Dakar XC owners, check your swingarm bearings
While trying to find the source of a loud snap sound that I've been hearing occasionally, I took apart the main swingarm pivots. Both bearings were extremely rough and barely turned. Removing the seals revealed that they were badly rusted. The threaded rod connecting the two aluminum bearing seats was caked in dried dirt. One of the bearings was corroded to it's bearing seat and came out of the swingarm during disassembly.
A quick inspection revealed the source of the problem: the seat tube drains into the swingarm pivot, but there's no outlet, so any moisture that gets in there eventually works its way into the bearings, rusting them. Since there's no outlet, I don't see any point in letting it continue to drain into the pivot, so I plan to plug the drain hole. About the only upside to this is that they didn't let the seat tube drain into the much more expensive bottom bracket bearings.
If you haven't checked your bike, you might want to do so. Disassembly requires two T-50 Torx wrenches, which are available at a reasonable price at auto parts stores. The bearings are not difficult to remove, requiring just light hammer and punch work to remove them from the swingarm (support it carefully when driving the bearings out). Pressing/driving the new ones into place should be easy. A quick search online revealed the the replacement 6903RS bearing are under $6 each. I'm going to check with some local bearing suppliers tomorrow for replacements and will post back here with any other tips or comments I have after I finish the repair. One thing I will definitely be doing is coating all the parts with CorrosionX before I reassemble them.
coo man! thanks for the heads up on that... i just built an XC pro so i will keep my eye on that
Update #1, bearings hard to find
I tried several local bearing suppliers (the part # is 6903-2RS, but any 30mm x 17mm x 7mm bearing will fit) and a few Jamis dealers with no luck, so I ended up ordering the bearings from VXB Bearings (http://www.vxb.com/), who seemed to have the best prices. I wanted stainless bearings, but they only had carbon steel at the low end ($5.95, but they'll rust like the factory bearings) and the next step up were combination stainless/ceramic ($14.95). Since it's not a rotating bearing - at least not in the same manner as hub or bottom bracket bearings - there's no advantage to ceramic, but at that price I figured I'd give them a shot. They should be in next week, but for now I'm going to have to reinstall the original bearings after cleaning and re-greasing them. I'm sure they'll last until the new ones get here.
i should see about replacing the bearings in my mavic crossrides with those ceramic ones.... for that price.... i could buy one every so often until i had all i needed to do the wheelset and then change them.. supposedly ceramic are supposed to roll better right?
Yes, but not enough that anyone can actually tell the difference, at least according to the industry experts that were inteviewed in the August issue of Moutain Bike Action. The consensus was that they aren't worth the huge premium that bike component companies charge for them, but since the prices at VXB are pretty reasonable, there's no reason not to try them.
Originally Posted by jpelaston
Update #2 - Another discovery
When reassembling the swingarm with the old bearings, I found that the bushings that go between the bearings and the frame aren't thick enough. With the bearings fully seated in the swingarm and the bushings in place, the space between the bushings is .073" (1.85mm) wider that the swingarm mount on the frame. This means that the bearings are severely side loaded when the assembly is tightened down. Not good. I fashioned additional spacers from a .045" thick stainless fender washer and a piece of .025" stainless sheet, which filled the gap nicely. Not perfect, but certainly good enough for a mountain bike. Still, I'm amazed that Jamis doesn't fit these parts better.
Last edited by Bnystrom; 08-11-2007 at 05:23 AM.
I would double check that those bearings are completely seated into the swing arm. Based on my experience (with a 2005 Dakar XC expert) the bearings came from the factory sitting too high, partly due to there being some paint over-spray in the bores of the swing arm. I made a bearing press with some very large fender washers nut and a bolt and managed to get the bearings seated better although not perfect. See: Cracked Dakar XC Expert pivot for my experience. Even the replacement swing-arm I got from Jamis had this same problem. At this point I'm not clear if the crack was cause by the stress of having the bearing pressed into a slightly undersized hole (due to paint over-spray?) or the extreme side loading when tightening down the pivot bolt.
Originally Posted by Bnystrom
I'm one step ahead of you...
Thanks for the heads-up, but I already removed all the paint from the bearing seats before I re-assembled the swingarm and I'm certain that the bearings are pressed in fully. Gaps between the bearings and the seats big enough to cause the spacing discrepancy I have (nearly 2mm) would be very obvious.
New bearing installation
My new bearings were shipped from VXB in CA on the 10th and arrived in NH on the 13th, thanks to the speed of the good 'ole USPS, which is cheaper and takes half the time that UPS does. The ceramic/steel hybrid bearings were extremely smooth, as expected.
I drove out the old bearings again with a hammer and a punch (a 6" long socket extension plugged into a 16mm socket backwards). Installation of the new bearings was pretty easy using a bearing press made from the following items:
- a 1/2" fine-thread bolt ~3" long
- a matching nut
- thick washers for both ends (use a larger 5/8" washer on the inside of the swingarm for extra support, in addition to a 1/2" washer)
- a 7/8" or 22mm socket, which is the perfect diameter for pushing in the bearing at the edges only, so you don't side-load them
- multipurpose grease
Here's the installation procedure:
1. Clean the bearing seats in the swingarm and lightly grease their circumference.
2. Assemble the following parts, lightly greasing their contact surfaces: Bolt, washer, socket (closed end facing the bolt head), bearing.
3. Insert the end of the bolt through the bearing seat in the swingarm and add the following parts: large washer, smaller washer, nut (again, greasing their mating surfaces).
4. Tighten the bolt until all the parts come together.
5. Carefully line up the socket with the outer edge of the bearing, then start tightening the bolt or nut, whichever is easier. Handling two wrenches and keeping things aligned can be tricky, so an extra pair of hands can be useful here. Take your time and make sure that the socket and bearing are properly aligned as you tighten the bolt. This insures that you won't damage the bearing during the installation process.
6. Crank the bolt down until the bearing is fully seated (you'll feel it). It may not be quite flush with the outside of the swingarm (mine aren't).
7. Repeat with the other bearing.
Once the bearings were installed, I checked to make sure they weren't binding, then installed the swingarm along with the spacers I made to fill the excess gap between the factory inner spacers and the frame. I made it a point to coat the mating surfaces of the parts with CorrosionX to prevent future corrosion (grease is OK if you don't have any CX). I also put some grease on the factory inner spacers where they contact the bearings. This serves two purposes; it holds the spacers in place during assembly and the grease creates an additional seal on the inner side of the bearings, where water/mud might otherwise accumulate. With everything installed and tight, I checked the swingarm action. Where before it would barely move, it now swung freely and smoothly. I'll keep you posted on how well the bearings hold up.
The next project will be to replace the bearings above the dropouts, as they're feeling a bit rough, but I think I'll wait and just replace all of the rest of the suspension linkage bearings at the end of the season, when I strip the bike down to rebuild the shock and the fork. The bearings in my MegaExo bottom bracket may need replacement then as well. Although I don't anticipate having any problems with them, I'll check the wheel bearings too. I'll post my experience with all of this when the time comes.
I just received by 2005 Dakar XC Comp frame from rbikes.com, and checked out what you have been talking about. I haven't taken the swingarm assembly apart, but I look into the frame through the bottom bracket race, and I can clearly see a drain hole that goes up into the seat tube. It doesn't look like the swingarm assembly blocks the entire seat tube, so I'm pretty sure water can drain down the seat tube and around the swingarm assembly into the bottom bracket area. I stuck a cable up the hole and it easily went around the swingarm assembly and into the upper seat tube area.
So is this a recent frame change? What year is yours? Thanks for all the research by the way...
That's pretty interesting
My frame is an '06 XC Pro, but I have no idea if it should be any different from yours. I didn't see any drain hole on the outside of the bottom bracket, but I'll certainly check it again when I get home. Perhaps there is an internal channel in the casting that I didn't see. There is definitely a hole that leads from the seat tube into the swingarm pivot mount, but it wouldn't make any sense to have it if there was a drainage path to the outside through the bottom bracket.
Does your frame have bearings or stainless bushing for the swingarm pivots? If it's the latter, it may be the same frame that was used on the '06 Dakar
Sport, which is slightly different from the XC Pro frame.
Update: I looked at a few more Jamis frames online and the '05s I saw had a seat tube and downtube that connect directly to the bottom bracket shell. My '06 has a large casting that combines the bottom bracket, swingarm mount and attachment for the oversize downtube. That probably explains the differences you saw.
BTW, I made an error in my first post. The bearings are not up against the frame, so water that collects in the frame would not have to flow through the bearings to get out. It would have to flow out along the threads of the center bolt that holds the swingarm on.
Last edited by Bnystrom; 08-16-2007 at 12:57 PM.
I just looked at the 2007 XC Pro frame, and you're right, it's completely different. Hmm, I wonder if that is a difference between the 2005 and 2007, or between the Comp and the Pro. I remember reading that the difference between the Comp and the Pro frame is just the shock, and possibly an added web at the seatpost....but looking at yours, it would seem they beefed up the swingarm / BB section of the frame for the Pro model, atleast for 2007...
There was a big change in the frames for '07
The shock is now mounted lower in the frame, with the forward shock mount moved from the top tube to the downtube. The shock linkage pivots now off a bracket attached to the seat tube rather than the top tube. I can't tell if the '07 has the same bottom bracket casting as the '06 or not.