Tank Slapper Over Rumble Strips

[Galligaskins]

Anyone suffered from a tank slapper going over rumble strips?
Just come back from an hour or so out on my 3½ and the handling is impeccable. Except that when I was on a dual carriageway approaching a roundabout in the outside lane, and for a few hundred yards there are quite severe rumble strips. Doing around 70, I rolled off the throttle as I came to the rumble strips and instantaneously the bike went into a massive tank slapper. How I held on I'll never know. Never experienced anything like it and I've had some evil handling 1970's Japanese bikes in my time. I genuinely thought I was going to be spread across the dual carriageway as there were cars all around me. It seemed interminable as the bumps go on for a long way and the shaking was blurring my vision. By the time I'd coasted down to around 40 or something it settled down and I went round the roundabout perfectly well and the rest of the ride home was fine. I suppose it could be a tribute the the Morini's chassis that I stayed upright but, equally, it perhaps shouldn't have happened in the first place.

Tyres are nearly new TT100s running at 32/35psi. There is no play in the swingarm. I rebuilt the forks over winter and they're running the right amount of droop/compression. Also, the head races were attended to. About the only thing I've not serviced is the rear shocks although up to now they've not given any concern. As I say, in all other situations the handling has been superb.

I've regularly driven over the rumble strips and always found them very harsh. They're on the A6 heading north into Loughborough, at the Mountsorrel roundabout if anyone wants to watch out for them. I get the feeling they've been recently recoated as they looked quite fresh today and were even more bumpy but that might be the difference between the Morini and a car.

Needed to share with someone but didn't want to worry my wife!

[MickeyMoto]

What rear suspension are you using? Could be a damping problem? Or, are the rear shocks soft?

[Galligaskins]

The shocks are the original (1977) Marzocchis so could well benefit from a rebuild or replacement with Hagons that many people seem to like. To be fair, they don't feel soft or lacking in damping but it might not be a bad idea to give them a closer look off the bike.

I googled tank slappers on rumble strips and it seems it is a recognised thing, even with modern, well set up bikes, with a least one fatality reported elsewhere in the country and efforts to have them removed.

[MickeyMoto]

I bought a pair from Falcon in Wareham. So far, so good. They do not recommend rebuilding the originals.

[Morini Jen]

Hello Howard.

Sorry to hear of your scary experience today. There could be all manner of explanations for why it happened. It is a good idea to take a close look at your rear shocks. If one of them is leaking a bit of oil it can be hard to spot with just a cursory glance. Do you have a steering damper fitted? I've never had one on my 3½ so am not qualified to comment.

Still,that said, you said that you had been riding for an hour or so, and today was a hot and sunny day. I'm wondering if your tyre pressures might be on the high side?

You said that you were on the A.6. near Mountsorrel. Is that the stretch that goes over the River Soar? Looking at it on Google Maps, those rumble strips seem to go on for a long way, possibly 500 yards, also the rumble strips become closer together the nearer you get to the roundabout. You were rolling off the throttle rather than braking hard, but even so there would be increasing forces on your front forks with each passing yard. I'm sure the late great John Robinson would have a scientific explanation for it (I failed my Physics 'O' Level).

Still, if your bike appears to be fit and well, if it were me I would drop the tyre pressures to, say 28 front/30 rear and have another ride in the sunshine and approach the same section of road again. Perhaps, traffic permitting, a little nearer 50mph. See what happens and then check your tyre pressures when you get home.

Ride safe,
Jenny.

[Galligaskins]

Hi Jenny,

I don't have a steering damper fitted. I think it would have broken the mounting bracket if I'd had one, the oscillation was that violent. I used to have one on my Sport, years ago, and always thought the bracket was weedy and flexed a lot.
Yes, you've found the stretch of road. I've been out this morning and driven over it in the van and it doesn't seem half as bad as I remember, and hasn't been recently recoated. It certainly didn't feel severe enough to generate the reaction it did on the bike, so I can't blame that.
At the time the tank slapper was happening the thumping through the forks was considerable but that might have been because the wheel was not pointing straight ahead when it hit the next strip. But it does make me wonder if everything is set up a bit stiff. In my efforts to minimise front end sag which I felt was steepening the fork angle leading to the flighty steering I used to have, I may have overdone it. We'll see. Time for some more experimentation.

[3potjohn]

I rebuilt my Marzocchis with little drama. Well they don’t feel worse….
https://www.suspensionzana.com/index.ph ... Itemid=114

Kit Marzocchi 9mm

[norbert]

I think this tank slapper is a fenomen that can happen when after lifting the front wheel it comes down to earth not in line. As far as I know you should grab the handlebar only slighty and acerlerate (perhaps also leaning back to take off weight from the front wheel) without breaking! That is not the natural reaction, but it seems the best way to solve the situation. I don´t think it has to do with a tecnic fault on your bike if before and after everything works fine.

About 45 years ago, quite unexpierienced with my first real moto (Suzuki 250 GT, the next one and til today: only Morini) to me happened the same: acelerating somehow strong I pased a manhole cover lifting the front wheel, without willing to do so. When it came down the bike behaved like a rodeo bull. It took about three seconds getting realy worse till I was down to the ground. I wondered like you do now. Fortunately it never happened again to me.

[Morini Jen]

I think norbert could be on to something there as it brings back to mind the one and only time my 3½ misbehaved itself. It was around 1998, the first year of my ownership. The bike had clip-ons and rearsets and the original Marzocchis. 

It was at the end of the M.45, at the Thurlaston roundabout, aiming to join the A.45 towards Coventry. I was going around the right hand bend in a reasonably sporting manner and was just at the point of picking the bike up to go round the left hander, probably subconsciously weighting the left footpeg and handlebar, when all hell broke loose! The bars flapped from side to side very violently and I felt like a superfluous jockey atop a completely out of control motorcycle. I thought I must have hit something in the road although I couldn't recall seeing anything. Still, I had little choice but to wind on the throttle, as norbert mentioned, to get around the left hander and that was when things settled down again.

It really shook me up, especially as the Morini had handled impeccably up to that point. Once home I checked the bike over and that's when I noticed that the right hand rear shock was leaking some oil. I ordered a new set of Hagons which were o.k. but not as good as the original Marzocchis in my opinion. I have ridden a 3½ with Konis and they were really good too.

So, what's the conclusion? If the right hand shock sprang a leak half way round a right hand bend, would that be enough to send the bike into a frenzy? It doesn't seem likely. Could I have hit something in the road? Much more likely, and it would mean that the front wheel could have been in the air, with me pushing the left bar, so that when it landed, the road would have pushed the front wheel back the other way. Also, if the back wheel hit something, it too could have been off the deck further adding to the jollity. Maybe the shock absorber seal popped at the same time?

I hope you get to the bottom of what happened to you, Howard. On the bright side, nothing like that ever happened to me again.

Jenny.

[Galligaskins]

Hey John,

thanks for that link. I may well give that a try when I've had a closer look at the shocks. They're probably not the root cause of the tank slapper but they are nearly 50 years old so would likely benefit from some TLC.

Norbert, I think you're right. Once the tank slapper had been initiated, the rumble strips sustained it. Accelerating in that situation is very counter-intuitive though, although I did sit up - not in an attempt to stabilise the bike, I think it was more surprise and looking around to 'choose my spot' where I was going to hit Thankfully it didn't come to that.

[norbert]

I think, the most important thing in that situacion is not to break on the front wheel! Guess that was, what I tried with that GT250 and ended up on the ground inmediately. Being that surprised I supose that my intuicion said: Stop

Now that we know that this horrible situacion might be easier to handle

[Galligaskins]

Well, I popped the shocks off this afternoon while I've got no work on, and they're fine. Damping feels good over the whole range with no loss over small displacements. No sign of any leaks. I'm almost disappointed. They're scruffy and could do with a repaint, and I'd like to get the springs coated or replaced as the chrome is quite pitted but in terms of function I can't fault them.

[Parker3865]

One of the letters in this month's Bike magazine mentions a tank slapper on rumble strips approaching a roundabout. On a modern bike as well.