Advice on SEVERE tank slap

Howard

Well Known and Active Forum User
VOC Member
Hi Vinparts
The advice is sound, as testified by others in this thread.
I did say that it works in all but the worst cases - "10 foot above the bike at 95+ mph" sounds like a pretty bad case to me.
 

hooterman

Well Known and Active Forum User
VOC Member
tank slapper

Hello
This is my first time try at using the new forum re: tank slapper I have had this nasty experience it was whilst riding a BMW R100rt heavly loaded with panniers etc all weight well to the rear of the bike only doing 15-20 mph in the Great Glen Scotland I was able to accelerate out of it but it made me wonder "To much weight aft" same with caravans ?

Hooterman
 

overthehill

Active Forum User
Non-VOC Member
- looking at fitting a hydraulic damper without the benefit of any drawings the right hand side of the bike looks the easiest place to fit it - with the head bracket offset to the left there is more space on the right - however all of the ones i've seen are on the LH side of the bike - any particular reason?

i assume that the details are in ATY as i cant find anything in FYO

cheers
mark
 
C

carlhungness@verizon.net

Guest
Tank Slapper

Hi:
The steering damper may well cure the "dynamic coupling" known as a tank slapper. I encountered one myself as I foolishly slapped the speedometer at 45MPH when it snapped a cable! Later at about 5 MPH I saw two ripples next to each other in the road and deliberately rode over them, gave the right handlegrip a little shake, and induced a 1-2-3 tank slapper I could control. SInce, I have always ridden with the damper snug on the highway and have encountered bumps that may have put it into the dynamic coupling mode if it had not been for the snug damper.
 

Tom Gaynor

Well Known and Active Forum User
VOC Member
hydraulic dampers

They're always on the left on Nortons. You need another reason?
I can send you drawings of my set-up, just ask. I can even tell you where to buy the raw material to make it a morning's job............
Tightened mine up on a run over the weekend, and got a satisfying "slow roll" at low speed, proof positive that it works!

- looking at fitting a hydraulic damper without the benefit of any drawings the right hand side of the bike looks the easiest place to fit it - with the head bracket offset to the left there is more space on the right - however all of the ones i've seen are on the LH side of the bike - any particular reason?

i assume that the details are in ATY as i cant find anything in FYO

cheers
mark
 
Last edited by a moderator:

timetraveller

Well Known and Active Forum User
VOC Member
Just reading through this 'historic' correspondence as it has now surfaced again and notice that one thing was not mentioned. Going back to the original posting; was the length and type of the lower supports for the Vincent hydraulic damper checked? There are two lengths, short for Vincent dampers, and I believe Konis, and long for Armstrong (Series 'D') and possibly others. A month ago I was working on a 'racing' Vincent to be used at Goodwood. The Koni damper was fitted to long lower brackets and at full extension the mudguard did not just foul the racing exhaust pipes, which are where the original magneto cover would have been, but totally locked up on them. Steering would have been impossible.

The comment about too much weight on the rear is correct but that is light steering, not a proper speed wobble. I went down to St Tropez on my Knight many years ago with a young lady and lots of camping gear on the back. The steering was so light that some of the heavier gear had to be put on the fuel tank. On the other hand I did have a proper speed wobble at Cadwell Park about 40 years ago while racing there. I wrote about it in MPH after Russell had his 'near death experience'. Believe me, at 100mph in a proper speed wobble you will not be able to hold on to the handlebars. Even while pumped up on adrenalin whilst racing the bars become a blur and no normal human could possibly try to control the bike or open the throttle etc. There is enough kinetic energy in a Vincent twin at 100mph to keep the speed wobble going for about 20 to 30 yards (the marks were there on the track) so at high speed you would need a lot of road. In my case I was not thrown off until I had gone straight on at the end of the Cadwell straight and was on the grass. The bike stayed upright with me sitting bolt upright and staring in amazement at the whole front end going from lock to thumping lock. And no ladies and gentlemen, my scars cannot be inpected for a small, or even a large, fee. :p
 

stumpy lord

Well Known and Active Forum User
Non-VOC Member
tank slappers

- looking at fitting a hydraulic damper without the benefit of any drawings the right hand side of the bike looks the easiest place to fit it - with the head bracket offset to the left there is more space on the right - however all of the ones i've seen are on the LH side of the bike - any particular reason?

i assume that the details are in ATY as i cant find anything in FYO

cheers
mark

Mark,
try page 288 of forty years on item114.
if you need any further help please let me know.
norm.
 

Hugo Myatt

Well Known and Active Forum User
VOC Member
Check Armstrong dampers. They do not all have the same extended length. Some definitely need the short eyebolts. Also some lower rear mudguard stays (possibly patterns or Harpers?) are longer than they should be. Fitting them upside down with the bolts above the stay will give a little more clearance.
 
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Tom Gaynor

Well Known and Active Forum User
VOC Member
Long vs short

Something that gave me disproportionate grief was trying to find out which was which. It's simple to see which are short and which long IF ONE HAS BOTH TO HAND, but try to find the dimensions to allow identification of the one set you have...........
A friend remarked that "anyone who talks of "controlling a tank-slapper" has never experienced one." The phrase that stuck in my mind was "you have about as much control as a rat in the jaws of a terrier". As one knows if one HAS experienced one, the analogy is almost perfect.
Just reading through this 'historic' correspondence as it has now surfaced again and notice that one thing was not mentioned. Going back to the original posting; was the length and type of the lower supports for the Vincent hydraulic damper checked? There are two lengths, short for Vincent dampers, and I believe Konis, and long for Armstrong (Series 'D') and possibly others. A month ago I was working on a 'racing' Vincent to be used at Goodwood. The Koni damper was fitted to long lower brackets and at full extension the mudguard did not just foul the racing exhaust pipes, which are where the original magneto cover would have been, but totally locked up on them. Steering would have been impossible.

The comment about too much weight on the rear is correct but that is light steering, not a proper speed wobble. I went down to St Tropez on my Knight many years ago with a young lady and lots of camping gear on the back. The steering was so light that some of the heavier gear had to be put on the fuel tank. On the other hand I did have a proper speed wobble at Cadwell Park about 40 years ago while racing there. I wrote about it in MPH after Russell had his 'near death experience'. Believe me, at 100mph in a proper speed wobble you will not be able to hold on to the handlebars. Even while pumped up on adrenalin whilst racing the bars become a blur and no normal human could possibly try to control the bike or open the throttle etc. There is enough kinetic energy in a Vincent twin at 100mph to keep the speed wobble going for about 20 to 30 yards (the marks were there on the track) so at high speed you would need a lot of road. In my case I was not thrown off until I had gone straight on at the end of the Cadwell straight and was on the grass. The bike stayed upright with me sitting bolt upright and staring in amazement at the whole front end going from lock to thumping lock. And no ladies and gentlemen, my scars cannot be inpected for a small, or even a large, fee. :p
 

clevtrev

Well Known and Active Forum User
VOC Member
Something that gave me disproportionate grief was trying to find out which was which. It's simple to see which are short and which long IF ONE HAS BOTH TO HAND, but try to find the dimensions to allow identification of the one set you have...........

5/8 " and 1 " mounting face to centre of hole.
 
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