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Front vibration/woble when brake applied

  • Thread starter Thread starter bexabarr
  • Start date Start date
B

bexabarr

Guest
Hey guys again.

I just got a new toy, a 1981 gs750. You can see some pics if you want here

http://www.thegsresources.com/_forum/showthread.php?t=108015&page=2

well the things is that every time I use the front brake at 20mph and above the bikes stops but in the process the handlebars and front end begin to vibrate/wobble a bit. I thought it was the air in the forks... so I added 15psi (the air nipples are right on top of the forks) that helped a little bit. Then what I did was renew all the brake fluid and bleed both calipers. It improved a bit, but it is still there. Anyone have to deal with this before? Have any suggestions? thanks guys!
 
A couple things come to mind. Rim bent/out of true, tire worn, or a warped rotor.

E.


bexabarr said:
Hey guys again.

I just got a new toy, a 1981 gs750. You can see some pics if you want here

http://www.thegsresources.com/_forum/showthread.php?t=108015&page=2

well the things is that every time I use the front brake at 20mph and above the bikes stops but in the process the handlebars and front end begin to vibrate/wobble a bit. I thought it was the air in the forks... so I added 15psi (the air nipples are right on top of the forks) that helped a little bit. Then what I did was renew all the brake fluid and bleed both calipers. It improved a bit, but it is still there. Anyone have to deal with this before? Have any suggestions? thanks guys!
Click to expand...
 
I thought about the warped rotor.. but they look fine. But I guess "looking" fine probably doesn't mean didly jeje I'l check them with a flat block of wood (does that make sense?).

The bike runs incredibly smooth, could it be something to do with the calipers. There are many little weights on one side of the wheel, maybe this was an attempt by the last owner to make that stop??

Could it be shot forks or something too? I know nothing about forks so I'm just throwing it out there...

Thanks earl you are alway quick to respond and have what seems to be infinite invaluable info to give!
 
bexabarr said:
There are many little weights on one side of the wheel, maybe this was an attempt by the last owner to make that stop??
Click to expand...

this would suggest a bent rim, in itself not a problem unless the area where the disc bolts onto the rim is bent too

i would bet on a warped disc and would check that first

also, make sure that the steering bearings are correctly tightened
 
Check your break rotors with a travle indicator, a block of wood won't do the trick. Place the bike on it's centerstand, fix the front wheel to one side and set your indicator on the rotor, turn the wheel and watch the indicator for runout.
V
 
Sounds like rotor, A local machine shop should be able to check & fix it pretty reasonable. Had mine one done on a KZ 1300 for I think it was $25
 
My first thought was also a warped rotor, but the other things Earl mentioned must also be checked.

You can do a rough check on the bearings by simply grabbing the wheel near the top and trying to rock it side to side. If you hear a sound, any sound, or if you detect play of any kind, you need to adjust or replace the bearings. Have someone hold the handlebars when doing this.

Do this check with the front wheel on the ground and do it again when the bike is on the main stand.

Wheel true is also open to an easy check, after you have checked the bearings. Put the bike on the main stand and fix the bars to hold the front wheel straight ahead.

Get a piece of 6x6 for a base, plus a brick or two for additional weight/stability, and a 2-foot piece of 2x4 for an upright. Fasten them together securely and attach a piece of chalk to point sideways at a height as close as possible to the middle of the wheel.

Again, fix the handlebars so that they do not move at all. Move the nice new tool to the rim of the wheel until it barely touches. Rotate the wheel. If the wheel is true, the chalk should scribe a line completely around it.

Repeat the exercise with the tire, just because you are already there, and because it will show you if there is a problem with the tire itself. Do it on the sidewalls and across the tread face.

You can even do this with the rotor, but chalk will not work as well as a marker for this as you must have only very light contact. The brake rotor will cause the results mentioned when it is only very slightly off true, so if this test shows considerable irregularity of the rotor you will likely need to replace it.
 
Another possible cause (hey, we're all just guessing, anyway 8-[ ) is a dirty rotor. Spray liberally with brake cleaner and a CLEAN rag. Repeat. I have even better results if I take the pads out, spray them down, too, and even sand them lightly to remove a surface glaze. Spray the pads again after sanding and put everything back together.


.
 
you guys went all out today on the posts lol- well it seems that it could be one of many things. So obviously I'm going to start with the easy things and move on from there. Thanks again.
 
A quick indicator for run out can be made from a piece of coat hanger or plain wire. Stick a piece under a bolt near where you're going to measure and bend it so it's really close to the disc or wheel, a few thousands will usually do. Turn the wheel and adjust the wire to just touch at a high spot. Then slowly turn the wheel and watch for a gap between the disc and the wire. The service limit for disc runout on my '79 GS is .012". If yours is more than that you should be able to see the gap and judge accordingly. Maybe have a set of feeler gauges handy for reference if you're not used to judging gaps that small by eye. If it's out enough to shake the front end you should be able to see it with this method. If in doubt borrow some dial gauges.
 
Hey sandy thanks so much for the info. That is exactly the info I needed, I was trying to figure out where I was going to get a dial gauge from. Perfect timing!
 
Here is what I have found out so far:

There is no bearing play

The left brake is sticking, the right one makes very minor friction noise but doesn't hold the wheel back that much.

I made a home made gauge with a coat hanger and found that while the brake rotors are perfectly straight the wheel is a little bit out line (very little, and probably explains the weights on one side).

I'm going to clean the caliper that is sticking (I have never done this) and see if that improves the situation a bit.

any other thoughts? Thanks again.
 
bexabarr said:
The left brake is sticking, the right one makes very minor friction noise but doesn't hold the wheel back that much.

any other thoughts? Thanks again.
Click to expand...

i had a really sticky brake on one side once. when breaking at high speed the bike would pull towards one side (and not keep straight as it should)
it also had some wobble
things improved when i took the callipers apart and cleaned them

see how you go but you still may need to have the rotors skimmed (it's very had to get a required precision measure with just a home made tool)
 
I think your right about not getting it right with the homemade tool. I'll probably have to get that checked out. I took the caliper apart (after a long fight) and cleaned it well or what I thought was well. It looked clean to begin with, except for a bit of rust spots her and there. I put it back together and found that the brake sticks just as much. I have to use the bike to get to work tonight, I'll be extra careful and see if I made any improvements.
 
bexabarr said:
Here is what I have found out so far:

There is no bearing play

The left brake is sticking, the right one makes very minor friction noise but doesn't hold the wheel back that much.

I made a home made gauge with a coat hanger and found that while the brake rotors are perfectly straight the wheel is a little bit out line (very little, and probably explains the weights on one side).

I'm going to clean the caliper that is sticking (I have never done this) and see if that improves the situation a bit.

any other thoughts? Thanks again.
Click to expand...

Sticking pads is common on the GS bikes. They tend to collect dirt, oil, and debris behind the pad.

Cleaning them, literally, is dirt easy.

You need a spray can of commercial brake cleaner and some old newspapers.

Do NOT use rags because there is a possibility of spontaneous combustion as they can store both oil and heat and brake cleaner is very flammable.

Put the newspapers under the tire, insert the extended tip on the nozzle then spray liberally. Spray from the back as well as the front of the pads.

You need the newspapers to soak up the black junk that will run off.

Once clean, you are done. This stuff dries/evaporates very quickly, but be sure to let the pads dry for several minutes before riding.

Spread the newspapers to dry and then dispose of them.
 
bexabarr said:
I think your right about not getting it right with the homemade tool. I'll probably have to get that checked out. I took the caliper apart (after a long fight) and cleaned it well or what I thought was well. It looked clean to begin with, except for a bit of rust spots her and there. I put it back together and found that the brake sticks just as much. I have to use the bike to get to work tonight, I'll be extra careful and see if I made any improvements.
Click to expand...

hm, not clear from your post, did you disassemble the caliper itself?
that's what i needed to to to get some results as the piston needs to move in and out freely (it did look ok from the outside and it did seemingly go in/out ok)
however, before doing this, it may be easier and wiser to first get the rotors checked for warpage by a shop (so to eliminate them as prime suspects)
 
The calipers slide on 2 pins. these pins get pretty gunked up after years of normal riding. These pins are the 2 bolts, that aren't mounting bolts, on the outside of the caliper. They should have little rubber boots on them. Remove them one at a time, sand off any rust or corrosion, grease them real good & put them back in.
 
umm... I did remove the whole piston, but I did not grease it when I put it back in. I also damaged the dust cover a little bit (first time I ever did this with little instructions). I didn't think greasing the piston was a good idea... should I have, Is that why it might still be sticking? If that is the case I will do it all over again if I have too.
 
bexabarr said:
... I didn't think greasing the piston was a good idea... should I have...
Click to expand...
no grease needed on the piston, the brake fluid does that
ideally you'd use new o ring and dust cover, but if they're in good condition you can get away with cleaning them with the brake fluid
you do need to grease caliper axle bolts though

if it's still sticky after this, sorry, i have no more ideas (but i'm sure some more experienced people here will have)
 
It actually improved a lot.. I'm going to give them one more cleaning with brake cleaner and then grease the axle bolts like you've suggested. Thanks guys again and again.
 
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