Light front end / tank slappers at 85+mph (136kmph) after new tires?

[ducaticowboy]

Hey folks,

Got new Dunlop Trailmax tires recently (80 miles on them so far) and noticed that as soon as I hit 85+mph, the front end gets light and goes into a wobble / tank slapper. This is regardless of the road. 

 

If I sit on the passenger seat at the same speed and take weight off the front, it doesn’t happen (or it’s about 90% less intense). 

 

They balanced the tires at the shop, and I’m getting mixed messages from different mechanics saying it could be balance while others say balance issues typically happen at 60mph or wouldn’t cause that at 85mph.

 

Another mechanic told me to check the fork neck bearings but as this is a brand new bike, that doesn’t make much sense. Yet another mechanic said to check alignment. 
 

I’ve lowered the tire pressure, same issue. Besides a defective tire, the only other thing I can think of is that I also installed the extended puig touring windscreen - do you think a taller windscreen can cause wobble on the front? I have inspected the front tire while it is rolling and I can’t see any wobbling from it.

 

Other than that, is there anything else I should be looking at, like a suspension adjustment on the front for rebound or compression?

 

One very experienced friend said to put in a bottle of ride-on self balancing tire sealant, yet others have told me this is a bad idea. 

 

I’ve talked to several mechanics and none of them are in agreement with what to try + they are all booked for weeks. Is it possible that a balance issue only appears at that speed?

 

Any tips on how to investigate this are greatly appreciated.

[Canzvt]

So all things being equal, on a near new motorcycle, weight transfer would likely be the issue. In the case of the original poster, weight transfer to the front due to aerodynamics (different windscreen) is likely the cause, possibly added by the change in tires/tire pressure. However, the T7 is known for having out of true wheels/tires from the factory as well.

 

If you spend enough time reading through Motorcycle Tuning: Chassis: https://www.scribd.com/book/282481826/Motorcyle-Tuning-Chassis, you will learn about the dynamics at work. Fore/aft weight transfer in combination with tire selection/pressure, aerodynamics, wheel true/balance, rake/trail, etc, etc can all be causes for the dreaded headshake/wobble.

 

Again, all things being equal, suspension set-up can also contribute to headshake/speed wobble but in a manner not intuitive. If the rake of the forks is too steep, meaning the rear is too high, it not only puts more weight on the front wheel, it decreases the trail (castor effect - see book above) meaning the wheel will have a higher propensity to oscillate. Having the rear too low reduces the rake, increasing the trail and increasing the castor effect, reducing the risk of oscillation if there are NO OTHER CHANGES.

 

A chopper's increased risk of wobble/headshake is largely due to reduced rigidity of components from the tire contact patch to the steering stem, coupled with added stress to the chassis steering stem/chassis stiffness, not necessarily suspension set-up/geometry.

 

[gone2seed]

Take it back to the shop and let them ride it. If it is balanced, bead is centered on the rim, tire is not defective; have then check for loose steering head bearings.

[Canzvt]

For those interested in really finding out what is going on with tank slappers from a forensic engineering perspective, take some time to read this...https://www.robsonforensic.com/articles/motorcycle-shimmy-crash-expert/

 

[Jon James]

Have you fitted crash bars? If so, if they have not been tightened up correctly they can bring an uneven stress to the frame and even though slight, can result in the frame not being perfectly 'square' and can influence the balance of the bike. 

 

If you do have have crash bars, perhaps loosen all the bolts and tighten as you would a car wheel, evenly all the way round in stages to the correct torque settings....

 

I wish you luck finding the solution.....

[Jarrid]

27 minutes ago, Canzvt said:

For those interested in really finding out what is going on with tank slappers from a forensic engineering perspective, take some time to read this...https://www.robsonforensic.com/articles/motorcycle-shimmy-crash-expert/

 

Good article, makes sense. The other piece to my issue is when I remove my rear panniers the problem goes away until a much higher speed. When I have them on, but empty, I feel the oscillation start at 130ish KPH. Without them on it doesn't happen until 165KPH. With either scenario, if I move my weight back the oscillation goes away. The Puig High windshield seems to add to the issue as well. 

[Canzvt]

Your panniers have the effect of aerodynamically lightening the front end/weighting the rear as they increase the frontal area of the motorcycle due to their width from the bike, and act mostly on the rear wheel. The aero on this bike (and most others) is massively turbulent. You moving rearward probably has more to do with 'cleaning' the air or changing the turbulent focal point a bit than moving the weight rearward, but only a full analysis (requires a wind tunnel, smoke wand, you, your bike in its various forms and some scales) would show the actual causes. Adding weight to your bike (loading the bags) likely changes the oscillation frequency independent of the aero, making it more confusing to figure out.

[Jarrid]

Just as an update, I took my front Mitas EO 9 off and put the stock Pirelli back on today over lunch.  The best I can tell in the short ride back to work is the front end tank slapper is 100% gone! Brought it up to speed (passed 135kph) and it felt solid. I'll go for a longer ride after work and do a few more runs. I will put on the side bags as well as they seemed to expedite the problem over running without them.  I still have the rear EO9 on and no changes to anything else. Perhaps I had a bad front tire as the front EO9 seemed to be even worse than the Dunlop 606 I had on before it. Unfortunately the temps have dropped here and the one thing did notice before is that the slapping did not come on as early on cold or wet asphalt and seemed to be worse the hotter it was outside.  

 

[palada]

I've been dealing with an irritating speed wobble over about 85mph lately too. Yesterday I tightened up the steering head and it's now fine. While waiting for a ring nut wrench to arrive, I had loosened and re-straightened the front wheel, moved the forks up and down in the tubes, screwed with preload, etc. None of those helped (or at least didn't eliminate it). Tightening the steering head did.

 

I ordered the wrench from Amazon and it worked fine.

Amazon.com

 

 

 

[Jarrid]

Good to know. Going  back to the stock tire eliminates every issue for me except the issue of wanting  to run other tires. I plan on putting on some aggressive tires for next season and will check the steering head.  Maybe a damper would help but I like  the stock quick steering response in the tight single track so not quit sure if I would spend the cash for one yet as an experiment. 

[Ray Ride4life]

On 6/6/2021 at 6:53 PM, ducaticowboy said:

do you think a taller windscreen can cause wobble on the front?

I don't think it, i know it.
Also have a Honda Crosstourer with an MRA Vario screen and in the highest setting i can't go as fast as when i have it on a lower position.
Do you have panniers or a topcase? When i bought it it had a full aluminium H&B set on and after removing the topbox it even got better and after changing the side panniers for the Mosko Moto panniers which are a bit slimmer and less boxy i even could set the screen back to the highest setting.
It still gets light in the front at really high speeds but i don't like those speeds for more than a quick overtake anyway.
But i think i noticed in some YouTube video that the T7 has conical bearings from factory and those have to be adjusted properly and in some cases readjusted. This can be the case if they not put in correctly which can happen easily even by a professional, it's a matter of 10ths of a millimetre and meybe even hundreds.