Steering Column U-Joint Difficulties

[Gyro]

Hey all,

I've been hard at work on packaging the steering subsystem for the past few weeks, and I've been running into some apparent problems with the steering column. I'm using a floor-mounted rear-steer setup, so the rack is relatively close to the front roll hoop (along the car's length).

Being that I don't have a lot of room, I've been researching various joints to make the steering column fit in the damn frame. What I keep coming up with is that the usual U-joint is only rated for about 35 degrees max (for the well-known Apex joints, anyway), and more importantly, that the max torque rating at that steep of an angle is almost useless.

I'm basing my standards here on having read Steve Fox's article for FS Germany, linked here (http://www.formulastudent.de/public-relations/fsg-news/news-details/article/steves-box-of-tricks/). Basically, he stipulates that the entire steering system be able to handle a 75-100 ft-lb torque input from the driver with no failure.

So I keep being told that the joints I'm considering are not going to be strong enough to get the job done (and I have included double U-joints in this group, to increase the max. operating angle), yet I also see designs that appear to use these same U-joints (or even joints that I would consider less suitable than those I have been looking at) at equal or greater angles than in my design, and apparently they work fine.

So my question is, where's the catch? Are the nominal ratings on these joints extremely conservative or based on high-speed applications, or do teams just use them beyond their rated capabilities, and trust that they'll be okay?

Or is there something else altogether that I'm missing?

As always, any input is greatly appreciated. All you guys out there are a big help to a first-time designer like me.

Cheers!

[spierson]

Take a look at Apex joints, mil-spec series. You should be able to find something suitable there.

[Rohanda]

Hi Gyro,
Two years ago we ran into a similar issue and solved it using 2 U-joints which later broke.
Last year we used one of these
http://secure.chassisshop.com/partdetail/C42-365/
and it worked out rather well, as we mounted the U-joint as close as we could to the steering wheel.
Hope this helps

[Gyro]

Thanks for the replies, guys.

Spierson: As mentioned above, I've looked at those. Somehow they still don't appear to be suitable.

Rohanda: You were able to use that single joint for a rear-steer setup? I can't get the angles to work out acceptably. Where was your rack positioned relative to the steering arm connection?

[Rohanda]

Gyro,
The front of our car was quite large, so I was able to lift the rack off the bottom of the frame by almost and inch and a half, and Our steering wheel had been angled upward by about 15 or 20 degrees to reduce some of the strain on the joint.

I can e-mail you some photos if you would like.
Our setup was not ideal, but was acceptable for us.

[Dr Tron]

we use two apex mil-spec joints, bottom mounted rear steer and dont seem to have any issues. also then angle per joint seems to be about 15-19 degrees, also something we did last year (and are not doing this year) is to run the u-joints back to back up by the steering wheel, this year we will run on by the wheel and one by the rack to prevent thing from being over constrained.

[Adambomb]

Originally posted by Kyle "steering and suspension":
this year we will run on by the wheel and one by the rack to prevent thing from being over constrained.

This is also good from a safety standpoint, as it gives the steering column the ability to collapse in a frontal impact. Not only that, but you can phase the two joints 90 deg. from each other to make the output more linear. (or phase them together to make it less linear...)

[Dr Tron]

yes, problem with phasing some of these mil-spec joints is the fact that they are covered with a boot making it hard to visualize

also we have used these joints as long as i remember (2years) and I haven't seen a failure yet

[Gyro]

It seems to me that, if the issue is dimensional constraints from the frame, that putting both joints (or the one double joint) as close as possible to the steering wheel would make the most sense.

But even having done that, and having also moved the rack forward about an inch and a half from being in-plane with the tie rods, I'm looking at angles of about 25-30 degrees in each joint. So is there something that I'm still doing wrong, or is it a frame problem?

[Big Mo]

Solution: "bus-driver" steering wheel setup.


Or, you could just make/buy a 90-degree gearbox in lieu of apex joints.

[Azab]

Originally posted by Rohanda:
I can e-mail you some photos if you would like.
Our setup was not ideal, but was acceptable for us.
it would be great if you e-mailed me some photos, omarahmed@aucegypt.edu http://fsae.com/groupee_common/emoticons/icon_smile.gif