Finally got the correct tools to service my bikes,tore into the bottom bracket of my univega,and found it in fairly got shape,still had grease,and had a plastic tube covering the shaft-any special reason for it? I cleaned and re-packed the bearings,and re-installed everything as I found it.

it's a dust shield, or more than that a "muck-and-grit shield". Keeps any cr*p from down the seat tube out of your bearings: better than nothing, use it.

Keeps the bearings/spindle somewhat clean from debris and water to a certain degree.

The tube seals the bearings off from anything that might enter from the down and seat tubes and anything from the chain stays, as well. You would be surprised what can actually get into the bearings from these entry points. You were wise to return it to the way you found it.

I thought that was it's purpose,the bicycle maintence thing is new to me-chapped my butt to have to buy more specialized tools when I have 20+ years of automotive tools

Shocked, might even be a better word!



I have posted that picture a number of times and the plastic tube is not really a great guard against rodents. Road grime and water, however, are a different story.

hahaha-no rodents!but for a $10 yard sale bike it was in really good shape-little rust-oh.................Thanks to al for the replies!

Personally, I detest those plastic accordion things, and based on observation, I think the only thing they are good for is to help direct dirt toward the bearing races and keep it there.

Instead, I feel it's better to coat the inside of the shell with a a little extra grease, and to inhibit entry of any foreign matter by either putting a cork in the seat post (as below), or choosing a closed-end micro-adjust version.

I like that idea! Anyone try, as a more "stealth" variation, filling the seat post with household aerosol insulating foam or some other product?

We thought this is the cork to your extra water rations...we were thoughtful and filled it up for you on our last ride!

Puts a whole new twist to the saying "Put a Cork In It" ...I like!

The cork not only keeps moisture out but in as well. Let it breathe. Those shields are useful for those of us with bottom shell cut outs.

Do not fill the plastic tube with grease.

Or you could drop oil down your seat tube every so often. Not that I've ever done this, but it might not be a bad idea. Now that I mention it, I'll try it on my 3-speed. If oil comes out on both ends of the spindle, I'll consider it a success.

The seat tube is open to the other frame tubes and their vent holes - so "putting a cork in it" will not entrap moisture any more than those ubiquitous micro-adjust seat posts.
It will however keep dirt and insects out (as well as the occasional unfortunate small animal) which can be attracted by the grease in the bb.

Also, if you are afraid of water and mud collecting in your cut-out bb, you should be VERY afraid of those plastic accordion sleeves because there is now even less chance of it draining out.

So lose the plastic, use the cork!

Yeah, those cutout were trendy but they sure collected a lot of grit and grime. I used to seal mine with silicone.

-G

hhhmm -not a bike genius,but ,the bike in question was built in 84 and bottom bracket looked great-plastic thingy went back in.

The picture is as-purchased condition. I have no fear of water and corruption getting in there as I am quite diligent about keeping my bikes clean and lubricated, especially this one. If properly installed, the sleeve can be a deterrent and assist in warding off the "stuff".


I assume that putting a cork in the post followed the one in the fork crown?

I assumed it was the plastic thing that prevented the bearings from rusting out on this one

Not a fan of those. Just finishing a bike and the only place that showed any rust was in the steerer. Moisture held in there by that damned wooden French plug. That said, I am a fan of plastic sleeves in BBs. I believe they work as designed, for BBs with cutouts.

This works really well as long as one is diligent and does it every 100 miles or so... quite a few old my old Raleighs are oilers and a few others get oil instead of grease via the seat tube which then gets a cork put in as a dirt stopper.

As a preliminary treatment, a little oil down the seat tube can refresh grease that has turned to sludge and then makes servicing the bottom bracket and cleaning much easier.

I see lots of old bikes that have never seen the light of day but have been stored so long the grease has ossified.