pstock

"Why" you were probably not wondering "is peter swapping on and testing every set of 20inch wheels he has? Instead of cleaning up the garage like he promised"

I swapped from short arm Tektro Mini-V calipers on my Bike Friday (because the attachment thingee was just touching the rear tire) for regular length V calipers.
but on the new calipers I noticed how close to the LH crankarm the LH caliper was sitting , when installed and adjusted tightly. the crank arm barely clears it. while one the RH side there is plenty of clearance, about 25mm.

Why?

I checked the hardware for extra spacers but they were the same on both sides.
moving the spring pin to a different hole has no effect.

Finally, I measured how centered the wheel was. and .... Yes, from the outside of the seatstay to the top edge of the brake track was about 5 or more mm greater on the RH side than the Left.
and the wheels are well seated in the dropouts. Now.

which made me wonder if that wheel (#1) was not "dished" correctly.
Maybe.

so I pulled two other 406 rears and tired them.
one measured about centered. a difference of just 1mm
but another was equally off. 39mm on one side 44mm on the other side.

so what now?
pop the wheels into a truing stand and test them there?
is there any other reason the V-brake calipers could be so off?

RB1-luvr

I find it difficult to get the v-brake arms symmetrical. I always have one at a different angle than the other. You've adjusted the small screw on the right arm to try to pull the left arm in?

Andrew R Stewart 

If the wheel sits centered in the frame and if the brake bosses are located in the same places of either side the brake pads generally are in the same spot in the arm's slots. The photo doesn't show this as the left pad is high up it its slot and the right pad far down. This miss matching pad position can be an indication of a wheel that is not centered, for whatever reason. It is hard to see well enough with these photos to confirm this. And our ability to best understand these positional relationships is less.

I would go back to a base line of what you can confirm. Is the axle bent? Is the rim centered/dished WRT the axle lock nut edges? Does the wheel seat fully in the dropouts? Once the wheel is confirmed to be "right" only then would I begin to explore the frame aspects.

I suspect there's an issue with the wheel and/or how it fits in the frame.

BTW the very first aspect I looked for is the pad mounting spacers/orbital adjusting washers were of the same thickness on each side of the bike. They are. Andy

KCT1986

A thing to check is the distance between the posts that the brakes mount unto. Shimano MTB V-brakes are usually speced to use an 80mm spacing standard. Narrower spacing would require that the rims braking surface is also narrow. Matching narrow post spacing with wide rims would cause the brake arms to be splayed out too much (even with all the brake pad mounting spacers 'outside'.

Also the design of the early V-brake with the 'parallel push' system moves the pads further inwards from the arm, which makes narrow post spacing even more problematic. The BR-M739, which appears to be what you have, needs to be splayed outward more than usual than other models.

So, it seems that the frame may not have been designed to work with MTB V-brakes (long arm) and the rim width combo you're trying. The M739 makes it worst.

pstock

Update.
so to recap.
the brake pads are equidistant from the rims so it is not a question of spring tension.
which pointed me to my wheel not being centered. maybe due to the dishing being off.

and measuring from a point on the seat stay to a point on the rim, Left and Right, gave me different readings. about 5mm off.

but a 2nd wheel gave the same result - LH V brake caliper arm being way to close to the crank arm (and visibly being too far Left)

Could I really have two off-dish rear 20" wheels?

So I put both these wheels in a freshly calibrated truing stand AND checked them with a dish-checking tool . and they are both perfectly symmetrical. apparently correctly dished.

so what does that leave me?
the frame is not straight?
I did put the rear derailleur into the spokes on this bike once and bent the derailleur hanger (but only it seemed VERY slightly as everything still functioned afterwards, it wasn't like I ripped the hanger off the frame or trashed the wheel)

I am baffled (and reading up on how to check frame straightness)

pstock

I wish it were as simple as spring tension adjustment.
but the brake pads are correctly gapped Left and Right and the arms spring back equally.

pstock

Ahh, yes. that top photo was mostly meant to show the orientation of the tire relative to the frame (that hole and the silver bolt) which all looks "Off centre" to my eye.

at that point I had been uninstalling and reinstalling the caliper arms a few times and because the fit is very tight I had to remove and reinstall the pads a few times.
I might not have had them aligned correctly when I took that photo.
I'll check that "height in slots" tomorrow.

john m flores 

This was my Pocket Crusoe after mounting new brake pads.


And this is after someone here advised me to re-arrange the brake pad spacers to create a bigger gap. Note that the pulley is not technically required (I converted the bike to flat bars) but helps create a tighter pull vs. a traditional noodle.

Kontact

The left pad is at the top of the adjustment slot and 90 degrees to the arm. The right pad is much lower, but angled up at least 30 degrees above perpendicular.

I don't see how you can tell anything about the wheel or the brakes with the pads so wonky. Raise and straighten the right pad.

pstock

that first photo was just to show the tire orientation. I had just reinstalled the caliper arms to make sure the spring pins were in the same hole (which they weren't but that didn't change anything either) and so I had to remove the pads to fiddle with the arms. I took that photo before I had correct repositioned the pads.

BUT you are right. looking at things again just now, with the pads correctly installed and positioned on the brake tracks one pad (LH) is at the top of the slot while the other (RH) is much lower down.

I confirmed that the axles are firmly seated in the dropouts Left and Right
Last night I checked the two sets of wheels I tried and they seemed both dished correctly

Now I am completely confused.

could the dropouts be at different heights? so the wheel, when installed would be akimbo/off center?



RH pad correctly positioned on the brake track

but RH pad is low in the slot.

RH axle fully seated.

LH pad correctly positioned on the brake track

LH pad positioned HIGH in the slot

LH axle fully seated

hokiefyd 

Your pictures confirm that the left side brake pad is slid all the way up to the top of its adjustment slot and the right side brake pad is slid all the way down to the bottom of its adjustment slot. Aligning the brake pads the same way in each arm should help the situation with one arm splayed more than the other (or at least rule pad adjustment out as a possibility).

As to why the arms are so splayed out in general, I think the XT arms (with the parallel push design) probably contribute to that, as the parallelogram action is just physically larger and moves the pad further from the arm, even in the at-rest position. They're also designed to clear larger MTB tires, so their design is such that they curve outward by intention. It looks also like your pads may be pretty bulky, with the replaceable rubber insert design. You can find low profile threaded brake pads (such as these Thinline pads from Kool Stop).

Using thin pads combined with a brake arm with a relatively svelte profile (such as this M530 from Tektro, just as an example) should improve your crank clearance.

pstock

But if I slide that RH Pad higher in the slot it would be hitting the tire instead of the rim brake track when applied.

Kontact

It shouldn't, if you bring the angle down as well.

The question is whether you want to ride the bike or diagnose the wheel problem. Did you try putting the wheel in backwards to see if it offers the same way?

soyabean

Does the OP have any new pads they can try?

Old pads can have been installed wrong and wear down lopsided, skewed.

They can't really be re-seated unless you shave them down back to parallel, which is what I do often to dumpster finds. I do get bulk jobber pads for about $1 each, but I still reuse whenever I can get away with it.

mpetry912 

I looked at the pics and I see several things but this is the big problem. Either the brake posts are assymetric (unlikely) or you have something else set up wrong on the brakes. I disagree with your contention that "if the pads are equidistant then spring tension should be the same". Disagree.

Not to mention the possilbe pedal / brake interference. As I think of it perhaps one post is bent ?

/marpk

Kontact

The wheel is off center.

Aubergine 

Yes, it seems to me that the first thing to check is the wheel dish. If the wheel is correctly dished, then check the alignment of the rear triangle of the frame. If the frame and wheel are straight, then and only then start fixing the brake alignment.

soyabean

Were there always V-brakes there?

I've only seen U-brakes with road pads by the chainstays.

Otherwise V-brake arms are just too dangerous, not just for barely clearing the crank, but for cyclists' foot.

Kontact

That's not a U brake.

bboy314

U brakes use different posts than linear pulls, and are mounted to a different part of the frame relative to the wheel.

grumpus

Take a wheel, check that the axle is not bent, the bearings are properly adjusted and the rim is true. Fit it to the frame and measure the distance from each chainstay to the rim (this will probably be easier without a tyre fitted). Now remove the wheel and fit it backwards, measure again. What are the measurements?

Kontact

Or maybe he could put the wheel in backwards.

pstock

Yes I want to ride the bike .
but I am heading off on a Two week excursion in about a week and so I would like to try to resolve wheel and brakes before I leave.

what do you mean by "bring the angle down"?
​​​​​​

bboy314

In addition to the pads moving up and down in the arm, they can be angled up or down, using the convex/concave washers. This is generally done to match the angle of the rim relative to the brake arm, so the pad engages the rim evenly.

grumpus

I do believe that's what I wrote, only in some detail so we might hopefully get some useful data.