Front wheel hop when braking - suspension arm bushes, or... ?? Any help?

It's the wheel hop that has me scratching my head. Pulsating, etc., could be warped rotors, worn A-Arm bushings, etc. But for wheel hop, I would remove the Centramatic balancers, torque the front wheels correctly in 2 stages, and give that a try. We've had some wierd front end issues go away by removing the Centramatic balancers.

Check tire pressures while you're at it.

A bit of a shot in the dark. But not hard to do and virtually free.

I would immediately suspect a problem with the ABS system kicking in, or out, when it shouldn't.

I'd get the dealer to flush the brake fluid AND flush the ABS unit, using the BUDS system. I'd have them check the ABS sensors and rings on the wheels, at the very least clean them or test them for correct signal pulses when the bike is in gear and moving down the road.

Incorrect signals from any of the wheel sensors or contaminants in the brake fluid in the ABS module could be causing issues. Even a faulty ABS module or ECU could be an issue.

Check the connectors from the ABS module to the ECU for corrosion, the same for all three wheel sensors to the ECU harness. Once you have cleaned them all out with electrical connector cleaner and a tooth brush apply a little dielectric grease before re-connection.

I would seriously get the dealer to look at it as the BUDS system will be able to test the sensor operations and allow a full brake fluid change including the ABS unit as well as just the regular parts of the brake lines.

As the Nanny control system relies heavily on correct ABS function, I would treat the issue seriously as you don't want that part of the bikes automatic systems to go wrong when you most need them.

Jetfixer said:

Thank you both for your replies. Ron, during a search for this problem you had posted somewhere that the arm bushings should be lubricated. Can you elaborate on that for me? The tires are set at 18 and even from side to side. Wouldn't a failed Centramatic cause vibration all the time? This thing is smooth while riding. It's just the braking that causes the wheel hop. But you're right, it's an easy thing to try. I wish I could get a video of this in action. It truly is unbelievable. It's so violent, I'm not kidding about it trying to throw off the fender.

mecsw500, I have BUDS and started there. No faults in any systems. This bike has the stock exhaust and I don't think the ABS is kicking in; I can't hear it running. I personally did the fluid flush on this bike a year or so ago, so maybe I'll give that a try also.

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I'd start with BajaRon's advice first. He's far more expert than me on Spyders. His idea is a far simpler and more likely thing to start with. At the very at least you can rule out another variable for zero cost.

I thought ABS as I did have a problem on a 1997 BMW R1100 RS with the ABS system that made braking a sort of on and off pulsating feeling. It turned out to be an ABS module fault. This was a really difficult problem as BMW no longer made the modules and replacements were almost impossible to find at a price that wasn't absolutely insane even for a used one.

Water from not changing the brake fluid often enough and rubber particles from the brake hoses had scored the pump pistons and made the thing behave really oddly. The ECU kept thinking it needed to operate. By the time I'd figured it all out the pump was beyond repair and there were no service kits available. ABS unit problems were so common after the bikes aged, some folks bypassed the ABS system altogether, but I wasn't that brave. I eventually found a new unit in Germany which was only double the list price of a new one when they were in stock in the US. The electronics one those bikes were not very sophisticated, but the unit never threw a fault.

You are right though, if you listen carefully you should hear it running if it's doing something.

I hope you find the problem out because that would be more than mildly annoying.

Jetfixer, Swap the front tires and wheels from your bike and see what happens.

Jetfixer said:

An update. I tried Ron's advice and removed the front Centramatic balancers. Actually did a 3 step torque on the lug nuts. By hand, then to 40, then to final 77 lb/ft. with a calibrated torque wrench. The problem was marginally improved, but still unacceptable. I believe that whatever is the real cause is happening at such a speed that the balls inside the Centramatics could not react fast enough and could be considered to be compounding the problem. I think I'll try another brake bleed tomorrow...

Any other suggestions would be greatly appreciated.

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OK, another crazy thought. Could it be the back end? If that is rising and lowering could it appear to be shifting weight on and off the front.

So, perhaps check the rear wheel bearings, rear brake pads and lines, rear disk run out, shock mounts, belt tensioner, rear wheel run out, and rear disk run out. If the rear is getting unloaded as the brakes come on, then a problem at the rear might be transferring to the front wheels as the front loads up and the rear unloads. Also, check at the Schraeder valve under the rear seat to make sure your rear airbag has pressure in it. It should be about 30 lbs or so, depending upon you weight. If it's 0 lbs or up near 90 lbs that's going to make the rear end do very funny things under braking as you have a ride height air bag problem. My 2021 Spyder had dozens of trips to the shop to sort out the rear suspension problems that kept failing from new. It kept losing air.

The other thing to check is to make sure neither the front or the rear shocks have lost fluid and that they have proper levels of damping, compression, and rebound; and that one of the fronts hasn't lost damping compared to the other.

Perhaps the belt condition (cracks and teeth condition) is causing it to play up when coming off the throttle as you brake, making it skip teeth. Check the front pulley splines as there were recalls on that for splines rounding off. You would notice it more under deceleration than acceleration perhaps?

Everything might be fine until you get the drive train really loaded up under braking? The braking is going to load up the drive train a lot including the belt, the front pulley, and the clutch as it runs against the engine compression.

Does it do it if you bang it if you like run along in third gear at say 4,000 revs and then snap the throttle closed and at the same time bang it down quickly into first and not use the brakes? If it did it even a little bit that would kind of eliminate the braking system.

Did you check the front rims for run out?

Now the Centramatics are removed, perhaps one of the front tires is way, way out of balance and the imbalance is transferring from one side to the other through the sway bar, the tire balance not being noticed until the tire deforms under braking? You could put 30 lbs of air in the fronts as a test to see if that cures it, or makes it better, as you will have far less tire deformation under braking?

I'm throwing things against the wall here to see what sticks. Sorry if I'm heading off in some strange directions, but it seems a really weird issue and not one I've experienced before, except on my old Beemer.

I'm sure we are all fascinated to know what it is, as probably you are too.

My thoughts are a faulty abs unit. A tech that is an expert on brake issues and abs needs to examine the entire brake system.