do.
thank you
The Driveshaft Shop

[dragoontwo]

This thread has crushed my want for a single piece shaft from DSS. It sucks that your shaft failed, but thank you for bringing this to our attention.

[NoHaveMSG]

Originally Posted by ZDan

So Vivid is selling DSS 3" aluminum driveshafts for the FT86 that are only rated to 110-115mph?

Seems like an extremely bad idea on both their parts...

Especially after you have called them before complaining of high speed vibration and they tell you “just turn it 90 on the yoke” or, “ it may need some finer balancing.” I should cut an in car clip and post it. You can hear it buzzing once you get into those speeds.

[ZDan]

I think the benefits of a lighter-weight driveshaft may be overestimated anyway. At low speeds in lower gears, engine acceleration rate may be high(ish), but the driveshaft is spinning and accelerating at rates less than a third (1st gear) or half (2nd) of engine rpm. At higher speeds acceleration is much lower and benefit of somewhat lower-mass driveshaft isn't going to amount to much there either. I haven't been tempted to run anything other than the stock driveshaft.

[NoHaveMSG]

Originally Posted by ZDan

I think the benefits of a lighter-weight driveshaft may be overestimated anyway. At low speeds in lower gears, engine acceleration rate may be high(ish), but the driveshaft is spinning and accelerating at rates less than a third (1st gear) or half (2nd) of engine rpm. At higher speeds acceleration is much lower and benefit of somewhat lower-mass driveshaft isn't going to amount to much there either. I haven't been tempted to run anything other than the stock driveshaft.

Especially when it could potentially snap in two at tripple digit speeds.

[x808drifter]

I'd love to see someone do a YouTube video about this.
Also them saying up to so and so a speed is kinda useless.
Critical speed is based on the RPM of the drive shaft not how fast you are going.
Your speed is going to change based on your final drive.

If I mathed this correctly; The rpm the driveshaft would see with the stock gears in a MT twin at 7,400 rpm in 6th would be 9,650 (rounded up 3ish rpm)

[blsfrs]

Originally Posted by x808drifter

I'd love to see someone do a YouTube video about this.
Also them saying up to so and so a speed is kinda useless.
Critical speed is based on the RPM of the drive shaft not how fast you are going.
Your speed is going to change based on your final drive.

If I mathed this correctly; The rpm the driveshaft would see with the stock gears in a MT twin at 7,400 rpm in 6th would be 9,650 (rounded up 3ish rpm)

In my communications with DDS they did ask for tire size, final drive ratio, top speed on dyno, and hp/torque. Pulls on the dyno were in 5th to 7000 rpm which works out to 125 mph.

[Grady]

Originally Posted by x808drifter

I'd love to see someone do a YouTube video about this.
Also them saying up to so and so a speed is kinda useless.
Critical speed is based on the RPM of the drive shaft not how fast you are going.
Your speed is going to change based on your final drive.

If I mathed this correctly; The rpm the driveshaft would see with the stock gears in a MT twin at 7,400 rpm in 6th would be 9,650 (rounded up 3ish rpm)

That is my math also. I contacted Verus and they stated the balanced their CF shaft to 10k rpm. I do get a buzz around 130 but don't worry about it. I only get to just over 110 on the track I like to go to.

[Ultramaroon]

OR, here's a thought, ditch the one-piece and go back to the properly engineered OEM driveshaft.

[blsfrs]

Originally Posted by Ultramaroon

OR, here's a thought, ditch the one-piece and go back to the properly engineered OEM driveshaft.

This may explain part of the reason for 2 piece driveshafts in our cars. Computer generated models for critical speed rpms.

[dragoontwo]

Originally Posted by blsfrs

This may explain part of the reason for 2 piece driveshafts in our cars. Computer generated models for critical speed rpms.

Do you happen to know the approx length needed for our car for reference?

[NoHaveMSG]

These also make terrible paper weights, what an awful product


Here is a good post on CS of the driveshaft. Shorter and fatter has a higher critical speed, so if I was a driveshaft shaft I should be good to 100k rpms

https://www.ft86club.com/forums/show...8&postcount=31