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

[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!!!

 

[SMSW]

When you apply your brakes, you are loading the front wheel/forks assembly. How many miles on the tire? If the tire has something wrong internally, I would expect you should have seen this from the first mile on the tire. If your wear pattern is akin to cupping, braking exacerbates the bumpiness of the ride.

 

[Slydynbye]

What pressure are you using in the front tire? My minimum is 38-40, some like 42.

Since you have ruled out a lot, it might be a steering head bearing.
Raise the front off the ground and check for ANY front to back movement of the wheel and forks.
Also at 65k miles it could be time to service the fork Bushings

 

[tinboatcapt]

Check to see if a rotor is warped or bent. They can warp from mishandling during a tire change or from excessive heat. The symptom is a fast vibration, and better breaking if the lever is applied, released, and reapplying them. the spongy first application caused by the warp pushing the pads away from the rotor.
Just a thought to check.
Jim

 

[Obo]

I know you said you replaced the rotors but check for dirty bobbins. They should rotate freely and let the disks float.

 

[IndyRob]

When you apply your brakes, you are loading the front wheel/forks assembly. How many miles on the tire? If the tire has something wrong internally, I would expect you should have seen this from the first mile on the tire. If your wear pattern is akin to cupping, braking exacerbates the bumpiness of the ride.

About 2000 miles on the tire. Plenty of tread left based on the tread wear indicators.

 

[IndyRob]

What pressure are you using in the front tire? My minimum is 38-40, some like 42.

Since you have ruled out a lot, it might be a steering head bearing.
Raise the front off the ground and check for ANY front to back movement of the wheel and forks.
Also at 65k miles it could be time to service the fork Bushings

I do 36 front/42 rear per the specs on the decal on the bike, and I check them before any long ride. While I had the front tire off the ground today I did check for any looseness of the head bearings. Nothing! Fork bushings is a possibility but they are not leaking any fluid, so I sort of ruled that out.

 

[IndyRob]

I know you said you replaced the rotors but check for dirty bobbins. They should rotate freely and let the disks float.

Yep did that. They both float a little bit front/back and side to side.... seems all normal and all the bolts and pins etc were tight. I cleaned the new rotors thoroughly with brake cleaner before installing, and used a small amount of brake grease in places where the Honda manual said to. Been riding 45 years and doing most of my own wrenching, so I am pretty sure I have the pads in correctly. This one has me stumped though.

 

[IndyRob]

When you apply your brakes, you are loading the front wheel/forks assembly. How many miles on the tire? If the tire has something wrong internally, I would expect you should have seen this from the first mile on the tire. If your wear pattern is akin to cupping, braking exacerbates the bumpiness of the ride.

Yeah I agree... but I ran about 2K miles on the tire before I started getting this issue... weird. As far as I can remember I didn't hit anything hard that related to the start of this issue, and as I mentioned I don't see any out of round when I spin the tire with the front jacked up just (like 1/32" off the ground!).

 

[dwalby]

I know you said you replaced the rotors but check for dirty bobbins. They should rotate freely and let the disks float.

N/A on the 1100, only the 1300 has floating rotors.

 

[dwalby]

Yep did that. They both float a little bit front/back and side to side.... seems all normal and all the bolts and pins etc were tight. I cleaned the new rotors thoroughly with brake cleaner before installing, and used a small amount of brake grease in places where the Honda manual said to.

he was asking about the rotors, I suspect you're talking about the calipers/brake pads, because the 1100 rotors don't float.

 

[dwalby]

nobody has mentioned the anti-dive mechanism yet. Not sure if that could be contributing to your symptom, but worth looking into.

 

[dwalby]

I ran about 2K miles on the tire before I started getting this issue... weird.

based on all the things that have been ruled out, the tire is still a very likely suspect.

 

[IndyRob]

Check to see if a rotor is warped or bent. They can warp from mishandling during a tire change or from excessive heat. The symptom is a fast vibration, and better breaking if the lever is applied, released, and reapplying them. the spongy first application caused by the warp pushing the pads away from the rotor.
Just a thought to check.
Jim

Yes per my original post, both rotors were replaced with new ones. The old (original) ones were flat based on my check with a machinist's level but I replaced them anyway since they still had 65K of riding on them. The new ones also tested flat before I installed them and I thoroughly cleaned them prior to installation to make sure I got any manufacturing grease etc off.

 

[IndyRob]

nobody has mentioned the anti-dive mechanism yet. Not sure if that could be contributing to your symptom, but worth looking into.

I considered that! Any way to test that? Appears to only be on the left side fork/disk of the bike. ??

 

[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!!!

On a TOTALLY different note.. while driving the Tail of the Dragon on Sunday, I lost ALL of my rear brake! Makes for a "pucker moment" for sure! It had been a little spongy when I left Indy but was still working fine. I nursed it home from Asheville NC using downshifts for rear braking (while enduring a "bumpy" front brake.... always fun especially since it poured down the whole trip back!) . Is this normal? I ordered a rear master cylinder rebuild kit today assuming this was the issue. I had plenty of fluid in the reservoir but I admit I do not change it out every year or two since the bike is garaged all the time and I (sadly) only have 65k on a 30 year old ST. I do bleed it and refill every 5 years or so. It's getting so I can't trust the bike to do a long trip. :-(

 

[TerryS]

I'd do a run-out check on the front brake discs to confirm they are flat. Attach a dial indicator gauge to a fork leg and spin the wheel with the indicator on the disc. I know you replaced the discs but if the mounting surface on the wheel is not flat then the discs won't be either. Given the shudder only happens when you brake I think the most likely cause is variation in braking.

 

[jfheath]

Short suggestion. Check the caliper bracket mounts top and bottom.

Reasoning.....

The anti-dive is on one fork only. Left in your model. It switched sides after CBS/ABS was introduced. It is activated by the movement of the brake caliper when the brakes are applied.
I am relying on fading memory here, so please excuse any inaccuracies, but get the gist of my comment..... but I think that the small bottom caliper bracket mounting bolt is fastened through the anti-dive plunger. When the brakes grab, the caliper rocks to move the plunger - inwards I think. This seals or partially prevents the movement if the fork oil, so that the fork cannot compress under braking.

I think that both mounts need to be able to move freely to allow this to happen. The bolts will be tight, but the caliper bracket is not clamped but is free to move/rotate on the bolt. I don't remember how this is achieved - whether there is a bush or a bearing in the upper mount bracket.

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 - hasn't developed a 'step'.

I'm going to speculate but you have just had wheels off, so presumably the mounting bolts have been removed for this. Im guessing you refitted old bolts and applied loctite ? Liquid loctite can get everywhere - I never use it and use the lipstick paste instead. Whatever, check that your mountings allow the caliper bracket to rotate freely, and are not compromised by loctite, corrosion, seized bearings etc otherwise your anti dive mechanism may be compromised.

Changing fluid.... Amount of usage of brakes between fluid changes is not an important factor in the deterioration of the brake fluid. It absorbs moisture from the air, even in a closed system. After 2 years fluid will have become 'too wet' to be safe.
But once it absorbs moisture starts to crystallise and turn to a sludge - think of those disgusting coloured ice drinks. This is more likely to happen in a system where nothing has moved than one which is in regular use. I change fluids every 12-18 months. Usually after the worst of winter.

 

[Upt' North]

On a TOTALLY different note.. while driving the Tail of the Dragon on Sunday, I lost ALL of my rear brake! Makes for a "pucker moment" for sure! It had been a little spongy when I left Indy but was still working fine. I nursed it home from Asheville NC using downshifts for rear braking (while enduring a "bumpy" front brake.... always fun especially since it poured down the whole trip back!) . Is this normal? I ordered a rear master cylinder rebuild kit today assuming this was the issue. I had plenty of fluid in the reservoir but I admit I do not change it out every year or two since the bike is garaged all the time and I (sadly) only have 65k on a 30 year old ST. I do bleed it and refill every 5 years or so. It's getting so I can't trust the bike to do a long trip. :-(

Did the rear brake come back to you when it cooled down? Or did it just go to brake hell and stay there.
I'm pretty convinced it's your forks/anti dive. The fluid in there may be about as good as the brake fluid and/or internals have failed. Make sure you change the rear brake fluid annually, it gets hot back there and they do fail, I know from personal experience. It's never fun.
Some on here are seasoned in the art of fork disassembly and will walk you through any problems.
Good luck.
Upt.

 

[IndyRob]

I'd do a run-out check on the front brake discs to confirm they are flat. Attach a dial indicator gauge to a fork leg and spin the wheel with the indicator on the disc. I know you replaced the discs but if the mounting surface on the wheel is not flat then the discs won't be either. Given the shudder only happens when you brake I think the most likely cause is variation in braking.

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.