Tank slapper bent my rear brake lever

[egleaves_imp]

Had a little scare this morning on the way to work. As I’m approaching my freeway exit I get over with plenty of time and fall in line with other traffic slowing for the exit. We had collectively slowed to around 50mph when my bike went into a violent wobble that must have lasted 2 or 3 seconds. My feet were thrown of off the pegs and the rear started alternately beating the insides of my legs while the bars whipped back and forth. I thought for sure I was going down, but I kept my cool and reacted without panicking. My legs are killing me, but I kept the bike up.

Speeds were about 50mph and slowing gently, no hard braking, no known road obstacles. It felt like the rear brake was locked up for the whole wobble. Riding the last mile to work felt odd every time I applied the rear brake….once I parked the bike I found that the rear brake lever is bent up and out which drastically changed its position relative to my foot. That’s why things felt different.

Could that lever have been bent (out and up) by my leg smashing into the side of the bike? It wasn't like that when I left the house. Think I can just step on it to bend it back, or would I be putting weird stress on other components that I can’t see?

I'm still trying to replay the whole thing in my head in the hope that I can determine the cause of the wobble. The bike has never done that, and aside from cars ahead and behind me conditions were pretty favorable.

 

[misterprofessionality]

From experience: you most certainly could have bent that lever with your leg if the wobble was violent enough, but I think it wouldn't be a question for you to ask us, because you'd have some kind of very obvious bruising/injury on your leg as a result. I haven't had a Tank Slapper (and frankly, i thought they could only happen at high speeds, so this is pretty terrifying to read), but I've had an incident where a car *very nearly* murdered me by splitting (yea, a car lane splitting) past me on the freeway, hitting my leg, causing my foot to catch the pavement, and slingshotting it back into my bike. My brake lever was bent at the time, too, though it was on a CBR500.

 

[chync_imp]

Wow. Have you made any mods to your front end? Or raised the rear? I thought the rake on the cb1100 was long enough to make a tank slapper very unlikely.


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[egleaves_imp]

From experience: you most certainly could have bent that lever with your leg if the wobble was violent enough, but I think it wouldn't be a question for you to ask us, because you'd have some kind of very obvious bruising/injury on your leg as a result. I haven't had a Tank Slapper (and frankly, i thought they could only happen at high speeds, so this is pretty terrifying to read), but I've had an incident where a car *very nearly* murdered me by splitting (yea, a car lane splitting) past me on the freeway, hitting my leg, causing my foot to catch the pavement, and slingshotting it back into my bike. My brake lever was bent at the time, too, though it was on a CBR500.

Yup. Those are surfacing now. Left is worst than the right.

Wow. Have you made any mods to your front end? Or raised the rear? I thought the rake on the cb1100 was long enough to make a tank slapper very unlikely.


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No mods, but my front tire is nearly bald. New tires go on tomorrow.

 

[misterprofessionality]

From experience: you most certainly could have bent that lever with your leg if the wobble was violent enough, but I think it wouldn't be a question for you to ask us, because you'd have some kind of very obvious bruising/injury on your leg as a result. I haven't had a Tank Slapper (and frankly, i thought they could only happen at high speeds, so this is pretty terrifying to read), but I've had an incident where a car *very nearly* murdered me by splitting (yea, a car lane splitting) past me on the freeway, hitting my leg, causing my foot to catch the pavement, and slingshotting it back into my bike. My brake lever was bent at the time, too, though it was on a CBR500.

Yup. Those are surfacing now. Left is worst than the right.

Wow. Have you made any mods to your front end? Or raised the rear? I thought the rake on the cb1100 was long enough to make a tank slapper very unlikely.


Sent from my iPhone using Tapatalk

No mods, but my front tire is nearly bald. New tires go on tomorrow.

there it is. no traction on that front. it was just skating along on the road while your rear tire was pulling away. Just like a wonky shopping cart. still, at 50mph? the low speed is probably the reason you were able to recover.

 

[VLJ_imp]

In addition to your bald tire, I've noticed one other thing about the CB that's different from any other bike I've ridden, and I noticed it while riding in your neck of the woods, up I-80 to Auburn. While traveling in a straight line, pushing on either grip produces the expected result. The bike just moves solidly in that direction. However, if you waggle each grip back and forth, yep, this results in something that feels very much like a tankslapper. I tried the same test on my XSR900, and it didn't get nearly so out of shape. The rubbery-feeling oscillations of the CB are crazy, they're so wild. They're so dramatic that if I kept waggling the bars, the bike would easily oscillate right out of control. It doesn't take much.

Like I said, I've never felt anything like it on any other bike. It's easy enough to avoid, and I guess it's just a characteristic of the chassis.

Moral of the story, don't waggle the bars even a little, and don't run bald/underinflated tires.

 

[nhawk7504_imp]

That doesn't sound good at all. Harleys had the same issue. Incorrect inflation and balding tires can definitely cause it. I buy tires before I "need" them. Glad you made it through that.

 

[Nortoon_imp]

Is it part of the skinny tire syndrome?

I had a terrible time on a sandy road with my CB1100 with fairly new tires.

 

[Lord Popgun]

Motorcycle physics can be strange and confusing. Wish I knew more about them.

Here are a couple of links that explain in English (vice math) some reasons for the slapper.

How to avoid a tank slapper or speed wobble - webBikeWorld

Speed wobble - Wikipedia

 

[PowerDubs_imp]

Only time I have ever experienced tank slappers is coming down out of a wheelie on sport bikes.

I've never had it happen on any of my other street bikes, or dirt bikes for that matter. Definitely not on my CB1100, and yea I've done wheelies on it and will be even more inclined now with the larger rear sprocket.

Now I am curious- going to go look up the front rake to my old FZR (25) vs the CB (27)

 

[egleaves_imp]

I’ve read of it happening for a few common reasons:

Coming down from a wheelie
Bad head bearings
Bent rim
Tire out of balance
Sudden steering input

My tire is suspect #1 , but it seems really strange that it happened at relatively low speed, while decelerating, and under light braking. Basically everything you’re supposed to do in order to mitigate the wobble is what I was already doing. Still trying to figure out why it happened. I’m sure the bald front tire wasn’t helping, but I have a hard time believing it was the cause in this case.

 

[Lord Popgun]

I just took a look at my rear brake and I would not recommend kicking it back into position. You could bend the pivot mount or maybe crack the lever and not be able to see it. Looking at mine there seems to be a place it could crack and you not see it. Hard to tell.

I would remove it and bend it and examine it closely before re-installing.
I’d examine the switch and linkage too as you never know when a big shock occurs.

 

[egleaves_imp]

I just took a look at my rear brake and I would not recommend kicking it back into position. You could bend the pivot mount or maybe crack the lever and not be able to see it. Looking at mine there seems to be a place it could crack and you not see it. Hard to tell.

I would remove it and bend it and examine it closely before re-installing.
I’d examine the switch and linkage too as you never know when a big shock occurs.

Thanks, popgun. That's exactly what I was wondering about. I won't try to bend it back until I get home and can detach it.

 

[jtopiso_imp]

I just took a look at my rear brake and I would not recommend kicking it back into position. You could bend the pivot mount or maybe crack the lever and not be able to see it. Looking at mine there seems to be a place it could crack and you not see it. Hard to tell.

I would remove it and bend it and examine it closely before re-installing.
I’d examine the switch and linkage too as you never know when a big shock occurs.

Thanks, popgun. That's exactly what I was wondering about. I won't try to bend it back until I get home and can detach it.

I assume it is not made of aluminium. If it is, don't even try to bend it back: it will be brittle and unsafe to use even if you get it to look good.

If it is steel, probably you'll be ok, unless you can find some cracks.

 

[dave]

Did your front tire wear unevenly (like mine did), with high and low spots on the sides? If your answer is yes, then I suspect the tire caused the wobble. My CB1100 started to head shake when the tire wore like this, and I replaced it. I switched to a different brand (can't remember off the top of my head right now) but the head shake completely went away, and the new tire is wearing much better (I'm on the third front tire now). A friend of mine crashed his CB750 back in 1981 because of unexpected head shake. Luckily he survived, but he sold the bike immediately.

 

[jtopiso_imp]

Had a little scare this morning on the way to work. As I’m approaching my freeway exit I get over with plenty of time and fall in line with other traffic slowing for the exit. We had collectively slowed to around 50mph when my bike went into a violent wobble that must have lasted 2 or 3 seconds. My feet were thrown of off the pegs and the rear started alternately beating the insides of my legs while the bars whipped back and forth. I thought for sure I was going down, but I kept my cool and reacted without panicking. My legs are killing me, but I kept the bike up.

Speeds were about 50mph and slowing gently, no hard braking, no known road obstacles. It felt like the rear brake was locked up for the whole wobble. Riding the last mile to work felt odd every time I applied the rear brake….once I parked the bike I found that the rear brake lever is bent up and out which drastically changed its position relative to my foot. That’s why things felt different.

Could that lever have been bent (out and up) by my leg smashing into the side of the bike? It wasn't like that when I left the house. Think I can just step on it to bend it back, or would I be putting weird stress on other components that I can’t see?

I'm still trying to replay the whole thing in my head in the hope that I can determine the cause of the wobble. The bike has never done that, and aside from cars ahead and behind me conditions were pretty favorable.

I am trying to think on the causes:

-worn tyre means smaller tyre radiuis, so smaller steering trail.
-braking means bike dives, so lower trail too.

Still, I'd say this would mean the bike isn't as stable as it could be, but is that enough to cause a tank slapper... :-? I wouldn't think so!

Check your pressures (again: affect your tyre radius->steering trail), and check your suspension preload settings (rake -nose down attitude- will decrease trail too).

And, if you find the root cause, please let us know!

 

[the Ferret]

Just out of curiosity, when you slowed down to 50 mph you didn't sit up and let go of the bars momentarily did you?

 

[egleaves_imp]

Wow. Have you made any mods to your front end? Or raised the rear? I thought the rake on the cb1100 was long enough to make a tank slapper very unlikely.


Sent from my iPhone using Tapatalk

Just out of curiosity, when you slowed down to 50 mph you didn't sit up and let go of the bars momentarily did you?

I had a proper grip on the bars…..not too tight, firmly positioned, elbows bent slightly. Good thing too, or I don’t think I would have recovered it.

I checked the pressure this morning and it was down to about 27-28, so I brought it back up to ~35psi. It seems like I lose about 1 or 2 psi per month and have to top it off again. I do believe that more frequent checks are in order.

Did your front tire wear unevenly (like mine did), with high and low spots on the sides? If your answer is yes, then I suspect the tire caused the wobble. My CB1100 started to head shake when the tire wore like this, and I replaced it. I switched to a different brand (can't remember off the top of my head right now) but the head shake completely went away, and the new tire is wearing much better (I'm on the third front tire now). A friend of mine crashed his CB750 back in 1981 because of unexpected head shake. Luckily he survived, but he sold the bike immediately.

No scalloping, if that's what you mean. The profile is evenly bald out to the chicken strips.

 

[VLJ_imp]

Quote:The profile is evenly bald out to the chicken strips.
The same could be said of most CB1100 riders.

 

[emptysea]

What tires are you running?