Side to side when braking

When i use the front brake my Mk3 goes unstable, side to side, I don't know what else to do.

I have installed:

- CNW Head steady
-Madass front break disc kit
-Centered the front weel
-Isolastics adjusted
-Roadrider 325 x 19 front on 1,85 rim and universal 100/90 x 19 on a 2,50 rim.

When the bike runs over 60 km/h everything is Ok.

If I don't brake everything is OK

If I take my hand to the helmet... Or use the front brake, the Handlebar goes crazy.

What I am missing?

Regards
Antonio

Have you checked for play in front wheel bearings or steering stem bearing?

Are your wheels in line, is your rear tyre worn.

Yes check you straight ahead fore and aft wheel alignment.

The one item in your “givens” that confuses the issue is that the phenomena goes away at 60 kph. And I’m confused about the hands to helmut statement.

Without those statement I would have looked at brake rotor pulsation on brake apply feeding through the system to the handlebars. Any slop in the wheel bearings would have exacerbated the problem.

Brake pulsation is usually due to a “hard spot” on the rotor caused by corrosion or heat and affecting flatness with a resultant one/rev. Check flatness of disc.

However - it goes away at speed? Totally or Is it digital or a linear improvement.

Have to noodle on this a bit more.

I had this problem on a BMW R100rs years ago. It eventually turned out to be the head bearings just needed to be slightly tighter on adjustment. If on the other hand the bike weaves over 60 plus they are too tight. The only problem you have is if your bearings are stock fittment they aren't adjustable.
The only other things I can think to try are tyre pressures, wheel balance, & swing arm play (bushings &/or isolastics.

Martyn.

NorCat said:

When i use the front brake my Mk3 goes unstable, side to side, I don't know what else to do.

I have installed:

- CNW Head steady
-Madass front break disc kit
-Centered the front weel
-Isolastics adjusted
-Roadrider 325 x 19 front on 1,85 rim and universal 100/90 x 19 on a 2,50 rim.

When the bike runs over 60 km/h everything is Ok.

If I don't brake everything is OK

If I take my hand to the helmet... Or use the front brake, the Handlebar goes crazy.

What I am missing?

Regards
Antonio

Click to expand...

Antonio, has it just started doing it? if so did it start after making a change?

Or done it since you've had the bike?

Check hub bearings (front and rear), head race bearings are ok, disc runout, disc tightness, lower yolk clamps and top nuts for tightness, tyre is seated correctly, and in the right direction, and the pressure is correct. Wheels balanced. Caliper tight, pads worn down to their limit?

So,... The bike tracks fine until you apply force to the bike by using the front brake...

If your problem was the swingarm, or the isolastics, or some other boogie man, you would have the problem all the time, not just when the force applied to the bike by the front brake... A Steering head bearing issue seems most likely, maybe something broken in the fork slider internals, or lastly a wheel bearing.

here are a couple of good article on this :



lots of things can excite the oscillation. Steeering bearings, wheel bearings, alignment, tire condition, weight distribution... all will have an impact.

George

Replace the front tire

A friend of mine has a T160, one day he was complaining about a ‘weave’.

I took one look at the front tyre and said “you need a new tyre mate”.

He was adamant that it was ok. I didn’t see him for a while, and next time I did, he’d clearly forgotten about our earlier conversation “you‘ll never guess” he said, “I fitted a new tyre sorted”...

Have you torqued up the front spindle 60 ftlbs and the the pinch bolt in the rh leg to 15ftlbs. If they are not tight you can see the front mudguard twist when you pump the forks with the brake on.

Esmerela said:

Have you torqued up the front spindle 60 ftlbs and the the pinch bolt in the rh leg to 15ftlbs. If they are not tight you can see the front mudguard twist when you pump the forks with the brake on.

Click to expand...

Worn fork brushes will/might do it. But is more likely to cause fork judder.

Lots of good suggestions here. And the question about was the bike ok before and what have you changed should be carefully analysed.

My reading of your description is that the bike is weaving uncontrollably under braking. That sounds a lot like tyres and steering head. But if it is shuddering under braking that is more likely suspension.

I'd first check everything for tightness. How's the fork oil? Have you changed anything in the front suspension?

Be careful to tighten the front axle pinch bolt only to the recommended torque. They will snap !!!! I have done it

Fast Eddie said:

A friend of mine has a T160, one day he was complaining about a ‘weave’.

I took one look at the front tyre and said “you need a new tyre mate”.

He was adamant that it was ok. I didn’t see him for a while, and next time I did, he’d clearly forgotten about our earlier conversation “you‘ll never guess” he said, “I fitted a new tyre sorted”...

Click to expand...

Yes my T150 is very tyre sensitive, if I start getting any sense of tracking or working against me on the road, I know that I am due a new tyre.

Many thanks for all your contents and recomentations, the bike has had this bad habit since I bough it.

I will change the steering and front weel bearings and make sure the weel axel and triple clamps are correctly tight as first step as I'm not sure of their perfect state.

Change tires goes second as the front is good and the rear has 25% wear.

The weel axel had play on the fork slider and was modified for proper fitment, i will recheck if that is well enough.

The disc brake is perfectly square, I took off the original disc and fit a floating system with a 13 mm pump conversion, it had a very poor Norvil disc with a Nissin caliper, de adaptor was not high quality either and took it off to fit a Madass 140 full kit with first class fitment to make sure that was nothing left on this department.

Weel alignment is as good as it can be, new rims and isos was the first thing done, the installation of the CNW head steady was the last thing on this department

The fork seams OK and the rear suspension is with new Hagons adjusted too max spring rate, i would like to adjust them to the middle position for better comfort but if I soften them the movement goes worst.

Regards
Antonio

A weave when accelerating or braking can often be due to steering geometry, or tyres. If everything is slack the bike usually just collapses a bit, but holds a straight line. I would never ride a motorcycle which does not have an hydraulic steering damper. The quicker the oscillation, the harder they work, - Friction dampers are more linear.

There's one more thing I would like to mention as a possibility. When you tighten down the front wheel axle bolt, the "pinch bolt" on the opposite slider has to be loose. This is so the shoulder on the axle (shown below as part #3) has to take up all the space between the inner races and the various wheel spacers so it fixes the wheel bearings in position on the axle by holding the pair of inner races and spacers under pressure. Once the axle nut is torqued, it's recommendes to pump the front end suspension a few times which allows the slider with the pinch bolt to align itself on the wide part of the axle, after which the pinch bolt is tightened. This is just how the commando design works to achieve both wheel bearings fixed in place on the axle, and parallel fork sliders. Things like the spacer between bearings (part #9) can NOT be omitted, nor can any other spacer parts or the system will not work properly.

If you didn't do it this way, you can have one of two issues. Your inner races and spacers may have space between them causing the wheel to slide around horizontally on the axle. OR, your sliders are not parallel to each other and when your front end compresses under braking, the disc is being pulled horizontally against the caliper, causing the mayhem you are experiencing.

It's just another thing to check...

"The weel axel had play on the fork slider and was modified for proper fitment, i will recheck if that is well enough. "

What is this about? Wrong axle, too large hole in slider? This could cause your problem

NorCat said:

-Roadrider 325 x 19 front on 1,85 rim and universal 100/90 x 19 on a 2,50 rim.

Click to expand...

concours said:

Replace the front tire

Click to expand...

+1 Concours.

If the Roadrider's DOM is current (less than 5 years) get an appropriate Roadrider for the rear, if not get a set of tires that have the same maker and model. The term "universal", to me, has a connotation that spells compromise at best and danger at the least.

Check front/rear alignment after getting a matched pair of tires on. I use a pair of 8' fluorescent lights, the string method works well if you have the judgement; the concept is simple enough in either case.

I wish you well, but the handling description you wrote makes me ask if your estate is properly documented.

Best.