Need some quick advice: recently (like, 2 weeks ago) I bought an '84 Motobecane Grand Jubilee off CL in what seemed like okay shape, not ridden in awhile but complete. This model has a Columbus 'inexternal' frame, very lightweight. I took it to the LBS for wheel truing and a basic tune-up, but although I've ridden it a couple times since then (not for very far either time) it still has some issues. The rear wheel keeps wanting to rub against the dropouts, I can adjust the QR to keep this from happening, but the tire still seems to come awfully close. And the chain rubs the FD on one spot during the revolution, no matter how I tinker with it. Otherwise, the ride just doesn't feel as smooth as I remember from the Moto I owned in the early 80s, or as my '74 Super Course MkII, which I'd planned on getting rid of once I got the Moto.

On some advice in another thread I started, I inspected the frame for cracks around the seatstays. Nothing visible, but while I was inspecting the frame (and yes, I know I should have done this before I bought the bike) I found two dents on the underside of the downtube. One is about the size of a dime, maybe 2mm deep, the other is a bit smaller; they're in a line, maybe 1/3 of the way from the bottom bracket.

I can't tell that the frame is bent, at least not in any way that I can see. And the bike hasn't yet had the BB and headset overhauled, as I'm sure it needs - but I'm wondering if whatever dented the downtube has screwed up the alignment in ways I can't see, and if the integrity of the frame is threatened.

I'm seriously bummed about the dents, and kicking myself for not checking the bike out more thoroughly.My only defense is I'd had my heart set on getting a late 70s-early 80s Moto and this one seemed to come along at the right time. But if the frame needs serious repair I'm going to contact the seller and ask for my money back. I'm happy to do basic maintenance, but I don't have the time or expertise to handle a real project bike, and I paid what (to me) seemed like a reasonable price for a Grand Jubilee in easily fixable, working condition.

What do you think I should do?

Thanks for any advice, help, admonitions or sympathy...

We need to see pics of the dents to give any opinion. The chain rubing the fd is most likely a bent chainring, and can be straightened. you just had a tune so I would take it back to the shop at the last, and have them look at it.

Without photos, we can't really offer any informed advice.


Minor dents on tubes don't usually affect the ride or the structural integrity of a frame. Downtube dents are sometimes caused by the front brake when the fork is turned too far to the side. The thing to watch for is a ripple under the downtube up near the head lugs; this is caused by a head-on collision and is serious damage; it changes the frame geometry and weakens the tubing significantly.

It sounds like your chain rings are bent; they can be straightened. Perhaps your rear wheel needs to be trued. There comes a point when a bike needs so much work that it having a bike shop do it will cost more than the bike is worth; but it's still worth fixing if you are willing to do the work yourself.

Thanks for the advice. I'll try to post some pics when I get home tonight.

I don't think these dents were caused by the brakes, they're on the other end of the downtube, nearer the bottom bracket.

I wouldn't worry about the downtube dents. The downtube is normally under tension.

Dents can be partially or sometimes completely removed with tight fitting frame blocks with minumal damage to the paint;
https://www.bikeforums.net/showthread...-Roll-in-Dents

If you are going to re-paint anyway and they are not structurally deep, then just fill them with body putty, epoxy, solder or braze.

If the rear skewer threading is not partially stripped, then slipping axel is a sign that the dropout faces are worn so it is no longer parallel from front to back. They get worn from the axel slipping from a loose skewer and then once the slipping starts, it usually accelerates the wear on the dropout. Measure carefully with calipers to verify that it is worn and then use a flat file to thin the dropout a bit so that it is all consistent width and parallel. Also make sure you have a good skewer with sharp steel teeth that bite into the dropout, lesser skewers dont have bite and are more prone to slipping. With a good skewer and perfectly flat dropouts, a horizontal dropout bike will have no slipping even beneath the strongest sprinting gorilla.

One was to narrow down the problem with the rear end is to mount a known good/true rear wheel on it to see if there is anything wrong with thre frame alignment. If it checks out OK, then you know it's the rear wheel you need to dig into. If the wheel just gets to be a pain to get fixed (sometimes they get that way if they are messed up too much from many reapairs, some of which might have been botched up) , you might as well consider getting a different or new one to replace it. It could also be a good excuse for an upgrade. At least you will not have to live with any kind of mysterious problems that the previous owner might have had or caused on the wheel and it will let you start fresh with the bike to enjoy it. .......While you're at it, you might as well treat yourself to a front one too to match the new rear, if the budget allows......

Chombi

Check the alignment of the frame with some string. Tie the string so it exits through the right dropout up to the head tube and back to the left dropout. With the string wrapped around the frame like this measure from the seat tube to the string, if it is not exactly the same on both sides the frame is bent.

Eh. So I got home from work and looked at the dents again. Comparing them with pics of dented frames from old threads on C&V, I've concluded they aren't that bad, may have just been the result of a poorly placed Kryptonite lock or something. But as I was looking the bike over, I did the string test, and took off the wheels and checked the bike out from more angles. The frame is ever so subtly bent, and the fork is bent less subtly. The top tube has a variation of about 2-3 mm in height (see pics, I don't know if you can see it easily, but that's me with an absolutely straight level sitting on the top tube). The two sides of the front fork are not aligned with each other, and the fork is a few degrees off vertical. So the whole frame is slightly off-kilter. It's easy to miss it unless you're looking for it, but it does affect the ride. I did Sheldon Brown's "hands-off" test and the bike pulled sharply to the right as I was riding it. I did the string test (twice) and the variation from side to side was between 1/8" - 1/4". Sighting down the frame from front to back, I could see that the front and rear wheel sit at a slight angle to one another; the chainwheels are perfectly straight but sit at a slight angle to the bottom bracket, hence the FD rubbing.

So I contacted the seller, asking for my money back, and got a somewhat sympathetic email saying, essentially, "you've had the bike for almost two weks, it wasn't that way when I sold it to you". Bulll****. I just didn't have time, with work and family, to track down these problems till now - especially since the bike spent five days out of those two weeks sitting in a shop. The guy I bought it from was not an experienced or very careful cyclist, and seems to have used the bike as a beater around town. That's pretty much what it rides like now.

Anyway, I don't know that I'll get my money back, and ther's not much I can do about it. So I have to think about getting the frame and fork straightened. Do you think that's possible? Or have I just learned a very expensive lesson about buying used bikes?

(BTW, I don't know why 2 of the pics went in upside-down - those are all pics of the top tube)

Thanks for any advice, etc.

Roy

forget about getting your money back. it is your mistake all the way. even if it was 5 minutes after the money exchanged hands, it's yours.

secondly, many/most frames are not perfectly aligned. we need some better pics of the whole bike, the drivetrain, etc to get an idea of what's going on.

right now you have buyer's remorse, but hopefully it's just a few tweaks and everything will be running well. at least the paint looks pretty nice.

i have no clue what im supposed to be looking at in those pictures......

I build frames and also have no clue what I'm looking at. There's nothing in those pics that jumps out at me. The top tube may be bent, but it also appears as though the level is simply not being held parallel to the tube. So it all gets a heartfelt "?" from me.

Now, assuming that the OP doesn't experience the same problem with every bike he rides (A surprising number of cyclists will note that any frame will pull one direction or the other while riding no hands, because of imbalances in the rider) then the frame is indeed misaligned. This is generally not a difficult thing to fix, though if the OP isn't up to doing it himself, he will have to expect to lay out $100 or so for the task.

Oh, and I'm not a Motobecane expert by any means, but to the best of my knowledge, that frame should have Vitus tubing of either .9 or 1.0 wall thickness. Any actual dents would have to be pretty severe before they affect structural integrity. Take a look at the indentations on the chainstays. If the dents in the tubes are deeper than the dents in the stays, then you've got a problem. Otherwise, you've got a cosmetic defect.

I have seen frames bent like this, in general they can be aligned, but those were lugged frames. Basically the bike took a punch to the front I think, maybe not a crash, but a drop, like off a storage hook. Good chance the downtube dents are related to that too. From the written description, the downtube dents are the least of the problems. I suspect the hit pushed the head tube up in relation to the bottom bracket shell, it might be twisted too, and the fork is not a biggie to fix but will take some time.
Is it worth having it bent straight? Probably not. A creative way might be show up at a local framebuilder if there is one and with the naked frame and fork in hand and two decent six packs on a Friday night, he might do it for the libation, and if you are lucky finish before the bottles are ready for the recycler.

If the main triangle is off, it can be fixed with a vice and a length of 1" pipe. Chuck the BB shell into the vice (soft jaws are a very good idea) and eyball the frame from the front or back. The human eye is extremely good at seeing parallels. If the head tube isn't parallel with the seat tube, the unaided eye will detect it. Slide the 1" pipe into the head tube and torque it in the right direction. (You'll be amazed at how much effort this takes. If you throw your back out, you never met me.) A bit of trial and error should make it perfect.

If the rear triangle is off, it will be obvious with the string test. With the BB still clamped into the vice, simply pull the rear triangle (one stay at a time) in the right direction. If you can do the arithmetic, you should be able to fix it in one go. If not, trial and error will again get you where you need to be.

If the fork is off, you're screwed, unless you have the right tools. The fork needs to be perfect in several planes, and should really be set by someone who knows what he's doing.

I don't build frames and this makes sense to me. It appears the gap is oriented on the top of the top tube and it may or may not be the manner in which the level is being held to the tube ? I'd be for holding the level on the sides of any of the tubes to see whether the frame turns appreciably. I'd really take the wheels off and then using twine to determine how straight the frame is laterally. That much side to side might make a difference, but vertically, the level might be misapplied to the top of the tube and creating more gap than is actually there ? I bought an internet SS/FG and determined the fork was off and got a replacement for it. The rest of the frame tracked straight and with the new fork it was near perfect. The bike is a hi-ten Chinese cheapo bike, so I didn't expect it to be perfect, but not nearly as bad as the original bike came. It rides just fine to this day.

Thank God! I thought that was just me! I can't ride more than a few revolutions of the crank without needing to adjust my course. It's only gotten worse after a series of head injuries over the past 5 years (only half related to cycling...).

I know what's wrong, the level is bent.

LOL...this is funny....well that's why we're here to educated you zazenzach...here at the C&V forums we believe knowledge is power so here it is...

the picture of the fork is so we can see if the fork is bent or not...

the rest of the pictures are to see if the frame is still straight, the level is to provide a straight edge to the top tube, as you can see there's a gap in the level so something is bent or tweaked....

Just me, but there are only 3 planes to a perfect long thin rectangle and that's all a bicycle frame and it's forks make when assembled assuming the axles and wheels have been trued. There's the vertical (front to rear) plane. The horizontal (side to side) and then there's the 90 degree planes that the axles need to be for the bike to track straight like this from the top:

Front --|-|--------|-|----|-- Rear (the first | is the intersection of the front wheel, the last | is the intersection of the rear wheel. Bottom bracket (3rd |), head (2nd |) & seat tubes (4th |) need to fall in line with those points.)

If that is straight at the front & rear axles the bike will track in a straight line.

The key is perfectly 90 degrees for straightness. A fork with more rake can offset the negligible gap of the top tube from the top since the brake pad angles are adjustable. That much gap probably means the front wheel is about that much closer, give or take to the down tube at the outer circumference of the tire for that wheel clearance.

BTW, the human eye can be tricked with depth, not everyone has 20/20 or better perfect vision in each eye and that's where a high tension string/twine/wire stretched from 2 points nailed eliminates any doubt. I'm not so certain about lasers, because that might depend upon the perfectness of any lens ? But for the most part I'd think a laser is optically, perfect straight ?

You're probably just smarter than me. That isn't saying much, though, so don't gloat.

Re. alignment (in your subsequent post) my thinking was once exactly like yours. Then I started building and aligning frames and forks, and discovered that my thinking was really, really off.

Wow! Thanks for all the replies. Look, I'm really a newb at this, and appreciate the feedback. I was quite the cyclist about 25+ years ago, and want to get back into more serious riding. And it's true, I'm experiencing a LOT of buyer's remorse. The last time I bought a Motobecane was in 1979 (!) it was brand-spankin' new, and it rode like a dream. Thanks for taking it easy on me...

The level was just to illustrate the fact that the top tube is not straight (level has a perfectly straight edge). Either end I squared it to, the top tube bends away from the level at the other end. Seems to be a bit of a gradual "hump" coming back from the headset. And BTW, I ride pretty straight. Even on my old Super Course, I can ride with "no hands" for half a block before I have to adjust (not that I do it all that often, but even so...) I've never experienced a bike pulling to one side.

So, on Friday I'll take it to a local shop with a decent rep and see what they allow about straightening the bike out. Seems like the consensus is these things are probably fixable. If I wind up with a smooth-running bike, and don't just throw good money after bad, then I'm set, and I'll chalk the whole episode up to experience. I don't really want my money back so much as to have this bike in good, working order.

Sorry for the lengthy posts. I kinda panicked when I saw the damage to the bike and realized what I'd missed when I bought it.

FWIW, every once in a while a new tube will come from the manufacturer with a slight bend in it. A knowledgeable builder will roll every new tube on a flat surface to verify straightness before using it in a frame. I make no claims of being knowledgeable, which is how a bent tube found its way into a frame I built a few years back. It's not bent enough to be noticeable unless you use that straightedge, and because it was built into the frame from the start, the end result is still aligned correctly, kind of akin to what Fuji86 is writing about.

Which is all a long-winded way of saying that the bend, by itself, is nothing to be concerned about. The frame can be aligned correctly and ride perfectly, even though the tube itself may remain a bit bent.

Good to hear. Thanks, Six jours.

I always thought it was the imperfections of the road & planet Earth. A bike should roll perfectly straight when the cyclist is rolling the bike slowly on a perfectly level walkway or floor, handsfree from the handlebars. I usually try this with one hand on the seat and the bike & wheels as perfectly in alignment front to rear as possible. It's difficult to do because the front tire will want to come out of that perfect alignment after a few inches or a foot or so into the roll. I presume you could roll it backwards and the front wheel would stay in alignment longer as it's dragging/trailing instead of being the leading line ? Even on as perfectly level a sidewalk as one can find, there are still going to be imperfections. Even the tires have imperfections that may influence perfectly straight tracking. Controlling all of those variables is difficult if near impossible. Again locking the front fork a perfect 90 degrees with the vertical plane front to rear of the frame, the bike should roll perfectly straight.