Bouncing front tire during braking (only). Any ideas on the cause??

[John OoSTerhuis]

Also that anti-dive plunger mechanism is prone to wear - especially the hole through which the lower mount bolt fastens. Check its condition make sure the hole - which will have worn/elingated

Minor point: the ST1100s’ little OEM nylon bushing in the anti-dive plunger is oval in shape. By design, long axis oriented in the direction of movement fore and aft.

I’d thought about mentioning the anti-dive unit but rereading the OP’s descriptions led me to believe the problem was more likely brake/rotor related.

John

 

[Slydynbye]

Run the Front pressure up to 40 and see if there is a difference. I suspect it's the tire exacerbated by pressure.

 

[SMSW]

Based on what has been said here so far, I think a fair amount of TLC (i.e. maintenance) is way overdue. I'm talking about a fork overhaul, (check bushings, change fluid), change brake and clutch fluids, pull the rear caliper and clean it, rebuild your rear master cylinder, etc. These bikes need to be used regularly, and maintained by the book.

Your rear brake problems might have been due to boiling of water absorbed by the brake fluid. I'm assuming the rear brake got a lot of use on the dragon, and heated up.

 

[Uncle Phil]

If you 'bounce' the front end up and down, is it pretty stiff or does it feel like a pogo stick?
If it only does it when you apply the front brakes, does the amount of pressure make a difference in the vibration?
Can you cause it to happen while the bike is on the center stand and the front end slightly off the ground?

 

[motornut]

No, a stiff wire probably won't work. you need something a little more accurate, check at the auto parts store may have a loaner dial indicator you can use. also check your front wheel bearings. Is this under very hard braking, such as bottoming the forks?

 

[jfheath]

Minor point: the ST1100s’ little OEM nylon bushing in the anti-dive plunger is oval in shape. By design, long axis oriented in the direction of movement fore and aft.

Well, well, well. I know mine was oval on my very first 1100. I thought it was wearing through. I kept an eye on it and it never got any worse!! A long time ago.

 

[IndyRob]

Good idea. I'll do that today and report back with a follow up. I don't have a professional runout gauge but hopefully a piece of stiff wire will suffice.

So... it appears there is decent (maybe 0.030"?) warpage on the right rotor. The left one seems to track true using my very homemade runout gauge. Going to try to get some better rotors (EBC this time, instead of Amazon specials!!) and hope that fixes the brake bounce. I'm also set to replace the front tire. I was mistaken on my original post; it has more like 12K on it, not 2K miles as I had thought.
Q: Dennis Kirk shows that the rotor I purchased (EBC MD-1016) is for front right only. However the rotor does not appear to be "direction specific". In other words, there does not seem to be anything that would prevent me from using the same one on the left vs the right, so I purchased 2 and was going to replace the pair so they are the same. Any issues there? The ones I bought last fall were also like this, and I used the same ones on both the left and the right sides.

 

[IndyRob]

Based on what has been said here so far, I think a fair amount of TLC (i.e. maintenance) is way overdue. I'm talking about a fork overhaul, (check bushings, change fluid), change brake and clutch fluids, pull the rear caliper and clean it, rebuild your rear master cylinder, etc. These bikes need to be used regularly, and maintained by the book.

Your rear brake problems might have been due to boiling of water absorbed by the brake fluid. I'm assuming the rear brake got a lot of use on the dragon, and heated up.

Yes no argument there. I have 65K on a 30 year old bike. Although I store it inside, I am certainly guilty of not changing all the fluids every 2 years. Yes of course the brake got used heavy on the Dragon but I expected some fade but not total failure, and it never came back the whole way home... went straight to the ground ever since. So something definitely just totally failed there. Rear master cylinder rebuild kit is on the way. I had replaced the rear pads just last November, and again no leakage so pretty sure the master cylinder, which had never been rebuilt, finally gave up the ghost "under pressure" so to speak.

UPDATE: The master cylinder rebuilt fixed the rear brake loss. Looks like the internal rubber seal must have finally failed. This was my second rebuild...first at about 35K miles and second at 67K (and 10 or so years after the first one...). How often do these need to be redone? Why do they fail so must faster compared to automotive ones? Fortunately they are easy and cheap to do, but hate to be caught like that again, so what's a decent "pro active" rebuild schedule?

BTW I went ahead an rebuilt the front brake MC as well as the clutch master cylinder while I was "in the mood" for master cylinder rebuilds.

 

[dwalby]

given your inexact rotor runout measurement, I'd replace the 12k mile tire first and then see what happens.

The Honda spec is 0.012", less than what you're estimating, but not zero either.

 

[IndyRob]

Background: '93 Non ABS ST1100 with 65K miles.

Last fall I noticed a "shaking" from my front end which happens ONLY during braking. Naturally, I assumed I had warped a rotor, so while my bike was apart during my 40A alternator upgrade project I took the liberty to install all new rotors, pads, and even replaced the rotor "bolts" per the recommendation in the Honda shop manual. Well.... that did not seem to help anything! The motion I am getting is much more of an up and down than a side to side wobble, almost like you were riding over a series of small speed humps (only) while braking. The faster I am going the faster the bumping and it gets slower as the bike slows to a stop. It happens on both hard and light braking, and at any speed. No noticeable issues when not braking at all! The new rotors and pads seem to be wearing evenly so far.

Thinking it may be an out of round front tire, this morning I jacked up my front wheel to where it was just barely off the ground, rotated the front wheel, and I do not detect any out-of-round or obvious high/low spots. The tire is a fairly new Dunlop RoadSmart III. I did notice sort of an odd wear pattern which seems to have little "humps" in the tread in line with part of that tires' tread profile. They are very consistent around the whole tire and also consistent left side to right side, but still a little odd IMHO. Not sure if this is just the way the RoadSmart fronts wear??

Nothing else seems amiss, like it tracks straight and true while not braking, no shaking up to at least 84 (tested that on the way home from the Dragon yesterday...

Any ideas where to look next?? Hate to drop $150 on another tire if that's not the root cause. Thanks for any input!!!

 

[IndyRob]

Purchased 2 new EBC brand rotors from Dennis Kirk that said they were a direct fit for my 93 ST1100. Well the bolt holes DO line up fine with my wheel, but the diameter of the rotors is much smaller than where my pads would hit them. Sending them back!!

Does anyone have any good recommendations for front rotors? The last ones I got from Scamazon warped in the first 100 miles and were paper thin compared to the EBC ones from Dennis Kirk.

 

[dwalby]

Purchased 2 new EBC brand rotors from Dennis Kirk that said they were a direct fit for my 93 ST1100. Well the bolt holes DO line up fine with my wheel, but the diameter of the rotors is much smaller than where my pads would hit them. Sending them back!!

Does anyone have any good recommendations for front rotors? The last ones I got from Scamazon warped in the first 100 miles and were paper thin compared to the EBC ones from Dennis Kirk.

those sound like the ABS2 rotors, smaller diameter.

might as well put the other rotors back on for now and see if the new front tire helps at all.

edit: You want the MD-1101 (316mm), not the MD-1016 that you have now (276mm), and not the ABS2 version MD-1008 (296mm) The MD-1016 appears to be a rear brake rotor for something, I just don't know what.

 

[Uncle Phil]

I was just going to say it sounded like you got ABSII rotors ...

 

[IndyRob]

Well a new front tire did not solve the problem at all.... still getting a distinctive up and down "bounce" only when I apply the front brakes. I tried re-cleaning the pads and rotors yet again. For refresh... both have been replaced since this issue started at random late last year. Any other ideas???

 

[Mellow]

Warped Rotor?

 

[dwalby]

back to the replacement rotors you installed recently.

You claimed that the right rotor appears to be warped, which is surprising for a brand new rotor, but possible if you bought some budget rotors off of the internet. I re-edited post #32 with the diameters and part numbers of the EBC rotors to make it easier to read. the numbers were correct originally, but when I re-read the post it was a little hard to read and understand, now it reads better.

I think what you said earlier is if you spin the wheel the right rotor has a visible runout to it, whereas the left rotor doesn't. You also mentioned they were paper thin, which may explain why one of them warped so quickly. Sounds like you need to get rid of those and buy something appropriate for the application, like the EBC rotors.

 

[Corvair]

Purchased 2 new EBC brand rotors from Dennis Kirk that said they were a direct fit for my 93 ST1100. Well the bolt holes DO line up fine with my wheel, but the diameter of the rotors is much smaller than where my pads would hit them. Sending them back!!

Does anyone have any good recommendations for front rotors? The last ones I got from Scamazon warped in the first 100 miles and were paper thin compared to the EBC ones from Dennis Kirk.

Same problem. Bought new rotors and they were warped. New poem,s are $800. If you find a good alternative please let me know. Thanks Corvair

 

[Larry Fine]

Hey, Corvair. My first car was a '65 Corsa 140hp 4-speed.

Original engine was a 180hp, but someone had changed it.

 

[Sunday Rider]

This makes me think of Ralph Nader.