headshake/tankslapper problems....

[HyperDude]

Anyone encounter the same problem as described below? It happened on my Hyper S model.

At a recent track day, my Hyper would start a violent headshake/tankslapper at exactly 108 mph. It would happen anywhere on the track where that particular speed could be reached. I've since re-created the problem on the street. It never did it before. The only think changed is I've replaced the stock street-compound tires (Pirelli's) with a set of US-made Dunlop 209 race compound tires. I have a damper, but that only minimizes the problem. There has to be a reason for the headshake issue espcially as it can be recreated at the same speed.

Anyone try the Dunlop 209's (race compound) before? If so, encounter anythign similar?

Not sure if it's the tires or setup on the bike needs changed.

Thoughts?

Thanks in advance.

[GHEZZI]

Q1. Are the Dunlop slicks? or just street tyres with sticky compound? Reason I ask is that slicks have a totally different profile/carcass shape that alters ride heights, rake & trail. This may help the bike with better turn in and ground clearance, edge grip etc, but there are compromises.

Seek out a technician from each tyre company, the numbers on the side wall only hint at theoretical sizes. You need actual numbers of diameter and rolling radius in mm.

Q2. What tyre pressures were you running? There is so much B.S. within the industry about low pressures mean faster warm up and stickier rubber. In part this is true but often we end up with tyre operating temperatures that are way outside the manufacturers design parameters. This can lead to failure of the product to perform as the user expected.

Again, seek accurate information from technicians, not a bleedin sales rep!

Q3. What alterations have you made to the chassis dynamics? ie. ride heights front and rear, pre-load, damping etc. These all have a part to play.

For example, if you were a short arse and lowered the rear, one would expect the steering to be more stable (with increased rake & trail). But previous experience with chook chasers at speed has been that wind can get under the high front mudguard and cause instability. Some on this forum believe that raising the fork clamps up the fork leg to gain stability instead of tuning suspension is a good move.

Never treat the symptom, FIND THE CAUSE!

[PedroHM]

if those tires are TRUE race compound rubber then I hope you keep in the garage with warmers on all the time, they only work for a number of heat cycles and will be slippery as all hell when the oil comes out of them!

I had the same issue with a vibe in the front at speed, I just kept the gas pinned and it went away. taking out some preload solved it in the front.

to solve the problem you have to understand why a tankslapper happens, it happens when the forks can't react quick enough to the changes up and down in the road surface, this usually happens more when on the gas and there is minimal load on the forks, this basically means the energy has to go somewhere and it is translated into sideways motion at the bars (the slapper).

So in reading into this you may find that front rebound is too stiff and the tire can't make good contact with the ground when the gas is on or its too stiff and can't react to the light pressure at 108mph OR the back is squatting too much and causing the front to go light.

so it could be almost anything, I'd try recreating the problem and then shifting your ass back and forward on the seat to see if you can calm it down and then adjust from there.

[infidel]

I had something really similar on my SXV550, it happened at 145kmh. You could accelerate through it, but if you stayed around that speed at was shocking. It was very noticeable when i had 208RRs fitted, but got alot better after i fitted supercorsa SC2s. It was still there though.

I did my first ride on Saturday with a set of 208GPs on the hyper and i thought they were brilliant. The profile seemed to be a lot different to the 208RRs, and also the Diablo IIIs i had on there before. From what i can tell the 209GPs are the same as the 208GPs, but they are dual compound.

[Monstaman]

Chezzi is 100% on the money, your setup for you is most critical.

Then first thing is to check is the surroundings, by that I mean headset bearing preload, fork pinch bolts and axle tight, calipers tight, triple trees all tight.

Next, do to your rear suspension, check the rose joints, shock pivot and end fixings and lastly your swinger pivot bolts, give a wobble side to side on the wheel and eccentric as well, any movement or looseness of any of the above will transfer bad vibes.

If all of this is fine check your preload, compression and damping adjustments are equal on your forks and you shock is correctly rebounding and static sag is correct.

From there I would check the balance of your wheels, it is not unheard of to have a slight manufactuting defect which will cause an imbalance.

Steering dampers are only sticky plasters, they do help but your setup should be improved to the point that the damper is for the serious occassion rather than everday stability.

[HyperDude]

Thanks for the feedback guys.

The mounted Dunlops are DOT's (not slicks). I've used these particular tires for years (in addition to the NTEC's, etc.) for racing the big litre bikes and that's why I thought I would try them on the Hyper.

I've played with the pressure (starting at 31/30 cold) and dropping the pressures as needed. The bike still sits as it did when it arrived with no changes to the suspension, ride height. etc. My plans are to get this thing dialed-in during another track day. However I would not have thought the differences brought on since changing the tires would be so great. No doubt each tire model/make measures differently and has their own characteristics. I've just never witnessed such a sharp contrast in handling (shake/no shake) between two makes of tires. I just wanted to make sure no one saw something similar before I started drilling down to determine the cause.

I think I'm off to start on the setup of this bike. Does anyone have a link or suggestions to some baseline settings. No doubt, they'll need to be refined.

Thanks again guys. By the way, I've ridden and raced a multitude of bikes and believe this is one of the funnest bikes I've ever had a chance to lean over.

[HyperDude]

Thanks John.

I've actually found the same thing. The handling of this bike is affected heavily by one's position. As well, tugging on the bars to move one's body in the seat (as seen sometimes withnessed late in the day/race) can affect the stability of the front-end.

When this was happening, I tried adjusting my position on the seat as well as how upright I was. Unfortunately, the headshake was always there and very consistent regardless.

Thanks again bud.

[GHEZZI]

Dont know how people can ride on flat tyres, anything less than 33psi front cold and the Hyper feels heavy in the steering like having a flat tyre - when breaking deep into a corner and beginning the turn in process. I suppose I could ride slower, brake earlier and softer. NAHH!..... FUCK IT! ......I got HDD remember.

[PedroHM]

Originally Posted by GHEZZI

Dont know how people can ride on flat tyres, anything less than 33psi front cold and the Hyper feels heavy in the steering like having a flat tyre - when breaking deep into a corner and beginning the turn in process. I suppose I could ride slower, brake earlier and softer. NAHH!..... FUCK IT! ......I got HDD remember.

ditto, its no wonder people are raping rubber in 2000km's on the road. bad setup, low pressure etc.

[nwrob]

Originally Posted by HyperDude

Thanks for the feedback guys.

The mounted Dunlops are DOT's (not slicks). I've used these particular tires for years (in addition to the NTEC's, etc.) for racing the big litre bikes and that's why I thought I would try them on the Hyper.

I've played with the pressure (starting at 31/30 cold) and dropping the pressures as needed. The bike still sits as it did when it arrived with no changes to the suspension, ride height. etc. My plans are to get this thing dialed-in during another track day. However I would not have thought the differences brought on since changing the tires would be so great. No doubt each tire model/make measures differently and has their own characteristics. I've just never witnessed such a sharp contrast in handling (shake/no shake) between two makes of tires. I just wanted to make sure no one saw something similar before I started drilling down to determine the cause.

I think I'm off to start on the setup of this bike. Does anyone have a link or suggestions to some baseline settings. No doubt, they'll need to be refined.

Thanks again guys. By the way, I've ridden and raced a multitude of bikes and believe this is one of the funnest bikes I've ever had a chance to lean over.

I understand the Non-NTech 209s should be set at 30 front and 26-27 psi rear. The main thing that will affect headshake is rear ride height so take a look at that after pressures. It's possible the Dunlop is substantially taller than the OEM Pirelli so adjust as necessary. Also, ease up on the bars when the headshake occurs and see if that has any effect.