Advice on SEVERE tank slap

[mach1ne]

Hello everyone,
Can anyone give me their thoughts on the cause of tank slap on a 1951 series C Black Shadow. The bike has freshly refurbished Vincent shock absorbers, the wheel bearings are OK, the headstock bearings feel tight and smooth and when lifted off the ground , the Girdraulic forks have no perceptible play. It has a 19" front wheel with a new ribbed front tyre running at 25 PSI.
I'm only fairly new to this bike , but have covered about 1500 Km, without any other signs of major headshake. A couple of weeks ago, on a club run, it had a head shake that was that severe that it shattered both my wrists and threw me onto the opposite side of the road. My wife , who was following me , said I did'nt go up in the air , just straight sideways off the bike. The road was smooth and dry tar with an old road repair perpendicular to it, which had subsided slightly and had a small bump a few feet prior to the repair. I was doing about 80 Kmh on trailing throttle in anticipation of an upcoming tee intersection. The steering damper was only just tightened enough for round town riding.
Any information would appreciated.

Thanks
Dean
Canberra
Australia

 

[krautrapide]

Dean, have you checked your pivot bearings? When they become loose, the whole unit starts wobbling!

Krautrapide

 

[mercurycrest]

Hi Dean,
You may have simply found the right circumstances for a tank "slapper".. "On the over run" and a bump or two, etc.... BTW, although 25psi isn't excessively low, tires have changed by leaps and bounds in the last 50+ years. I suggest you put the amount of air specified on the sidewall of your tires, having a low tire didn't help any in your mishap.
Cheers, John

 

[rbove]

Tank Slapper

Dean-
Same thing happened to me Nov '06 on the BSA run in Marin, CA. F10AB/1B/3956 is running down the road, 40-50 mph, tires around 32-35 psi, throttling down just a bit when I ran up against a long low ridge in the middle of the lane. About 6 feet long, an inch high, and I was down before I knew it.
I've become better at making sure that steering damper is tight, but the main change I've made is in my clothing. I was wearing full leathers without padding, and if I have to go through it again I'd rather be wearing my new leathers.
Richard Bove
NOS4ATU

 

[mach1ne]

Hello Krautrapide and John,
Thanks for the replies. The pivot pins and bushes are good and the forks move freely , with the spring units disconnected, with no noticeable sideways or back to front play. As you suggested John, the tyre pressure should have been 5-7 PSI higher, according to the shop I purchased the tyre from. I don't have the bike here at the moment to check the recommend pressure on the side of the tyre. I also may have had to have the damper turned down tighter. I spoke to Terry Prince ( a reputable parts supplier and developer, over many years) and his suggestions were, wheel alignment, 21" wheel with standard width tyre and also a hydraulic steering damper.
What I'm trying to achieve here, is to get some understanding of the mechanism of the tank slap, so I can have some confidence that have been able to change the bike enough to ensure it doesnt happen again. At 55 , I don't bounce down the road particularly well.
On a side note, the club member that took me to hospital, on the day I came off, is also a Vincent rider, who's Rapide also did exactly the same thing in fairly similar circumstances.
Thanks
Dean

 

[mach1ne]

Thanks for the reply Richard. I denifinitly is a recurring theme.
Dean

 

[Tom Gaynor]

Tank slappers

This has been beaten to death on Jtan. Rather than argue about what caused it, make sure it never happens again, and FIT AN HYDRAULIC STEERING DAMPER. I have experienced several tank-slappers, none on Vincents, and they aren't experiences i'm keen to repeat. The last, on my 60th birthday, courtesy of a Guzzi Le Mans, put me in hospital for 10 days and bloody nearly broke my neck. So almost the first thing I did, and before i ever rode the bike, was to fit one to my Shadow. I reckon that indeed it might never happen, and indeed it ought not to happen. But if it does it could be fatal. Fatal isn't curable.

There are details in FYO or OTY, but basically you need a generic Kawasaki steering damper (about £40), a bracket on the two left hand head bolts to secure the body stud, and a plate where the friction damper is to carry the other end. The plate is located by the spigot that locates the friction discs (which are removed) and prevented from rotating by two capscrews into holes drilled and tapped into the two blind bosses conveniently placed in exactly the right place. It took about 4 hours to make everything (with hand tools) and fit it. You have to look very hard to see it.

 

[mach1ne]

Thanks for the reply Tom. Ive just been looking on the internet to see what kinds of hydraulic dampers are available. Forgive my ignorance , can you let me know what Jtan , FYO and OTY are, or where they can be found.
Thanks
Dean

 

[Howard]

Hi Dean

I'm surprised how common this seems to be - I've only ridden a Vin with Egli frame for any distance on the road, but I raced a Comet for a few seasons. The only tank slapper I had on that (and it was fitted with an hydraulic damper) was when the eccentrics turned to the sidecar setting (long story). They might be worth checking, sometimes it's easy to miss the obvious.

Regards

Howard

 

[mach1ne]

Hello Howard,
I checked the eccentrics when I got the bike and also afterwards, just to may sure that had'nt messed up. They are definitely in the solo position.
Thanks
Dean