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MT-9 Headstock steering bearings question

Posted: Sat Jul 13, 2019 2:32 am
by Korto
This may seem a stupid question, but how tight should the steering bearings on the steering rod and bearing nut (aka "Jesus Nut", for some obscure reason) be?

I pulled my headstock apart when I noticed how shaky the front was under braking and tightening the bearing nut wasn't helping (I could tighten it all the way to locking the steering, and it still shook), and I noticed a couple of things.

1) I had put the tapered roller bearings in what was probably upside-down. :roll:

2) The inside race of the bearing could be easily slid on and off the steering rod (including where it thickens out down the bottom) by hand, and the meeting between the inner race and bearing nut can only be called sloppy, with visible wobble between the two.

Now, I can buy the bearing nut connection being loose, but I would really think at least the bearing onto the steering rod should be a press fit.

Re: MT-9 Headstock steering bearings question

Posted: Sat Jul 13, 2019 8:35 am
by propwash
What tapered set did you use?

Re: MT-9 Headstock steering bearings question

Posted: Sat Jul 13, 2019 9:03 am
by Korto
Good question. I bought them from sidecar pro a couple of years ago, so I can't remember any details, but looking at his site, and the numbers on the bearings, it's these:
http://sidecarpro.com/BFA-tapered-steer ... aring.html
He told me it would also suit my bike.

So the internal diametre of the bearing is 34mm (possibly true, I find it difficult to get an accurate ID with calipers, OD's easy), but I'm measuring the OD of the steering rod at 33.84mm.

Re: MT-9 Headstock steering bearings question

Posted: Sat Jul 13, 2019 9:40 am
by propwash
Here is a link to the tapered bearings I use.
I prefer tapered in solos and ball with sidecar attached.
https://www.cycleworks.net/index.php?ma ... 4b2cfecb00

Re: MT-9 Headstock steering bearings question

Posted: Sat Jul 13, 2019 6:11 pm
by Korto
What benefit do you feel the ball bearings give over tapered rollers with the sidecar? Because the only difference mentioned online is that ball bearings have less resistance and are therefore better at high rpm, while rollers have a bigger footprint and so are better dealing with more weight. Since turning the handlebars would involve no rpm worth speaking of, I can't see any benefit to balls, but obviously you do find one in practise.

Unfortunately, however, this still doesn't actually answer my original question. Does anyone know whether the bearing is supposed to be a press-fit on the steering rod, or a slip-fit?

Re: MT-9 Headstock steering bearings question

Posted: Sat Jul 13, 2019 6:46 pm
by propwash
Slip fit, but should require a bit of tapping with wood to get it in place.
I find the tapers require more use of the damper than the balls with my sidecars.

Re: MT-9 Headstock steering bearings question

Posted: Sat Jul 13, 2019 7:22 pm
by Korto
Which is tighter than what I've got. Maybe if I wrap a strip of aluminium foil around it, to bulk it out a little...

Re: MT-9 Headstock steering bearings question

Posted: Sun Jul 14, 2019 12:51 am
by jeffsaline
Aluminum foil is just waiting to fail. Mearsure the thickness of the aluminum foil and then tap on it with a hammer. Then measure it again. It doesn't take much pressure to make the aluminum move/change dimensions.

Before using aluminum foil to fill the gap between the stem and the bearing inner race maybe consider using a center punch to raise dimples all around the stem where the inner race makes contact. Depending on the wear you might have to do 50 or more dimples. It should be an interference fit meaning the hole in the inner race should be smaller than the outer diameter of the stem. My guess is maybe 0.001"-0.003" interference is appropriate. Kind of depends on the size of the stem and the material from the stem.

Another solution that will be permanent is to build a weld on the stem and then turn it to the correct diameter. I've done the weld and turn fix and it is usually a pretty easy operation. I've never done the dimples on the stem but have used it to tighten the fit for a bearing race in an aluminum wheel hub on a BMW with sidecar. Got word from the owners when they returned home it was still tight over 2,500 miles later.

Best,

Jeff

Re: MT-9 Headstock steering bearings question

Posted: Sun Jul 14, 2019 12:49 pm
by Claus
I would say it should be between a slip fit and a press fit. I do not know the English word for it, in German it's called Übergangspassung.(transition fit?)
If you don't have a triple tree to swap, Jeff's idea to weld on and turn to fit would be ok. Or to turn the stem thinner and shrink a bushing onto it that makes it fit.
Both above mentioned taper bearing sets should fit perfect (should be for BMW pre 1969)

Re: MT-9 Headstock steering bearings question

Posted: Mon Jul 15, 2019 3:16 am
by Korto
<...grumble...>
Yeah, you're right. Aluminium's soft. It wouldn't last long, and then I'd have to pull the whole damn thing apart again.
I'll take it around the welder's tomorrow, tell him I'm looking for a fit that needs to be tapped on with a hammer.
I'll also get him to do the same with that bearing nut, too, unless someone's got a good reason why that would be a bad idea. I prefer the thought of the steering rod being held firmly top and bottom.