Mounting Rockers

[aknott400]

First time posting here but this forum has already been a great help to our first year team at the University of North Florida.

Searched the forum and this topic doesn't seem to be discussed much.

Coming up on an issue with mounting the rockers to the chassis and wanted to see what kind of feedback could be received from the good folks @ fsae.com.

Here is a mock up the rocker mount location. The plan so far is to cope and weld 4130 tube stock to the chassis for the mounting post. Then, tap threads into the the mount and run a single shear bolt through two bearings in the rocker and thread it into the mounting post.


http://i38.photobucket.com/albums/e110/knotslck/Delete1.png

http://i38.photobucket.com/albums/e110/knotslck/Delete.png

Seem to notice a lot of teams cutting a hole in the chassis and integrating their mount post through the chassis. Is this necessary for strength purposes?

Any input or tips on rocker mounting in general would be greatly appreciated.

Thanks in advance

[aknott400]

First time posting here but this forum has already been a great help to our first year team at the University of North Florida.

Searched the forum and this topic doesn't seem to be discussed much.

Coming up on an issue with mounting the rockers to the chassis and wanted to see what kind of feedback could be received from the good folks @ fsae.com.

Here is a mock up the rocker mount location. The plan so far is to cope and weld 4130 tube stock to the chassis for the mounting post. Then, tap threads into the the mount and run a single shear bolt through two bearings in the rocker and thread it into the mounting post.


http://i38.photobucket.com/albums/e110/knotslck/Delete1.png

http://i38.photobucket.com/albums/e110/knotslck/Delete.png

Seem to notice a lot of teams cutting a hole in the chassis and integrating their mount post through the chassis. Is this necessary for strength purposes?

Any input or tips on rocker mounting in general would be greatly appreciated.

Thanks in advance

[racer81]

Bending. Think About It.

[MegaDeath]

Do not use just a bolt for mounting the rocker.

Have the welded post come through the bearings and then pass a blot all the way through just to secure the rocker, but not to take loads from the rocker.

[Mike Cook]

A better idea, like the others mentioned is to have basically a stud or post integrated into the frame that sticks out. Have your rocker bearings ride on that and have some threads at the end of it for a nut to tighten everything up.

I'm not a single shear Nazi like some are around here who's only experience is reading carroll's books. Stress is stress, stress risers are stress risers, they are in every design. If you mount things in single shear you need to account for these things (just like in every design), which usually means extra large studs. This doesn't mean it is not the best, lightest design.

EDIT:

Looking at your images; make sure you check the rocker bolt heads and nuts for clearance on the frame through its full articulation. Looks like it might be close and you won't be the first or last person to make this mistake.

Second edit:

Looks like you plan on using two deep groove ball bearings for the rocker. I would advise against this. Because the rocker really has a limited range of motion, the bearings will ride on the same spot of the balls over and over. It will locally smush them (called brinelling). It's better to use a needle bearing to take the radial load. If you insist on this design, keep an eye on it and each time before you run the car, pull of the shock and pushrod from it, spin it around a couple times so that it rides on a different spot on the bearing. Goodluck.

[Pico]

Completely agree with Mike and Mega.

My 2 cents piece: These mounts need to be well jigged so make sure it's fool proof. The last thing you want is them to be crooked or the tubes running front to back to be out of line. Looks like things are braced properly but make sure the loads can be distributed to the next side. Again, you don't want this point to be the weakest link in your chassis stiffness.

[W1N3I]

We had a similar design the last 2 years. The post came through the whole point there.
But be aware of warpage. You need to be very accurate to align and to fix the post properly before welding.
If you have the possibilty you could maybe laser cut the notch. That worked out pretty well for us.

[aknott400]

Thanks for the comments so far. I agree that at a post should be used. Any more comments as far as the use of deep grove bearings or bearing advice in general? Seems to be a lot of differing opinions on bearing selection. Also, any input on how the post should be attached? Would laser cutting a hole in the lower frame and welding the post to the front and back of the frame tube suffice?

[MegaDeath]

I don't know anything about lasers so I can't help you there. But the way we have done it is to make a jig and have a plate with a through hole and a drill bushing welded into that for the diameter of the hole you want to drill in the frame and then drill through the frame with an endmill. If you do this, make sure to full weld around the node you will be drilling out as much as possible so as you're drilling the tube won't jump around and chatter as much, but do not full weld the node you will be drilling. If you full weld the node before your try to drill it out you will most likely want to punch yourself in the groin.

Full welding the front and back will suffice as long as the tube the post is welded into is strong enough, but you should already have accounted for that.

[Mazur]

Why would you put in a rocker post after you're finished welding the frame?

Why not design the rocker post to be at the center of a node, fixture it in position, and cope the tubing that will run to it?

[Rotary Sprocket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mazur:
Why would you put in a rocker post after you're finished welding the frame?

Why not design the rocker post to be at the center of a node, fixture it in position, and cope the tubing that will run to it? </div></BLOCKQUOTE>

You would want to weld the tubes surrounding the area that you are drilling that way the vibration from drilling doesn't break the tacks that are holding your tubes in place. Also this will reduce the amount of chatter from the endmill because the area you are drilling into will be more rigid. As MegaDeath said if you weld the exact area you are going to drill through, the nice heat treatment you did to the tubes will make it a bear to drill through.


You could fixture it into positon if you designed the post to sit in a collar or your post was big enough so that the tubes butted up to it nicely. For our application our rocker post was only 1/2" OD so fitting tubes to it would have been a treat. It was easist for us to just drill a hole in the tube where it was going.

[Mazur]

I see, didn't realize people were using such small diameters for the base of their post.

[Tim.Wright]

There is quite a good discussion on bearing choice here;

htt p://ww w.f1technical.n et/forum/viewtopic.php?f=6&t=9070&hilit=needle+bearing+rocker

I would tend to god for the needle roller but there are some arguments against it.

Tim

[dazz]

While I generally agree with Mike and Timo's comments on bearing selection, for a low cost, robust and simple design the deep groove ball bearing is hard to beat. If you decide to go with the deep groove ball bearing, I'd suggest you look at using a series that has a large internal diameter relative to the outside diameter. The smaller the balls in the bearing are, the more they will roll for a given amount of rocker motion. Thus reducing the severity of the brinelling effect that Mike mentioned.

[Mike Cook]

Nice link Timo....

I don't get it though. The one guy (riff raff) posts about how 'ignorant' it is to use needle bearings and how much better plain bearings are (based on the foundation that plain bearings have less friction). And at the end of the post someone calculates everything and shows plain bearings to have more friction by a factor of 10 over needle rollers. He then hand waves and says well its only 2 times as much. And slowly departs from the conversation.

Don't you love the internets?

But anyways, there was some good information in there, and plain bearings are probably not a bad choice. Whatever you pick, you need to keep it clean and build it to tight tolerance to eliminate slop. I'm sure our FSAE cars don't run as high of speeds and some other cars, but I have been running on the same set of needle bearings for over a year with no problems. I think I want to pull it apart and check out how they are doing now though.

[dazz]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
Whatever you pick, you need to keep it clean and build it to tight tolerance to eliminate slop. </div></BLOCKQUOTE>

Totally agree Mike! I reckon any bearing type in this application (provided it's sized appropriately) will likely outlast the life of the average FSAE car if it is kept clean, lubricated and free of moisture.

I haven't read the link, but could it be that the guys are mixing up friction levels of dry plain bearings (ie bushings) and shell bearings with pressure fed oil (ie big end, crank mains, etc.)?

And back to aknott400, Don't forget that there are all kinds of different materials that are available for plain bearings. A neat bushing design could save a bit of weight over bearings if you really are chasing every last gram!

[Tim.Wright]

It was an interesting post to read, not so much because there was a clear answer but quite the opposite. Some very different opinions, and if I remember rightly I don't think there was really a resolution to the question in the end.

I would steer clear of anything that introduces friction in the suspension system, so as a rule I personally would never consider a plain bearing on a rocker. Maybe on a damper end but never a rocker pivot as its too highly loaded. Its basically adding a friction damper into your system that changes in intensity as the vertical load changes. Good luck predicting how it will respond.

There was also an argument on the stiffness of the bearing as compared to a supposedly stiffer plain bearing. It is true that a ball bearing's stiffness is very compliant at low loads but as the load increases and more of the ball contacts the races, the stiffness increases a massive amount. To start in this high stiffness condition you can preload (or purchase already preloaded) ball bearings.

Tim