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  1. Brakes or Wheel Bearing? 
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    I have been asking some questions about brakes recently but I am wondering if the brakes are really the problem. The symptoms I am trying to get rid of all relate to the drivers side front wheel:
    • After a journey, sometimes I can feel heat radiating from that wheel (and not any other), mostly it seems from the brake disc rather than the wheel itself.
    • When braking hard, there is a shuddering. In some ways it is a bit like ABS having kicked in but it is only affecting the one wheel, there is no ABS light on the dash and the "pulses" are a little closer together than is typical for ABS and also not as strong. The effect seems more pronounced the higher the speed when starting to brake.
    • If the car spends any significant amount of time above about 50pmh a vibration starts from that wheel which is very similar to that when braking. It feels rather like an unbalanced wheel but a bit stronger and, unlike an unbalanced wheel, gets stronger the longer I continue to drive. If I stop for a while, it is gone again. I suspect this must be heat related, i.e. something is expanding with heat but what?
    • Even without the vibration there is a slight rumble that increases with increasing speed.


    So initially I thought the heat source was a stuck caliper and the inside edge of the brake disc being initially rusty while the outside edge was polished suggested it may not be acting very evenly, i.e. perhaps it isn't floating properly. However, looking at the brake disc now the rust has been removed from some patches and not others so it seems for some of its rotation it is touching the pad and for some of it not.

    I took the car to a local tyre centre and they said the caliper must be stuck as they were unable to turn the road wheel by hand. At that stage I was missing the locking wheel bolt key so we couldn't look any further. I now have a key and taking the wheel off at home I was able to rotate the brake disc by hand, independent of the hub. It does drag a bit but I am not sure whether that is excessive. With the brake disc diamter being less than that of the wheel and trye I would have less leverage turning that by hand than the trye centre guy when trying to turn the wheel so I am wondering if the difficulty they guy had was more to do with the hub than the disc.

    So now I am beginning to wonder. If the wheel bearing is knackered, might that completely explain the symptoms and the brakes have been a red herring? Could the wheel bearing be the source of the heat, and the thing that seems to vibrate more as it heats up? Would a defective bearing allow the wheel to wobble enough to cause the uneven wear on the inside of the brake disc? Or is the source of heat due a to wobbling wheel pressing the disc against the pads for part of its rotation thus heating the disc and, the hotter the disc gets and therefore the thicker it is, the harder it bears against the pads, i.e. kind of viscous circle. Does a defective wheel bearing explain anything else I have noticed here?
     
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  2. Re: Brakes or Wheel Bearing? 
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    Quote Originally Posted by Amphitryon View Post
    I have been asking some questions about brakes recently but I am wondering if the brakes are really the problem. The symptoms I am trying to get rid of all relate to the drivers side front wheel:
    • After a journey, sometimes I can feel heat radiating from that wheel (and not any other), mostly it seems from the brake disc rather than the wheel itself.
    • When braking hard, there is a shuddering. In some ways it is a bit like ABS having kicked in but it is only affecting the one wheel, there is no ABS light on the dash and the "pulses" are a little closer together than is typical for ABS and also not as strong. The effect seems more pronounced the higher the speed when starting to brake.
    • If the car spends any significant amount of time above about 50pmh a vibration starts from that wheel which is very similar to that when braking. It feels rather like an unbalanced wheel but a bit stronger and, unlike an unbalanced wheel, gets stronger the longer I continue to drive. If I stop for a while, it is gone again. I suspect this must be heat related, i.e. something is expanding with heat but what?
    • Even without the vibration there is a slight rumble that increases with increasing speed.


    So initially I thought the heat source was a stuck caliper and the inside edge of the brake disc being initially rusty while the outside edge was polished suggested it may not be acting very evenly, i.e. perhaps it isn't floating properly. However, looking at the brake disc now the rust has been removed from some patches and not others so it seems for some of its rotation it is touching the pad and for some of it not.

    I took the car to a local tyre centre and they said the caliper must be stuck as they were unable to turn the road wheel by hand. At that stage I was missing the locking wheel bolt key so we couldn't look any further. I now have a key and taking the wheel off at home I was able to rotate the brake disc by hand, independent of the hub. It does drag a bit but I am not sure whether that is excessive. With the brake disc diamter being less than that of the wheel and trye I would have less leverage turning that by hand than the trye centre guy when trying to turn the wheel so I am wondering if the difficulty they guy had was more to do with the hub than the disc.

    So now I am beginning to wonder. If the wheel bearing is knackered, might that completely explain the symptoms and the brakes have been a red herring? Could the wheel bearing be the source of the heat, and the thing that seems to vibrate more as it heats up? Would a defective bearing allow the wheel to wobble enough to cause the uneven wear on the inside of the brake disc? Or is the source of heat due a to wobbling wheel pressing the disc against the pads for part of its rotation thus heating the disc and, the hotter the disc gets and therefore the thicker it is, the harder it bears against the pads, i.e. kind of viscous circle. Does a defective wheel bearing explain anything else I have noticed here?
    It is feasible that vibrations could get worse as the (worn) bearing tolerances open up but in my experience you can normally feel a worn disc vibration through the brake pad (oscillating). Ive never seen a hot wheel from a bearing so from what you say id be changing both bearings discs and pads.


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  3. Re: Brakes or Wheel Bearing? 
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    Have you had the car jacked up and felt any movement from side to side or can hear noise as the wheel being rotated.As for the heat,could be sticking caliper.
     
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  4. Re: Brakes or Wheel Bearing? 
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    You've obviously got something generating friction there, but need to isolate various causes to fully define what.

    Firstly, if the disc has uneven friction from rust/ surface imperfections it will give you pulsing through the pedal generally. Ultimately you'll have to replace to bring within usable competent spec. But it won't, on it's own, produce random heat.

    If the wheel bearing is failing then the wheel mounting along with the disc and wheel assembly will not completely follow steering input faithfully. It, in more extreme cases, will flop about on the suspension components, and be guided by the caliper and brake pads. This would definitely give friction. One way of checking for wheel bearings on front wheels is to start the car, rotate the steering left to right a few times while stationary (don't touch the brake pedal at all!). Then jack up that wheel and try to rock the wheel top to bottom, and then left right. This should have pushed the brake pads back a bit into the caliper, so allowing you to see the true condition of wheel bearings withiout support from caliper, if that makes sense.
    A characteristic of this is, that when you next try to use the brakes, you'll get long travel on the brake pedal as it has to move the pads back into contact with the disc, along with a wobbly wheel of course.
    You'll also generally feel this if you've just wound it lock to lock to get in and out of a parking space.

    The caliper and pads also need checking for free movement. If they are binding it will very quickly build heat into the hub assembly. You'd need to remove the pads to check this competently. This would be the most common cause.

    It does sound like, from your description, it needs replacement discs and pads though to correct that intermittent corrosion you have. It'll become apparent when worked on for this if the pads and calipers are sticking.
     
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  5. Re: Brakes or Wheel Bearing? 
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    Quote Originally Posted by Brownieaj View Post
    It is feasible that vibrations could get worse as the (worn) bearing tolerances open up but in my experience you can normally feel a worn disc vibration through the brake pad (oscillating). Ive never seen a hot wheel from a bearing so from what you say id be changing both bearings discs and pads.
    The pulsing when braking can definitely be felt through the brake pedal.
     
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  6. Re: Brakes or Wheel Bearing? 
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    Quote Originally Posted by f927 View Post
    Have you had the car jacked up and felt any movement from side to side or can hear noise as the wheel being rotated.As for the heat,could be sticking caliper.
    So far I have only had the one wheel off the ground. With the the other three on the ground and the transmission in park the hub will not turn. I'd have to double check the handbrake works properly before taking it out of park when on a jack/stand. I will see if I can do this.

    I have tried rocking the wheel at 12 and 6 o'clock and there is no movement but Hannu5 makes an interesting point about the caliper potentially supporting the wheel and preventing it from moving. It's a twin pot caliper with both pots on the same side so quite tall - I can see it could inhibit rocking in the vertical direction.
     
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  7. Re: Brakes or Wheel Bearing? 
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    Quote Originally Posted by Hannu5 View Post
    You've obviously got something generating friction there, but need to isolate various causes to fully define what.

    Firstly, if the disc has uneven friction from rust/ surface imperfections it will give you pulsing through the pedal generally. Ultimately you'll have to replace to bring within usable competent spec. But it won't, on it's own, produce random heat.
    Yes, regardless of the state of the wheel bearing I am intending to replace the brake discs and pads. I think I may have been slightly misleading when I suggested the brakes could have been a red herring. What I really meant was could the wheel bearing be responsible for uneven wear on the brakes discs so the state of the disc is more of a symptom than the root cause. It doesn't change that the disc needs to be replaced but obviously I'd like to avoid the new one quickly going bad too.

    Then, on the question of the heat it was is a wobbling wheel making intermittent contact with the disc causing the heat, or is that due to a stuck caliper.

    Quote Originally Posted by Hannu5 View Post
    If the wheel bearing is failing then the wheel mounting along with the disc and wheel assembly will not completely follow steering input faithfully. It, in more extreme cases, will flop about on the suspension components, and be guided by the caliper and brake pads. This would definitely give friction. One way of checking for wheel bearings on front wheels is to start the car, rotate the steering left to right a few times while stationary (don't touch the brake pedal at all!). Then jack up that wheel and try to rock the wheel top to bottom, and then left right. This should have pushed the brake pads back a bit into the caliper, so allowing you to see the true condition of wheel bearings withiout support from caliper, if that makes sense.
    That's a very interesting point. I had not thought previously of the caliper supporting the wheel via the brake disc and making bearing play undetectable but it makes sense especially with this caliper being quite tall. I am trying to work through how moving the steering would back of the pads. The steering is rotating the hub carrier to which the brake caliper is fixed. The disc is sandwiched between the hub and the road wheel. The only way I can think it could work is when the bearing is loose such that the hub, brake disc and road wheel move together as one part, and the hub carrier and caliper move as the other part.

    Quote Originally Posted by Hannu5 View Post
    A characteristic of this is, that when you next try to use the brakes, you'll get long travel on the brake pedal as it has to move the pads back into contact with the disc, along with a wobbly wheel of course.
    You'll also generally feel this if you've just wound it lock to lock to get in and out of a parking space.
    Thanks for the reminder. I do always apply the brakes before setting off if I have had the pads out after getting a bit of fright when I was much younger and finding the brake pedal went right to the floor the first time I tried to use them. Fortunately I was slowing down in good time and the second time I pressed the pedal I got some braking.

    Quote Originally Posted by Hannu5 View Post
    The caliper and pads also need checking for free movement. If they are binding it will very quickly build heat into the hub assembly. You'd need to remove the pads to check this competently. This would be the most common cause.
    The pads don't seem to be binding against the frame of the caliper - once the pistons are back a bit, the pads slide out easily. The pistons also move, not especially easily, but then if they did there wouldn't be a threaded tool for pushing them back. To get enough clearance to remove the pads it is sufficient to insert a big screwdriver into the notch on the caliper so levering against the metal backing of the outside (non-piston side) pad. That also implies the caliper must be sliding on the pins to a certain extent but again I am not sure if the amount of leverage I am having to use is excessive.

    I am thinking the answer may be to start with the discs and pads and then re-assess.
     
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  8. Re: Brakes or Wheel Bearing? 
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    To clarify about bearing movement. Yes you are correct about the two assemblies (suspension upright plus caliper) as moving as one. This SHOULD steer the moving wheel assembly via the bearings having minimal play. But, if the bearings are loose the caliper will try to steer the disc to realign the wheel. Which obviously it shouldn't be doing.

    The point about brake pads being pushed back (although safety related, as you say) was that if the pads are significantly pushed back after rotating the steering from lock to lock while stationary, then that would be an initial indicator of the bearings having too much play, as that should not happen.
     
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  9. Re: Brakes or Wheel Bearing? 
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    Pushing the pads back is as you describe fairly stiff but they should go with constant reasonable pressure via a lever behind them.

    The caliper slide should be much much easier. It should, in comparison to the effort required to move the pistons, virtually flop back and forth on smoothly sliding pins.

    If the caliper slide doesn't move easily with no pads installed, then it will keep the non piston side pads in contact too heavily with the disc and give you unwanted heat.

    A very quick and easy check is to spin the wheel when lifted by hand, then go and tread on the brake pedal hardand release, then try to spin it again. It should very quickly release and spin easily, if not something in that caliper is causing it to stay in contact too much.
     
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  10. Re: Brakes or Wheel Bearing? 
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    Just read on your other thread that this is an Allroad, did I miss that earlier?

    Anyway, if it's fitted with a Torsen centre differential (our A6 Quattro is specced with one) and you just lift up only one wheel, then trying to turn that free wheel will in turn be trying to overcome the centre differential locking gradient (limits it from free rotation) as the wheel opposite is immobile from being in contact with the road. With both wheels lifted the opposite will turn backwards across the front and rear differential, depending on which end you are working on.

    The garage may have done this when they checked it, so giving an odd feeling.

    Not many general garages understand this.
     
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