Reducing free stroke on Shimano XTs
sorry, don’t know the answer but
small hands with brakes that have an inch of free stroke aren’t a good combination
Really – Why not ? Contact point closer to the bars; I’d have thought that might be a good thingPosted 7 years ago
Are they the same model? Some XTs are servo-wave and pull the pads close to the rotor really quickly compared to others
I know it’s been raised before on here. Does anyone have a solution for this yet(other than the filling-reservoir-with-pistons-not-completely- retracted option)?
I have 2 sets of XTs. Both are bled properly (bottom up). There is no air in them at all.
One set, the levers move, at most, half an inch before being solidly on. Other (newer) set, the levers move, over an inch (before being solidly on). The latter ones are on my other half’s bike and small hands with brakes that have an inch of free stroke aren’t a good combination.
Problem seems to be that the pistons retract back completely into the calipers as the brakes are released rather than just backing off the rotor a little bit….as (all) disc brakes (in theory) are supposed to. And strangely – this seems to be at its worst when pads have minimal wear.
The Shimano free stroke adjuster as other people have pointed out here doesn’t adjust the free stroke, merely alters the cam and hence the rate at which brakes come on – so that doesn’t seem to be the answer.
Any ideas gratefully appreciatedPosted 7 years ago
@ scaredy pants –
Because if you move them closer to the bars to accomodate small hands…. given that there is an inch of free stroke… the brakes won’t have even come on by the time the levers are hitting the bars
The servo wave action is what I was referring to when I mentioned the cam/free stroke adjusterPosted 7 years ago
There is a theoretically easy solution to this, which I have used to good effect on many brakes from many manufacturers (especially those awful brake-shaped-objects that the purveyors of SRAM seem to want to stick on every cheap bike going…)
1. Take the rotor (whole wheel really) out.
2. Observe the pistons/pads.
3. Squeeze the lever slowly so the pistons creep out.
4. Let go of the lever.
What we’re trying to do here is fool the system into believing there’s (effectively) a narrower rotor in there. Sounds simple, right? Well, the tricky thing is to do it just enough that the pistons are set in a new position (i.e. further out than they would be if you did this procedure with the rotor in place) but a place that is not so far out that the pads rub on the rotor (touching is fine, and even a little bit of ‘brushing’ noise is fine – it’s not slowing the wheel down in the grand scheme of things.)
That probably wasn’t explained too well, so shoot any questions you might have… 🙂Posted 7 years ago
Also: The amount of fluid in the reservoir of an open system has NOTHING to do with the actual operation of the brakes (unless there’s too little, of course). This is because, as soon as you press the lever just a tiny amount, the brake effectively becomes a closed hydraulic system until you release the lever again.
Just wanted to clear that one up as it seems to be a popular misconception.Posted 7 years ago
TJ – Interesting. Is it theory or for real? It’s always seemed to me that the pistons retract under the vacuum that’s generated in the pipe when the brake lever is released
I’m assuming you mean the pistons and seals in the caliper (as opposed to lever) I can see the seal in the caliper could drag the piston when it’s moving out, but actually pulling it back into the caliper again – I’m not so sure. That would surely need seals with some fairly large dimensions to be able to distort to the extent that they can pull a caliper back in about 1/2 a mm?? And the idea with seals is (usually) that they distort as little as possible
However – you got me thinking about stiction type effects – which made me drop the wheels out and squeeze both brake levers a little bit to try and overcome any stiction. Have now put wheels back in and …Wow….phenomenal difference! Seriously…massive difference
There has got to be be some sort of stiction/drag/lag type effect on the calipers. I’m not sure if it’s for reasons TJ said, but definately that kind of effect. With the rotors out, the pistons can move a little bit closer together and when they separate again, they don’t move quite as far apart as previously.
FluffPosted 7 years ago
TJ is correct. It’s the same reason why Bananaworld’s fix works.Posted 7 years ago
The piston glides against the seal, but with increasing back-pressure from the rotor equalling the pressure pushing it outward as well it grabs the piston and deforms, pulling the piston and pad away when you release. When there’s pad wear to be taken up then there’s no back-pressure so it just slides.
Isn’t it nice that we all agree? 😉
Woah there, bikewhisperer! I’ve, coincidentally, just read the whole thread on wheel tension etc. for the first time and would hate this thread to head the same way.
Before someone declares utter-wrongness, please explain what you mean by
with increasing back-pressure from the rotor equalling the pressure pushing it outward as well it grabs the piston and deforms
(Also: the seals have NOTHING to do with pulling the piston back in – the only thing pulling the pistons back in is the piston in the master cylinder retracting and ‘pulling’ the fluid back from the slave, thus retracting the caliper piston (I know, I know, it doesn’t really pull, but let’s not confuse things…))
EDIT: Another also: not wanting to blow my own trumpet or piddle on TJ’s fire, but the reason you’ve found your fix to work, Fluff, is nowt to do with the seals, and simply to do with there now being more fluid in the ‘closed’ section of the system, leading to the caliper pistons resting in a further-out position. Glad I could help, and that you’ve sorted your problem.Posted 7 years ago
Alright, calm down! It’s got nothing to do with spokes, superposition or anything like that.
There’s more than just one force acting on the system than your finger or the return spring. The seals acting upon the piston offer some friction and deformation against the piston. If they didn’t the system wouldn’t be able to self adjust.
As you said it’s a closed system once you’ve moved the lever a couple of mil. This is true irrespective of whether the pistons have adjusted or not. If the pistons just moved back and forth against a non elastic seal then they would never self adjust.
The adjustment comes because the piston seals grab the piston on the extremes of pressure or motion (forward and back) and allow some elasticity into the system to draw more fluid in when it’s open.
With regard to what I said, I should have made it clearer. The master cylinder spring does most of the work, but some is done by the deformed seal. When there’s a few thousand psi in the system the wet side of the seal is under the same pressure as the piston and deforms to move too. The bore is smooth but still lets the pressure through. That kind of pressure will cause the seal to deform and stick to the piston under deformation and draw it back as it relaxes. It’ll still have some friction against the piston when the master cylinder opens up, and if it’s moved at all (due to a differential in back-pressure compared to normal movement) then the elasticity of the seal will allow it to move out and draw more fluid through.Posted 7 years ago
Wait, so with a theoretically uncompressable seal, the brakes wouldn’t work…?
Blimey, they say you learn something everyday.
I only learnt it after talking to the importers about shitty Hayes HFX-9’s! Apparently the sticky piston is the one that doesn’t keep moving outward..
Interesting, does this work with Avid Juicy brakes?
It’s what they recommend. Take the wheel out and squeeze the lever halfwayPosted 7 years ago
Amen too. I accept your scepticism! It’s a slightly strange thing to get your head around. It might help to know that a very small volume change in the system from the pistons distorting would cause a large volume change in the system from the piston body… Hence the relative “vacuum” when the pistons retract.
Night!Posted 7 years agorudedogMember
bananaworld – Member
Interesting, does this work with Avid Juicy brakes?
theoretically easy solution… used to good effect… especially those awful brake-shaped-objects that the purveyors of SRAM seem to want to stick on every cheap bike going…
No offence intended….
cheers – although not sure why I would be offended that you answered my question 🙂Posted 7 years ago
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