Comet suspension

[davidd]

"The problem that halted my pursuit of the idea is this.
The top link comes close to the damper at full droop, so the coaxial spring top platform (perch?) would have to be placed lower down, limiting the length of the spring even more. I assumed the resultant short spring could not provide a full travel of 3 inches without being greatly overstressed, and therefore travel would have to be limited.
As my main interest is road use with maximum comfort and safety, this ruled the idea out for me. I did not look at stress issues in the bottom link and top damper mounting.

However, you may be able to find ways of creating more clearance and my guess at spring stresses and travel may be plain wrong.
The other thing is that from my limited experience of preparing and racing cars, I am aware that for track use reduced suspension travel can be perfectly acceptable.
When I have completed my model and I know the wheel spring rate with the springs in the existing location, I should be able to calculate the spring rate necessary at the damper to give similar results, if that is any help."

I just wanted to mention a few things on the coil-over idea. The above is a quote from Hadronuk for reference p.24. Several owners have used these systems and I would agree that the top link has a clearance issue as it goes to full extension. Two possible solutions: first, as Bill says, maybe this is a good place to have the travel limited by the damper thus creating a little clearance. Two, the Works Performance (Thornton) damper is an upside down damper, i.e., the body is at the top as opposed to the rod at the top, so I think that a collar and platform could be fashioned to give the upper link good clearance.

I see the spring as being short, but not unusually short, with a large gauge and few coils so coil bind is not an issue. I think the travel could be as long as stock, but shortening the extension may be worth shortening overall travel. You are the first to mention the ratio between the damper and the axle travel. Also the first to note the ratio varies with travel. It is something I knew of, but did not quantify it.

David

 

[davidd]

I suspect that we all now agree that the standard set up on a Vincent leads the bike to try to ride up over the forks during front wheel braking. This leads to maximum trail which should give the maximum 'castor action'. Something can then happen to cause the bike to go into a speed wobble but it does not do it every time one brakes heavily. So to me, the question is what is going on when there should be the maximum effect to try to get the front wheel to self centre that overcomes this. Is it a bumpy track or road which is the causative agent, working against a suspension which cannot respond because the braking effort is overcoming the attempt of the front forks to go down and hence the front wheel to move forwards against the braking forces? I'm pretty sure that there is still something in all this that I am not understanding. I think that I understand what I have written above, and if it is incorrect then let us all know, but what is causing the oscillation. Lack of suspension movement I understand, trail going to an overly large value I understand. The cause of the start of the oscillations escapes me at the moment. I've been there, done it and have the scars but still am left wondering. Help!

Slightly going off track on this. It would be possible with modern machining methods to make stainless steel claws which would screw into each end of the front springs, rather like the rear suspension on 'B's and 'C's. Given that these would also carry the bottom fork or top eye to fit the springs and their boxes to the bike then it could be organised so that the springs were able to give a restorative force as can happen on Bramptons, when the springs try to become over extended. Davidd's system would make this redundant but is 3" of movement enough on our present roads?

TT,

I think the answer may be above my knowledge, but I would tend to say "any un-weighting of the front end can cause a wobble". The road profile can cause either or both the unweighting or deflection of the wheel from center. Also remember that the fork seizure takes the springs and damper out of the equation, so whatever effect these other components could have on self-correction are removed. In my case the fork seizing cause a "hobby horse" motion that kept lifting the front wheel further and further off the ground until, I assume, the front wheel, like the shopping cart wheel touches down when it is near its maximum position to the left or right. Then the bike goes down. In the case of Glyn Johnson on his Flash at Montlhery, he crossed a span of concrete that was a repair and it was slightly lower than the rest of the track. Crossing off the repair unweighted the bike and the wobble began. So, I would say the oscillations begin mostly as a result of un-weighting of the front end, which is normally induced by road profile, but can, in the case of a Vincent, be induced by heavy front braking also.

I think cleats would work. I had thought of designing the spring instead of closed and ground, left open. You could then turn an aluminum plug with a thread to screw into the spring and pin both ends through the spring boxes. The spring would have to be designed noting that there are more inactive coils where the plug is inserted. I think Tim may be correct that 3" is fine for racing. One of the things that surprised me with the short springs is that when the road is bumpy you can be thrown into that extension area momentarily with little danger, I think. So far, with no pre load, the weight of the bike itself causes the bike to drop back to normal even when applying the brakes.

David

 

[Big Sid]

Sid here . The term anti dive as I see it , but perhaps misnamed . Became aware that the front fork on my Rapide would not depress when the bars were pushed downwards If the front brake was held on , but release the brake and they did . This was back in the early 50s . Observed that this occurred as the front wheel tried to roll backwards a tiny amount and felt that it was this that drove the effect .
Call it what you will but those were my observations at the moment some 60 years ago . You guys wernt around to correct me .
Bout time you did , been waiting a long time ! Sid .

 

[b'knighted]

Slightly going off track on this. It would be possible with modern machining methods to make stainless steel claws which would screw into each end of the front springs, rather like the rear suspension on 'B's and 'C's. Given that these would also carry the bottom fork or top eye to fit the springs and their boxes to the bike then it could be organised so that the springs were able to give a restorative force as can happen on Bramptons, when the springs try to become over extended. Davidd's system would make this redundant but is 3" of movement enough on our present roads?

Could the same retention be acheived by a pin through each spring box passing through the spring close to its ends. A short stiffening outer tube pinned to each spring box end may be needed to ensure that the thin tube doesn't tear.

 

[Big Sid]

Sid . More . I've always believed that the original choice of Avon front rib tires helped to reduce the excited castor induced wobble by virtue of its soft ribs natural tendency to return the front wheel to a forward path if momentarily deflected . This works best if the tire is not inflated too hard . Sid .

 

[Big Sid]

Sid . Certainly when the front wheel is deflected to one side the inbuilt castor swings it back towards center and here the energy forces it too far to that other side whereupon the action is repeated , then again if the energy is not depleted by a damper . The weight on the wheel driving the rogue effect . And again all British machines having a good turn of speed had ribbed tires , these acted to damp out the wobble . Note that our Vincents carried a greater proportion of weight on the front tire patch , it's motor carried closer to the wheel and weighing near 200 pounds itself . Sid .

 

[Bill Thomas]

Hello T.T. I have always said the trail is less at the top AND the bottom, I think this what I found, But like John, It was a long time ago !!. Any body know what the BMWs are like, The testers say they are strange, But they seem to work.

 

[greg brillus]

Interesting some of you talking about attaching the ends of the springs to the spring box's and so on. On the Bramptons when the forks are at full extension, the spring has no compression on it, only a bending force as the angular change of the forks swings forward and down away from the upper stem attachment point. So when the front of the bike is jacked off the ground, the forks are hanging down with no apparent upward pull from the spring ( or this restorative action as TT describes) So if you did install "Claws" at the ends of the front springs on the Girdraulics to hold each end, I can't see any point as the downward full extension is limited by the shocker's fully extended length......unless.....you set it up as per the rear end where the shock absorber does not limit total spring travel.....say when the rear of the bike is off the ground and supported under the back of the engine, where you can grab the lifting handle and compress and extend the back suspension without the rear shocker bottoming out as per the front....if jacked up in the same way.....is this what you mean..?? Would this not make the front springs too short......Cheers....Greg.

 

[Howard]

Just to add a bit of variety to this thread, this isn't just a girdraulic phenomenon, the scribbling below is from a modern Triumph forum. I think there are two topics - how do tank slappers happen and why are girdraulics more prone to them.

This also also states what I've long believed (and been castigated for saying previosly) that the best way of dealing with a tankslapper is to relax and let the castor straighten the bike like a supermarket trolley (maybe that's not the right analogy ) and before anyone says it, no I'm not brave enough to try it!


1. The “tankslapper” is a very frightening experience. Usually occuring when accelerating hard over bumpy pavement, a tankslapper ensues when the front tyre becomes airborne, then regains traction outside the rear tyre’s alignment. The resulting deflection bounces the tyre off to one side, followed by another bounce in the opposite direction as it contacts the pavement again. Unless the bike’s steering geometry is able to damp out the deflections quickly, the resulting oscillations from the front tyre as it bounces back and forth will swiftly gain in strength, causing the bars to swap from side to side with increasing ferocity. The oscillations can be violent enough to rip the bars out of your hands, and fling your feet off the pegs. You can guess what happens next.

2. The easy cure for this problem is a steering damper. Many sports bikes now come stock with one, as the radical steering geometry needed for quick handling can otherwise cause some instability in certain situations. While a steering damper is an easy fix, it shouldn’t be a cure-all; if you’re forced to adjust the steering damper’s stiffness (if available) until you can barely turn the bars in order to keep the bike’s handling stable, there is a problem somewhere in your chassis setup. A too-stiff steering damper can also cause handling problems by itself; if your steering damper is adjustable, and you find that your bike won’t hold a line (especially in slower corners), or gets into a small wobble or oscillation in high speed corners, try backing off the stiffness a little and see if it helps.

3. Not all sports bikes need a steering damper, however. Many have steering geometry setups that offer quick handling, while still providing the necessary stability to damp out any front-end oscillations. In most cases, one of the biggest contributors to a tankslapper is your body positioning and grip on the bars. Some people ride in a more upright position when carving corners, but when accelerating over bumpy pavement, that upright body position puts even more weight transfer to the rear, which causes the front end to get lighter. Also, the more upright torso means that your grip on the bars is tighter in order to stabilize your upper body. That firmer grip feeds more input into the front end, something it doesn’t need while it’s busy trying to damp out the inputs from the bouncing front tyre. It actually forms a vicious circle: you grip the bars tighter because they’re starting to flap back and forth, but that only feeds more input into the front end, compounding the problem further.

4. The easiest way to avoid tankslappers while accelerating over bumpy pavement is to—believe it or not—keep a relaxed grip on the bars. Relaxing your grip on the bars means you must lean forward in order to assist in keeping your torso stabilized. This helps put more weight on the front end, which keeps the front tyre on the pavement. Since you’re not using your arms to stabilize your upper body, get your weight onto the footpegs so that you can get your body as far forward as possible; this also allows you to grip the tank with your knees for more stability.

If you do get into a tankslapper, keep your weight forward and—as hard as this sounds—maintain a relaxed grip on the bars. Let the motorcycle’s chassis deal with damping out the oscillations. Don’t try to be a human steering damper; you’ll only make the problem worse. Tankslappers can definitely soil your undies; but if you’re able to deal with them correctly, you’ll usually ride through them before you know it.

 

[vibrac]

If you do get into a tankslapper, keep your weight forward and—as hard as this sounds—maintain a relaxed grip on the bars. Let the motorcycle’s chassis deal with damping out the oscillations. Don’t try to be a human steering damper; you’ll only make the problem worse. Tankslappers can definitely soil your undies; but if you’re able to deal with them correctly, you’ll usually ride through them before you know it.

Bears out what Ron said in the 60's I recounted in post #57