A reverse Salvador Dali; -or how to straighten wheel rims

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Brucey
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A reverse Salvador Dali; -or how to straighten wheel rims

Postby Brucey » 11 Feb 2018, 3:42pm

Do you have wheel rims that, erm, 'persistently' look a bit like these clocks...?

Image

then you may need some advice on how to straighten them....

In many cases if a wheel rim gets badly bent the quickest and easiest thing to do is to replace it with a new one. But in some cases it may be desirable or even necessary to straighten the old one. For example if the model of rim (or one with similar ERD) is not readily available, or you are in the middle of a trip and a replacement rim cannot be sourced locally in a timely fashion.

Well in many cases you can, using simple tools only, recover quite badly deformed wheel rims so that they are at least usable and in some cases little different from new rims. This post explains and demonstrates some of the techniques you can use.

The rim below is a French made weinmann/rigida ZAC19 double-walled, double-eyeletted rim in 700C (622) size, sourced from an LBS's scrap pile. To start with it was pretty badly bent, mostly laterally, so that it would 'run out' at least 2-1/2" if built into a wheel with even tension. In the first picture it is leant up against a 'gauge wheel', i.e. one that is pretty straight:

Image282.jpg
Before....


Once straightened it looked like this;

Image283.jpg
After....


It is true within 1mm. Some new rims are little better than that.

The method used was to:

1) gauge where the worst bend in the rim is and mark the outside of the bend apex (OBA). (This is where the rims are touching in the first photo)
2) lay the rim on the ground, so that the rim rests in two places (180 degrees apart) on blocks of softwood, in this case offcuts of 4 x 2" timber. One of the contact points is the marked OBA.
3) stand (in soft soled shoes, eg trainers) with your weight on your heels but with your toes over the rim, about 8" either side of the OBA, so that you have a three-point bend with a loaded span of about 16"
4) gradually transfer some of your weight from your heels to your toes
5) you will feel the rim deflect; first elastically and then plastically. It may take a few goes to pluck up enough courage to move the rim plastically; if so, no worries, you can always have another go. Far better to underdo it than to overdo it.
6) If you think you have done some good, using the gauge wheel, choose another bend (a different one will now be 'the worst') and repeat.
7) Carry on until the rim is straight.

Points to note are;
a) that at any given time the rim is likely to contain a mixture of plastic and elastic deformation. If you correct one bend there will effectively be another one (that could be of either sort) of the same sense on the other side of the rim. If elastic, it may mysteriously disappear once the opposing bend is removed.
b) Elastic deformations are usually nothing like as abrupt as plastic ones. The second worst thing you can do is to try and correct an elastic deformation by plastically deforming the rim; this will make the rim very likely to end up shaped like a thrupenny bit.
c) the very worst thing you can do is to mark the OBA badly so that the correcting bend is not centred over the original bend. This creates an 'S' bend in the rim, and in rims with any tendency to work-harden. it may prove impossible to rectify this fault.
d) the radius of the bend you are trying to correct may vary; in this case you can vary the width of the OBA support (eg by laying the rim diagonally across the corner of the timber offcut) and thus vary the radius of the correcting bend.
e) the width of the loaded span can be varied to allow for variations in rim strength and bodyweight.
f) there are two points on the rim that will behave differently from any other; they are the valve hole and the rim joint (esp if a pinned joint). A pinned joint can't be anywhere within the loaded span, and (to a first approximation) it is a bad idea if the valve hole is within the central 8" of the loaded span, unless the bend you are trying to correct is exactly centred over the valve hole.
g) considering f) above, pick and choose the rims you try and straighten carefully, and adjust your expectations accordingly if they are deformed near the valve hole or rim joint.
h) some rims work-harden when bent and straightened; these rims are more difficult to work with and they need more care with selection and placement of the OBA support.

[ Other damage; If the rim has dinked bead (like the one in the picture) don't worry about getting that out until the rim is straight. Lay the inside of the rim edge over a vice jaw at ~45 degrees, and (using a soft-faced hammer or a hammer and timber bolster) give it a few whacks until the dink is gone. If the rim is now slightly wider than it should be, this can usually be corrected with a gentle squeeze in the vice.

If the rim is slightly egg-shaped (eg through pothole damage), then this is best corrected when the wheel is whole. The best method I know of is to leave the spokes at full tension, and remove two or four where the flat spot is. Hold the wheel in your hand so that the damage is lowermost (NB on no account try to support the rim from below against the tyre beads; you will mangle the rim). Take a horizontal length of stout timber (eg 4x2") about 18" long, and support one end on the workbench, and the other end on the apex of the bend, i.e. where you took the spokes out. Using hammer with a head about 1-1/2lbs in weight, give the timber a goodly whack (as close to the wheel as you dare), and check the rim for roundness (eg against a known good gauge rim).

Now, it will feel like you are trying to break your wrist in the hand that is supporting the wheel. You may think you need to hold harder, else the wheel won't straighten; this would be a mistake, because the load of the hammer blow is taken by the inertia of the wheel. Also the fact that the spokes are still tight elsewhere in the wheel lowers the force required to push the rim out enormously. Once the rim has a local bulge that is ~1mm proud of the rest of the rim, you are done. The rim should pull back in locally when the spokes are replaced and tensioned.

Note that if the rim is particularly thin walled where the spoke drillings are, you may need a wider support where the timber bears in order to avoid denting.
Also, if the correcting bend is not quite the right shape to start with, you can fine-tune the shape by removing (or simply slackening) more spokes.]

You may think that I cheated with the rim above; well I did inasmuch as I chose one without bends at the valve hole or the rim joint, and without big flat spots in. The rim is also quite stiff, but not particularly strong, and is made in a material that doesn't work harden too quickly. So I loaded the dice in my favour. I also chose a rim that had been used in a wheel with a hub brake of some kind; the braking surface is both unmachined and unworn; I would perhaps find it difficult to buy a better rim new.... :roll:

As a little guessing game; I took the first picture, immediately worked on the rim, and then took the second picture without delay. My camera told me the elapsed time; can you say what you think it is?

Also, have a guess at roughly how many corrective bends I had to put into the rim to get it straight.

I shall reveal all in a day or two's time.

cheers
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Gattonero
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby Gattonero » 11 Feb 2018, 6:14pm

After 7 minutes the dent on the braking surface is still there.
It is by riding a bicycle that you learn the contours of a country best,
since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

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andrew_s
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby andrew_s » 11 Feb 2018, 7:08pm

6 min 23 sec
6 tries

brynpoeth
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby brynpoeth » 11 Feb 2018, 8:32pm

What are plastic and elastic deformation?
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Brucey
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby Brucey » 11 Feb 2018, 8:41pm

brynpoeth wrote:What are plastic and elastic deformation?


elastic deformation is what happens in a spring; the part recovers its shape.

Plastic deformation is what happens when you bend something to a new shape.

cheers
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colin54
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby colin54 » 11 Feb 2018, 9:56pm

A similar method demonstrated here, (starts about 3minutes into the video ).

https://www.youtube.com/watch?v=i73vVq7 ... 5Ik2AIf7dQ

Samuel D
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby Samuel D » 12 Feb 2018, 10:43am

The time is in the EXIF data, which I looked at. Given that and quick work, my guess is four corrective bends along the way.

It’s a good result. As you recall, I tried something similar recently and saw the difficulty of this job.

Is it not worth keeping the rim laced under low tension to encourage it to stay radially round as you work on the lateral faults? Although that may hide the plastic deformations underneath yet more elastic ones.

Brucey
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby Brucey » 12 Feb 2018, 11:19am

Samuel D wrote:
Is it not worth keeping the rim laced under low tension to encourage it to stay radially round as you work on the lateral faults? Although that may hide the plastic deformations underneath yet more elastic ones.


I say chaps, you didn't have to look at the exif data.... where's the fun in that...?. :wink: BTW I might have reset the clock in the camera between shots... did you think of that...?... :lol:

Having straightened very many rims in this way I can say that having the wheel laced (even with spokes that are fully slack) is of no advantage whatsoever when tackling typical lateral bends; in fact it is of considerable hinderance, since the wheel needs to be supported higher up to allow axle clearance (which is awkward) and whatever lateral loads you apply to the rim are likely to be shared (in an uncontrolled and unknown fashion) between the rim and the spokes, and this makes putting a controlled 'set' into the rim incredibly difficult.

The only benefits from having the wheel laced are

1) when you are doing an emergency repair; it spares you from having to take the wheel to bits or
2) when trying to tackle a bend that is near a pinned joint.

The latter procedure is only worth attempting if the wheel is still laced and still has a fair amount of tension in it. You may need to straighten the rim where it is bent elsewhere, and put some tension back into the wheel, all before tackling a bend at a pinned joint. Even then the outcome is far from certain; this is something to try when you have a few other rims under your belt, as it were.

I have dismantled deformed pinned joints, refaced them, and reassembled the joint before now. This will take a long time to do and the outcome is rarely a perfect one. I advise that this isn't worth trying except in truly exceptional circumstances.

cheers
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mig
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby mig » 12 Feb 2018, 4:34pm

it took 11 bends in 8 and a half minutes.

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531colin
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby 531colin » 12 Feb 2018, 7:18pm

I think that very few people are actually likely to try to straighten really bent rims, and even then its more likely to be as a "get you home" exercise than something you would do at home, in a nice warm dry workshop.
So, as a "get you home" exercise, I think that you are unlikely to did-assemble the wheel (and I don't even take the tyre off) .....however, I recommend slackening right off the spokes in the area of the bend, otherwise you are going to have to completely overcome the spoke tension in order to get the rim to take a set....you have to bend it past straight and past the plastic deformation point to get a set.
This is how I do it....
Mark the bend.....middle and both ends
Slacken off the spokes in the affected area...count turns.
Place the "middle of the bend" on a flat stone ( or whatever comes to hand)
Stand with one foot on each of the "ends of the bend" (a helping hand for balance is invaluable)
Grab the opposite side of the rim with both hands and pull up to straighten the rim....leverage is with you
.......yeah, I know its only now that you know which way up the wheel should go.......
I find its much easier to gauge the amount of force I am applying by pulling up on a rim than by how hard I am treading on a rim (or jumping on it...)

If you actually try this in the workshop, I find it works well with a trestle so I can support the "ends of the bend" on the trestle and hook the middle of the bend under the workbench.....push the free end of the rim down to straighten.

I recommend you don't ask your favourite wheelbuilder to do this!

Brucey
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby Brucey » 12 Feb 2018, 8:03pm

there are any number of ways that work well enough to get you home, but FWIW if you try and correct a lateral bend in a rim in a wheel in which the spokes are only locally slackened, there are several problems

1) the spokes restrict the deflection you can achieve in the rim, so that not all bends can be taken out this way. On the plus side you perhaps are unlikely to overdo it I suppose.

2) it is extremely unlikely that you will put the corrective bends in the right place; the rim is most likely to end up with a pair of 'S' bends that are then very difficult to correct properly.

3) if the rim starts to yield laterally near where the spokes are slackened, it is very likely to wind up egg-shaped, i.e. the tension in the other spokes will tend to push the unsupported section of rim outwards.

The last of these occurs because when the rim reaches yield, it has virtually no strength remaining in any direction.

BTW for a roadside repair, if you are strong (and you don't need to be very strong with a single-walled rim BTW) you can just use your knee against the protuberant part of a lateral bend and pull by hand either side of that. There is no need to deflate the tyre (it is more comfy on the knees if you leave it inflated), or even remove the wheel from the frame if you don't want to. If you want to help things along without the wheel egging too badly, just slacken the spokes on the knee side of the wheel only, local to the bend.

Both the above method and the one described by Colin might get you home but they also have another disadvantage; it is difficult to see or feel exactly how much displacement you have created in the rim. With the best will in the world they create basically uncontrolled deformations in the rim. By contrast the method I described upthread allows you to both see (vs the ground) and (because your heels remain on the ground) feel the amount of displacement very accurately; this leads to more accurate work, and better results. In fact I would go as far as to say that this is the only method I know of (and I have tried plenty, believe me) that stands a reasonable chance of producing a perfectly straight rim again in a reasonable length of time.

Regarding the 'is it worth it?' question; the repair is otherwise to remove the old rim and replace it with a new one. If you remove the old rim, straighten it, and refit it, there is no labour saving per se but if you are good at it, you can straighten a rim more quickly than you can buy a new one, it matches perfectly, it is there right away, you don't have to go someplace to collect it or wait for some numpty in a white van to turn up and hurl it in the neighbour's garden or anything...

If you do replace the rim, the old one can be straightened 'offline' i.e. at your leisure and you can use it as a spare.

cheers
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willcee
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby willcee » 12 Feb 2018, 8:22pm

Some years ago on a summer sunday outing myself and 2 chums were cycling up a very steep cyclepath beside a cliff overlooking the sea, just on the outskirts of a local resort town, in the summer all sorts take to cycling as we all know and a gaggle of young chaps were trying to get up this slope,tight enough for hard cyclists, they were in front of us all over the shop and one had ground to a halt causing a smack from his mates behind right on the side of his rear wheel, we passed by amid the shouts and laughter from those affected, stopping at the top where the path joined the highway again to see if we could assist, it was certainly banjoed, a BSO hybrid with cheap machine build wheels, he was hauling it up sliding the jammed rear wheel the last few yards, one of our crew asked if we could assist, if you like he says, throwing the machine on the grass, we removed the rear wheel , one of us stood on the wheel and another on the opposite side, it sprang back not true but rideable with the V brakes open, these chaps stared at us older blokes in total amazement, thanks he says, the oldest of us, a real wag, said to him, these boys work at bikes, but get it trued at a bike shop before you venture out again.. it all took about 5 minutes, but as a big guy he was untrusting of it taking him home.. so they walked the last few hundred yards to their guest house, wheeling the now mobile machine..Bruceys contri is informative and very useful, however this always depends on the strength of the rim, some so strong you will never get back to where you would consider a rebuild.. will

Brucey
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby Brucey » 12 Feb 2018, 8:52pm

to date I have yet to encounter a rim that would bend, but not be bent back.... :wink:

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Chris Jeggo
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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby Chris Jeggo » 14 Feb 2018, 8:03am

Search for 'P66 dali' in http://www.simondrew.co.uk/web/3_catalogue/b_prints/prints.html.

Straighten those!

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Re: A reverse Salvador Dali; -or how to straighten wheel rims

Postby francovendee » 14 Feb 2018, 8:31am

Brucey, I wish you'd posted this 2 months ago. I had 2 wheels that I decided were just taking up space in the garage so took them to the tip!

Normally I keep things like this indefinitely but pressure from my wife to have a clear out changed my approach, just wish I'd ignored her :(