Tank Slapper?

meinschaft

meinschaft

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I recently had new spark plugs installed when they put my new PR2 CTs on my Gen II. Most noticeable on my first ride was how much the handling was improved. It is like a night/day difference especially how planted and confident in feels in turns. The other thing I noticed is how much smoother/faster the bike feels. I saw the OEM plugs (with 18K+ miles on them) and they looked great; all the same light tan color.

So now that the mold-release is gone from the new tires, I wicked it up on my favorite straight stretch of backroad. I was WOT, approaching triple digits on the speedo as I shifted to third gear when the front end got light and I felt the handlebars starting to wobble, like an impending tank-slapper. I immediately got off the throttle and it settled right down.

So I guess I have two questions. Should a new set of plugs make such a difference, and does the tank slapper indicate a possible problem with tire install? The bike runs smoother than it ever has with no noticeable vibration/imbalance.
 
meinschaft said:
I recently had new spark plugs installed when they put my new PR2 CTs on my Gen II. Most noticeable on my first ride was how much the handling was improved. It is like a night/day difference especially how planted and confident in feels in turns. The other thing I noticed is how much smoother/faster the bike feels. I saw the OEM plugs (with 18K+ miles on them) and they looked great; all the same light tan color.

So now that the mold-release is gone from the new tires, I wicked it up on my favorite straight stretch of backroad. I was WOT, approaching triple digits on the speedo as I shifted to third gear when the front end got light and I felt the handlebars starting to wobble, like an impending tank-slapper. I immediately got off the throttle and it settled right down.

So I guess I have two questions. Should a new set of plugs make such a difference, and does the tank slapper indicate a possible problem with tire install? The bike runs smoother than it ever has with no noticeable vibration/imbalance.
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I would say it was just a perfect storm of conditions . Light front end, chatter in the road...
 
I don't think there is anything wrong. My 07 will start to wobble under hard acceleration on less than perfect pavement. My GPR damper is one of my favorite mods on my bike. It really gives me more confidence and a better sense of security.
 
Nothing wrong with tire install.
I was told once that that shake is caused by the front tire skipping and dropping speed compared to the rear.
I don't know if its true but sounds reasonable enough.
 
Sebastian F7 said:
Nothing wrong with tire install.
I was told once that that shake is caused by the front tire skipping and dropping speed compared to the rear.
I don't know if its true but sounds reasonable enough.
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You also have to remember that as your accelerating hard and the front lifts a touch and you are pulling back on the bars if you mistakenly get the front tire slightly off when it comes back down it will want to realign itself and cause a little wobble. I won't worry about it too much, the FZ1 isn't known to tank slap.
 
JMundy said:
Seriously?? why is that?
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When you load a tire that is already not holding a true line with the rest of the bike bad things happen. It is best to maintain of actually add throttle until the bike settles.

On a side note. If the head shake is really bad. Make sure you test the brakes before you actually need them. Bad head shake will actually send the pistons back into the calipers, thus requiring a quick pump to have any brake power again. Ask me how I know. :)
 
Let's not get confused. We are talking about two totally different things here. Tank slap. The violent lock to lock "slapping" of the bars is much different than head shake, which is what I believe the op is describing. Buggy describes what happens during head shake perfectly. The front end gets light and tries to self correct as the tire barely skips off the ground causing a slight back and forth motion of the bars. Just about every bike I've ever owned did this as the front tire was approaching lift off. Letting off the gas and replanting the tire always killed the shake.

Tank slap on the other hand is much more serious and is usually caused by the geometry of the bike reacting to another force, such as the loading of the forks. Some bikes are prone to tank slap while others are not. Johnny is absolutely right when he says don't come off the throttle. This will just load the forks more and overwhelm the front tire that is already searching for grip. Staying away from the front brake and slowly rolling on the throttle will usually cure a tank slap.

Although I have experienced head shake numerous times, I have not, in my 30 years of riding street bikes, ever had a tank slap. So my advice luckily does not come from experience. I love to read about the dynamics of motorcycles and I am pulling the info from memory. It seems to make sense to me. Any other thoughts on the subject are appreciated.
 
I have had what I think is a good head shake a few times on rough roads with my old FJ1200. A good power input always seemed to snap things back into it. Though my gut is telling me to break hard, pull over and clean out my boxers.
 
Kpd1031 said:
Let's not get confused. We are talking about two totally different things here. Tank slap. The violent lock to lock "slapping" of the bars is much different than head shake, which is what I believe the op is describing. Buggy describes what happens during head shake perfectly. The front end gets light and tries to self correct as the tire barely skips off the ground causing a slight back and forth motion of the bars. Just about every bike I've ever owned did this as the front tire was approaching lift off. Letting off the gas and replanting the tire always killed the shake.

Tank slap on the other hand is much more serious and is usually caused by the geometry of the bike reacting to another force, such as the loading of the forks. Some bikes are prone to tank slap while others are not. Johnny is absolutely right when he says don't come off the throttle. This will just load the forks more and overwhelm the front tire that is already searching for grip. Staying away from the front brake and slowly rolling on the throttle will usually cure a tank slap.

Although I have experienced head shake numerous times, I have not, in my 30 years of riding street bikes, ever had a tank slap. So my advice luckily does not come from experience. I love to read about the dynamics of motorcycles and I am pulling the info from memory. It seems to make sense to me. Any other thoughts on the subject are appreciated.
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I've had the headshakes a few times and only had on tank slapper. I was riding my KLR at a speed it was never designed to do with weight on the rear. So at around 85-90mph the handlebars start to shake and then instantly into a full tank slapper. The handlebars were moving so fast and violently that there was no way to hang onto them. I pretty much gave up and let go of them completely and started to look for a soft landing in the prairie. As the bike slowed it straightened itself out and I was able to grab the bars again. I had to stop on the side of the road and take a 5 min break.
 
I have experienced head shake on many motocross bikes and once on my 1992 Kawasaki Concours. The FZ1 is VERY stable and even more so with a decent shock.
 
OP - When it shook it's head did you shift into 3rd on the rise of a hill? If so that is very common. I've had that happen at tracks and at first it surprises you. Once you get in a couple of laps you expect it. On the road it can really dirty your undies! As Johnny said, don't do anything drastic when it happens. Typically you can ride through it.

I know FZ1 riders swear you don't need a steering damper however I really think every bike, except of my wife's GZ250, need a steering damper.
 
Hellgate said:
OP - When it shook it's head did you shift into 3rd on the rise of a hill? If so that is very common. I've had that happen at tracks and at first it surprises you. Once you get in a couple of laps you expect it. On the road it can really dirty your undies! As Johnny said, don't do anything drastic when it happens. Typically you can ride through it.

I know FZ1 riders swear you don't need a steering damper however I really think every bike, except of my wife's GZ250, need a steering damper.
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No. Where it happened is a long, flat straightaway; it's on my way home from a 100 mile loop I ride, and it's a relatively safe place (free of obstructions) to crack open the throttle to savor those brief triple digit blasts. As I said in my original post, the bike, now close to 20K miles, is running better than it ever has. This is the first time it scared me.
 
I would ask if you have adjusted your spring preload lately?
If so, to what setting?
What are your forks height in the clamps?
What pressure in the new rubber.

All these things can add up.
 
terdog said:
I would ask if you have adjusted your spring preload lately?
If so, to what setting?
What are your forks height in the clamps?
What pressure in the new rubber.

All these things can add up.
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That's a good question which is what I was thinking about for my bike.

When you set the pre-load what is a good setting for rider sag? I've seen 10mm from the bottom and up. You look about my size, 5'10" 190-200 pounds. My forks have about 50mm left at the bottom of the stroke but I have a lot of dive when braking.
 
Im 6' 0", 225.

I went for 40mm rear sag as my starting poin, ATGATT.
My front settings are a bit more "personaly derived".
Ive ended up having 7 lines showing on the preload rings, which gets me about 20mm of unused, with my forks raised 10mm in the clamps, running a 190/55 PR3.

I have a bud with a Gen1. We rode some twisties, and I was on any line, but he could barely ride the outside. Later on, we traded rides. I couldnt turn his! It was truely ugly. I looked at his shock and it was full soft on the spring. It was like riding a Harley!
We gotta work on it.

I tend to play around with the settings a bit. I see alot of people who dont even mess with thier stuff (you know what I meant). Ya gotta work it.
 
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