Surly LHT forward-back rocking on descents
 

Just got a LHT and took her for a 40 mile ride today. I’ve modified from stock by adding VO’s 45 degree stem, but that’s it. On descent at a pretty good clip and on road, but not smooth road, I noticed forward-backward rapid rocking motion. The wheels look true, the brakes aren’t rubbing, all seems fine. What is this?? Doesn’t seem to be the same as shimmy. Bike was not loaded (no racks even) and I’m about 180. Thanks for any hints! Bike broken? Surly master-designed flex that I just don’t appreciate? Magic gremlins?

I assume this bike has a threadless headset, and you said you replaced the stem. Did you properly adjust the bearing preload in the headset before tightening the new stem mounting bolts onto the steerer tube. It sounds like you have a loose headset. If this is the case, you will need to loosen your stem from the steerer, tighten the top cap properly to take slack out of the headset, and re-tighten the stem bolts onto the steerer.

Edit: I just looked at the VO stem design, and it looks like this stem eliminates the top cap, and replaces it with a new, longer bolt. This new bolt is what will be used instead of the top cap to add preload to the bearings in the headset.

+1

Before you/OP makes any adjustments. Squeeze the front brake and try rocking the bike forward and backward. If the headset is loose, you'll feel the bike/fork rock back and forth.

Check the web for proper tightening. You can load the headset too much.

As above, first thought was loose headset.

OP: If it turns out to be something else, do report back, would be good info.

When braking? My LHT with the model of tire I use can sort of “rock” at the front end with some braking if I’ve been lazy and haven’t checked tire pressure.

But I’m going with loose headset.

Go back to the bike store, that would be your best bet, internet unseen people's no really can help.

That's true..I forgot about that. If you've felt them both..they feel a little different..but you need to have felt them both first... If this makes sense, a loose headset feels more "discreet" rocking while the tire-induced feels more..spongy perhaps.

For the OP..still thinking a loose headset from the stem change.

I really do not understand what you describe as "forward-backward rapid rocking motion", are you saying that there is flexibility in how the handlebars and stem feel? Or are you saying the bike itself it rocking? Or something else.

If the headset issues do not solve this, please describe in more precise terms what the symptoms are.

My point is that with nearly all real life mechanics stuff, to learn actual real world "tightness" or "looseness", you 've got to be shown in real life what "good" feels like.

And you do have to have a certain amount of real life mechanical experience or at least inclination. I've shown people I know how to do stuff and it's promptly forgotten and or they continually repeat over or under tightening.
heck,I've screwed up doing stuff so many times it still makes me laugh.

all I'm saying is that interwebs mechanic teaching ain't really practical, have someone show you headset adjustment in person, it takes 10 seconds and hopefully you pick up on the feel for proper adjustment.

Seems highly likely. Should be an easy fix, but don't over tighten. I agree with those wo say to ask someone to show you how if in doubt.

Sounds like it is time for someone to pick out the right Park Tools youtube video again.

could you maybe return to place of your new purchase,
ask the mech there to take a peek....?

Re headset tightness, a mechanic I trust was going over my bike wheels before a trip, checked out everything quickly and recommended I tighten the headset a smidge, I have lots of spacers, he suggested tight until you can't twist the spacers anymore. Said that for cartridge bearings and touring, that tight is better. I had adjusted it just a bit looser than he suggested, but still with no play with rocking.

just thought I'd mention this, I've ridden toured a lot on this bike like this, and it appears fine, no stiffness etc

thanks for the quick reply - based on your posted and all the concurrences, sounds like it’s the most likely culprit. I installed and am learning as I go, so I’m not surprised I didn’t quite get it right. I’ll definitely report back after I get time in the garage.

brilliant - will do

tire pressure was not an issue. Thank for the suggestion to consider!

rocking as in the whole bike feeling like it is rocking fore and aft without an obvious cause from me or conditions (aside from less than smooth pavement and higher speed). I’ll report back after adjusting headset.

great point. The local shop that sold me has still a first re-adjustment visit and I can tack this on to witness the tasks.

thanks!

After checking headset, Try it on fairly new smooth pavement, then the bike should ride perfectly smooth.

Otherwise it could be a frame problem or perhaps something odd with the saddle or seatpost. In that scenario, unlikely that we can provide much help on this forum. Or a tire problem, but I think that would be noticeable from looking at the tire.

Anything else I can think of would likely cause some side to side motion that you did not mention.

And yes, check out the Parks Tools YouTube mechanic series, excellently made.
Will give you an understanding of headset adjustment etc, and their info is solid and generally very well explained

Another hint if you ever think you will have to ship your bike. If you have cantilever brakes, use your front brake cable hanger to lock your setting in after the headset is adjusted. If you have V-brakes or disc brakes, use a locking headset spacer. This will allow you to remove the stem and not worry about securing the fork and adjusting the head set when unpacking your bike. If you have a LHT, it comes stock with cantilever brakes so changing the stem should not have affected your headset adjustment. If it is loose, it came that way.

the stem can be removed from my wife's bike to facilitate packing, and the headset is secure with a locking headset spacer.
https://live.staticflickr.com/4361/3...61d3fe3f_c.jpg

thanks. My first thought was frame problem, but not because I know enough to diagnose. I’ll try to recreate the conditions again. It’s felt very smooth on smooth pavement so far.

that smooth-talking gentleman with the almost-handlebar ‘stache is reason I’m where I am! 100% agree.

interesting. That’s gonna make more sense to me after I watch more Park Tool videos.