V Brake centering

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elduderino
Posts: 16
Joined: 2 Jun 2016, 6:25pm

V Brake centering

Postby elduderino » 6 Aug 2016, 7:56am

Hi
I have Shimano Deore LX V brakes.
My problem is that after centering the pads(via the tension screws),& riding the bike, the front stay centered, but the rear do not.
I have tried two of the three tension pin holes(third gives no tension at all).I have tried bending the tension arms to increase/decrease tension over one side or the other. I have made sure that all moving parts are lubricated & moving freely.
Is there a solution for this, or is it time to replace the rear brakes for XT, or some other recommended make.
in all other respects the brakes work fine.

Regards

mercalia
Posts: 14456
Joined: 22 Sep 2013, 10:03pm
Location: london South

Re: V Brake centering

Postby mercalia » 6 Aug 2016, 8:08am

I have bog standard shimano V brakes C 1999 and they centre ok. Mine have just 2 screws one on each side to screw in or out. BUT it can be a bit of a fiddle, as when you screw in/out one screw it affects the other - there is a knack to it, you have to do a little at a time & jiggle the brakes and exercise them to see the effect as you do the adjustment. I dont think bending things a good idea. I assume the wheel is properly dished in the first place? just realise that setting up the brakes not as straightforward as putting a chain on ( only one way to do it ) and see it as some thing you have to learn to do with patience, you were maybe lucky with the front wheel

cromo
Posts: 64
Joined: 3 Aug 2009, 9:10am

Re: V Brake centering

Postby cromo » 6 Aug 2016, 8:28am

Is it possible that the short length of brake housing that runs from the cable stop to the V brake is too long or too short? This might be introducing a "variable force" that is acting on the V brakes. I say variable because the housing does move when the brakes are applied.

Brucey
Posts: 41384
Joined: 4 Jan 2012, 6:25pm

Re: V Brake centering

Postby Brucey » 6 Aug 2016, 10:11am

some possibilities;

- as Cromo suggests, is something fouling/pulling on the brake arms?
- are the brake arms binding at all on their pivots?
- is the main brake cable free-running or does it drag?

A flaw with V-brakes is that the centring force is rather small; the slightest problem will cause the brake to misbehave.

There are several different models of LX V-brake, which have different features. All however have what can be termed an 'integral bushing' i.e. the brake arm pivots on a bushing that is part of the brake arm, rather than around the brake boss on the frame.

I can't overstress this enough, but the integral bushings need to be **perfectly** free-running. That you have felt the need to be fiddling about with the springs is usually a good sign that at one point the bushing wasn't free running in one or other of the brake arms.

This usually afflicts the rear brake before it afflicts the front; I think that the wheels throw more crud over the pivots of the rear brake, and they bind/seize up first/worse (as does the cable itself). Usually the pivots can be made to become free-running again by using a penetrating spray lube (such as GT85 ) and working the mechanism back and forth for a few minutes. A semi-fluid spray grease is a better lubricant in the long term, but almost any oil will do provided a drop or two is added regularly.

Just for kicks I recently salvaged a set of shimano V brakes from the LBS scrap bin; they had seized/draggy pivots and at workshop labour rates it wasn't worth fixing them. The least draggy arm came free after a minute or two but the worst of them took about five minutes of waggling (with lots of spray lube) before it came free. [ NB. I mounted the brake arms one at a time to an old fork so that they could be swung through a decent angle without obstruction; it would have taken far longer to free them had the arms had restricted movement.] Lots of black gunge oozed out from the bushings as they were worked back and forth, but all four brake arms came good in the end.

The brakes I freed up were easily the worst I'd ever seen (on a bike that was being used), but they work perfectly now. I have little doubt that if your brakes are similarly afflicted, they could be made to work well again too.

However if you have been resetting the springs by bending them, it is possible that you have made them a different shape such that they are less likely to exert an equal force through the full movement. Some care may be needed to make sure the springs are perfectly balanced. [Tip; if you unhook the springs from the arms, they should sit at the same angle as one another; if they don't, the brake probably won't work properly and/or still has draggy pivots or something.]

If you are shopping for new brakes, I think that just now there isn't that much difference between the various shimano models on offer; you get better brake pads and different branding, but the arms themselves don't vary an enormous amount, ever since they ditched the (clever, but squeal-prone) parallel-push linkage design (as used on old model XT and LX etc).

Any new brake will benefit from a drop of lube from time to time, which could be where it all went wrong for your old ones.

hth

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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Sweep
Posts: 6703
Joined: 20 Oct 2011, 4:57pm
Location: London

Re: V Brake centering

Postby Sweep » 7 Aug 2016, 7:42am

Wonderful post as always brucey.

Thanks for the info that all shimano v brake units are now near as dammit the same. Have long wondered/suspected.

And this is i suppose another good argument for v brakes on expedition tourers. In the unlikely event that a unit is totally trashed, it shouldn't be hard to source a just as good replacement.
Sweep

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NATURAL ANKLING
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Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: V Brake centering

Postby NATURAL ANKLING » 7 Aug 2016, 9:11am

Hi,

Like suggested just play with them a lot and keep lubricated.

Your back wheel is not moving is it.............one to watch.
NA Thinks Just End 2 End Return + Bivvy
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.

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Gattonero
Posts: 3730
Joined: 31 Jan 2016, 1:35pm
Location: London

Re: V Brake centering

Postby Gattonero » 7 Aug 2016, 11:54am

elduderino wrote:Hi
I have Shimano Deore LX V brakes.
My problem is that after centering the pads(via the tension screws),& riding the bike, the front stay centered, but the rear do not.
I have tried two of the three tension pin holes(third gives no tension at all).I have tried bending the tension arms to increase/decrease tension over one side or the other. I have made sure that all moving parts are lubricated & moving freely.
Is there a solution for this, or is it time to replace the rear brakes for XT, or some other recommended make.
in all other respects the brakes work fine.

Regards


Sometimes the springs get too much tension set, and one of the two gets weak. Before bending the springs, is worth to completely release the grub screw adjuster on both arms (when present), and start over again in small increments.
Of course, you have to check that the noodle pipe is not touching, and is on a correct angle in respect to the brake housing, often and incorrect angle of the noodle or the housing too long/too short can pull one of the brakes.
Needless to say, the pads must not have uneven wear, as they tend to "stick" on the rim when they have grooves
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...

elduderino
Posts: 16
Joined: 2 Jun 2016, 6:25pm

Re: V Brake centering

Postby elduderino » 8 Aug 2016, 9:48am

Hi

Many thanks for the very informative replies.

Being a bit of a tweaker, I now have a lot more to work on. Will report back with results.

elduderino
Posts: 16
Joined: 2 Jun 2016, 6:25pm

Re: V Brake centering

Postby elduderino » 17 Aug 2016, 7:52am

Hi

Sorry for taking so long to get back, but needed to do a couple of test rides.
I took Brucey's advice about checking the bushings. I soaked the brake arms in paraffin oil, & then lubricated with GT85.
Also made sure that the springs were equal, & then tweaked with the screw's.
it seems to have worked , because have done a couple of rides, & the pads have stayed centered.

I know GT85 is not strictly a lubricant, but if i give the arms a paraffin oil soak/GT85 clean on a regular basis it should keep the bushings dirt free.

Again many thanks for the info.

Brucey
Posts: 41384
Joined: 4 Jan 2012, 6:25pm

Re: V Brake centering

Postby Brucey » 17 Aug 2016, 9:19am

elduderino wrote: I know GT85 is not strictly a lubricant....


GT85 isn't much different from a lot of other spray lubes (WD40 etc) in that it is mostly solvent but contains oil (about 15-20%) too.

If you spray some GT85 into an open container, you will see that after a few hours all the solvent will have gone, and you will be left with oil.

So GT85 is most certainly a lubricant; it just isn't the best lubricant for the job of keeping the pivots lubed. In the summertime it is OK but in the wintertime salty water seems to cut right through it.

Glad you have got your brakes sorted out, it sounds like you are on the right track now anyway.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~