Front-end shake under braking?

[ManDownUnder]

I have a spare pair of RF900 forks at home, most parts are good, and all bushes etc are there. One pitted Staunchion, apart from that they're ok (have 75,000km on them though)

Available for general consumption by RF owners... replacement costs only (I bought the whole things for NZ$200 out of the US so a bush should be around... $10?)

If you need 'em come and get 'em.

[riffer]

Nigel - plotting here. If you are feeling inclined, and mechanically minded enough, a morning between the two of you could sort this out.

I'd start with swapping wheel between your bike and wkingham's and test riding it. No shudder under braking - the problem is in his rotors.

Once that possibility is removed it should be easier (and cheaper) to fix.

Can you two sort that out between you?

[wkingham]

Thanks for the comment.

Riffer the questions you mentioned about the forks I only took the top of and did the seals and oil change so I did not have the damper bolt out. When I inverted the shock to drain the oil, the bush that sits uner the oil seal slid straight out. The last forks I did was on a ZXR750 and this did not happen. This is what makes me think that the fork bushes need replacing. I haven't lost anything out of them.

I lifted the front wheel of the ground and use a screw driver to measure if the discs were out. By my naked eye they look straight and not bent.

I am going to take the brakes apart and clean out the pistons etc this week to see if this helps. Will keep you updated.

[Pussy]

Give Robert Taylor a call, 06 278 4160, he will more than likely have some good advice for you

[BMW]

I have a RF900 that has an issue in the front end. When I brake it feels like the front forks flex forward and back causing the front end to shake.

I have replaced the steering head bearing (which was stuffed), front wheel bearing and fork seals and fork oil. The problem is better but still not perfect.

When replacing the fork seals the bushings slid out and I had to put these back in place. I have not had this happen previously while replacing fork seals and was wondering if this meant the bushes were stuffed and possibly causing my problems?

Any help much appreciated.

try the brakes. Warped disks.

[Robert Taylor]

Welcome to Kiwibiker Wkingham.

Okay, steering shake problems on RF's can be due to:

- insufficient tension on steering head bearings
- warped discs
- bent brake pad holder pin causing brake pads to not slide properly together
- brake caliper pistons not going in and out smoothly due to wear or shit in the calipers.
- tyre wear (do you have triangulation of the front tyre?)
- out of balance tyre
- wheel bearing worn

Tell us more about your bike. What's the state of the front tyre? Remember any of these type problems are REALLY exacerbated on the RF as it has a disproportionate amount of weight on the front tyre as opposed to a gixxer so they push that tyre hard.

I got brake shudder on mine (only between 80 - 100km/hr) which was traced to insufficient steering head tension. Tightening the steering, cleaning the brakes, replacing the front and balancing it have made a huge difference.

Who replaced the steering head bearings? One good test of steering head bearings is to let go at about 80km/hr. If you get tank slap its too loose. On my bike now its set up right you can go into a corner and let go of the bars and it keeps going around the corner. Until I got it sorted it stood straight upright. What does yours do? Don't underestimate the effect of a badly worn tyre on the steering too.

This man talks perfect sense. Warped discs are the most likely problem, even if when it is in early stages and not yet apparent with lever pulsation. You should have them checked proffessionally by a qualified motorcycle engineer with all the adaptors to rig up a dial test indicator. It takes surprisingly little disc runout / wear to ''tip the balance'' on a sensitive bike like this.

You didnt state how many kilometres this bike has done, which I suspect are now ''getting up there'' so you will have ''complimentary'' wear issues raising their head. When people successively fit replacement brake pads they are usually aftermarket. That is not neccessarily a bad thing but very often the shims that clip onto the back of the oem pads are discarded. These are fashioned in such a way that it attempts to equalise pressure over the full face of the pad. I am not saying that refitting these alone will fix the problem.

But, a little bit of disc runout, worn fork bushings, sticky brake piston action all adds up to a sum total and can tip the balance. Calipers over time will corrode inside their hydraulic and dust seal recesses, this build up squeezes the seals harder onto the pistons causing less than smooth action. In the UK where they grit the roads during winter this is a real problem.

Bear in mind that when you are feeling for steering head bearing play your bare hands cannot simulate road braking and deflection loads. My own personal technique for adjusting headrace bearings is to first slightly overtighten them and back off until they have just a faint hint of drag. You have to be real pedantic becaause every time you tighten the crown nut above the top triple clamp that can also influence total torque applied to the headrace set.

The bushings in the forks that reside immediately underneath the fork seals are a slight interference fit in the fork tube, they should not just fall in. If so you may need new alloy legs or a very good machinist. Aside from that you could also have sacked out fork springs, assuming they are the originals and k's are high. There is also the possibility that there is wear debris inside the cartridges trapping open the compression shims a little. This means the forks will blow through their stroke a little too readily, youll arrive at the more compressed part of the stroke a little earlier and that wont be a happy combination with the undesirable brake disc runout frequency.

Tyre wear will also affect it, some brands will have a much stiffer sidewall construction. That may excerbate or reduce the annoyance you are experiencing.

[riffer]

Robert raises an interesting point here regarding the shims on the brakes.

My RF does not possess these at all and I'm wondering how much of an effect this would have on braking performance. Please refer to the brakes fiche pic I've attached to see where they should go.

Also - wkingham - I'm still concerned about this "bush" that fell out of your forks because it just SHOULDN'T.

Can you look at the forks fiche picture I've attached to this thread and tell us EXACTLY which part it is?

[Robert Taylor]

Robert raises an interesting point here regarding the shims on the brakes.

My RF does not possess these at all and I'm wondering how much of an effect this would have on braking performance. Please refer to the brakes fiche pic I've attached to see where they should go.

Also - wkingham - I'm still concerned about this "bush" that fell out of your forks because it just SHOULDN'T.

Can you look at the forks fiche picture I've attached to this thread and tell us EXACTLY which part it is?

It would indeed be interesting to very objectively answer that question on brake shims, if anyone has some proven, credible answers I would be interested.

Ditto re that bush, no way it should fall out.

[ManDownUnder]

Also - wkingham - I'm still concerned about this "bush" that fell out of your forks because it just SHOULDN'T.

Can you look at the forks fiche picture I've attached to this thread and tell us EXACTLY which part it is?

If it is what I think is it... it's part #5 which (for me) slid out quite easily once the retaining clip and fork seal was removed. It's the golden coloured one (Phospher bronze I suspect?). If that's gone there will only be one point of support (at the bottom) for the stanchion as it slides up and down, so under braking the fork seal itself will be crushed, esp under heavy braking.

As mentioned - I have one at home. I'm happy to have a go at helping but don't have the necessary gear to exactly measure oil back into the fork etc.

[riffer]

How critical is this measurement of oil anyway? I've always used a plastic jug and measured to the nearest 5mm or so.

What sort of tolerance is there in this quality of fork for variability of oil level anyway?

[ManDownUnder]

How critical is this measurement of oil anyway? I've always used a plastic jug and measured to the nearest 5mm or so.

What sort of tolerance is there in this quality of fork for variability of oil level anyway?

Don't know but it we catch it all, and put it all back in, +5 mls to allow for stuff that stays on the inside of the jug...?

[vifferman]

It'll be interesting to see Eggs Zachary what this turns out to be. When I first bought my VFR it had a similar problem, which I thought was a warped disk, but it turned out to be badly (and unevenly) worn pads. I think the problem was exacerbated by steering head bearings that were on their way out. Replacing the pads fixed the problem, but shortly after that the head bearings needed to be replaced - they weren't loose, but notched in the 'straight ahead position', which manifested itself as a slight reluctance to tip into corners.
Like the RF, the VFR is front heavy, so anything that's "out of whack" in the front - even low tyre pressure - is shown up by vibration or steering problems. Get a couple of things that aren't enough on their own to register as needing fixing, and you get misbehaviour.

[wkingham]

Yes its the bush # 5 that slid out very easily when I inverted the forks. I still have it and have put it back in but was surprised that it fell out.

Last night I started to take the brake callipers apart. When I took the pads out and pumped the brake to push the pistons out one is not moving. Once I have managed to get the pistons out and cleaned out the callipers I will see how it is and if this has made any effect on the problem. May put new pads in at the same time.

[ManDownUnder]

Yes its the bush # 5 that slid out very easily when I inverted the forks. I still have it and have put it back in but was surprised that it fell out.

Last night I started to take the brake callipers apart. When I took the pads out and pumped the brake to push the pistons out one is not moving. Once I have managed to get the pistons out and cleaned out the callipers I will see how it is and if this has made any effect on the problem. May put new pads in at the same time.

So to confirm - you had a shaking issue while the bush was out of the fork leg?

or

The bush was always in the fork leg while you were riding (i.e. you had the shaking issue even with the bushing in the fork).


I'm in Kumeu. Come over on Sunday and we can swap front wheels if you want.

[Robert Taylor]

How critical is this measurement of oil anyway? I've always used a plastic jug and measured to the nearest 5mm or so.

What sort of tolerance is there in this quality of fork for variability of oil level anyway?

Refilling by quantity is very inaccurate. Overfilling and then syringing back to specified oil level / tolerance range is the only way you should do this. It will be detailed in the workshop manual. Setting the oil level sets the secondary air spring volume, and that very volume determines the latter reaches of stroke progressivity. Simple way equals mediocrity, the correct way equals a better result.

That bush should NOT fall into the fork housing.There is a problem with the bushing and / or fork leg that needs to be rectified.