FF: Forks Re-made (by Maughans) original Vin front damper

[davidd]

Is there a definitive statement about how the C twin links should lie ? I'm sure I read somewhere that with the bike on its' feet, unladen, the upper should be pointing slightly down and the lower almost level. To achieve this my (Vincent) damper is fully extended , and that's with longer eye bolts. Loaded until the damper is nearly halfway down its travel, the upper link is pointing well up ("alarmingly up I think Paul Richardson said") and the lower is well down at the back. All the hardware is standard C twin.
I'm going to try D outer springs but I can't see the orientation of the links changing much from my current 15" outers with 1.1/4"packers. All very odd, looking at photo's in the various books, some are up, some down, some level, grey flashes with the upper link really pointing down, they must have had a very long damper !
Regards, Terry.

Terry,

On a stock Girdraulic and fork stem the best position for the links is parallel to the pavement with you sitting on the bike. They can be down a bit in the back also, but parallel is ideal. As I mentioned above, the stock bike works better when the fork is in the compression area and worse when it is nearing full extension. The stock C springs were 15" and had a helper spring inside that added 15 lbs., if I remember correctly. The D spring is 16", but uses no inner spring.

Spacers can be dangerous, if they push the spring up so high that your fork is nearer full extension. You might consider small spacers if your springs have sagged, but with new springs there should be no need for spacers.

On a stock bike keeping the lower link level to the ground will help with the handling and braking. If you change to a modified steering stem the link position is not as important because the geometry has changed.

David

 

[fogrider]

I had an original front shock on my bike. I had re worked it myself with a modern seal and also I had installed a check valve to reverse the flow as outlined in 40 years on. It worked OK but it was harsh over really sharp bumps. This last summer I installed an AVO front damper and the harshness is gone. The suspension is almost as good as a modern bike. I have the lower links sitting level when I am on the bike and I have a hydraulic steering damper. I have not had any bad behavior from the suspension or steering in the 8000 miles I have covered so far. I don't find the front fork ever bottoms. (the rear does) I think the standard set up with a modern shock and a steering damper very good for the touring rider.

I'm quite happy with the re-built damper, with just the outer springs and the packers the forks work well enough ,it's just that the range they work in has reduced trail and that's a recipe for tank slappers. To put the trail in the right range means having the damper nearly topped out even with longer eye bolts. Accuse me of heresy, but it seems clear to me now that the original design was flawed ( although brilliant at the time ) and that the new modified steering stem set up is how it all should have been in the first place.
I think I will just live with what I've got and a compromise on settings.
Regards all, Terry.

 

[fogrider]

I'm quite happy with the re-built damper, with just the outer springs and the packers the forks work well enough ,it's just that the range they work in has reduced trail and that's a recipe for tank slappers. To put the trail in the right range means having the damper nearly topped out even with longer eye bolts. Accuse me of heresy, but it seems clear to me now that the original design was flawed ( although brilliant at the time ) and that the new modified steering stem set up is how it all should have been in the first place.
I think I will just live with what I've got and a compromise on settings.
Regards all, Terry.

I should have added an important aspect of this topic: the forks 'locking' when you brake hard. Some clearly have found this can create nasty "offs". This is related to the lie of the (mainly) lower link. Set where EMG Stevens says in KTB, there is some compliance with the forks on braking, and around 4" of trail, the damper is about 1/3rd down, also fine. Pretty tank slapper proof. Have the lower link rear spindle down and a lot of trail is lost. Around 7 degrees up has been quoted on here but it reduces trail to around 3" which is less than many modern superbikes and a lot of damper travel used up, reducing bump absorbtion.
Lower link sloping down (upper level) gives big trail, some 6", but the damper is almost topped out and restricting rebound.
With standard C inners and outers, links where EMG says, is good , the springs are stiff and keep you in the best area. The D springs in mine gave an improvement to comfort, but moved the links down, reducing trail slightly. So far no slapper, I'll stick with this set up for now.

 

[timetraveller]

Sorry Mr fogrider, Sir, but some of that is not right. When one brakes really hard with the standard set up then the bike rises up over the front forks until the damper is fully extended and the wheel is as far back as it can go, giving about 5" of trail and the lower mudguard stay very near the mag cowl. There is no longer any movement in the front suspension and neither the trail nor the suspension movement can be recovered unless the brake is released. Gentle braking does not produce this effect which is why so many of us never realised just what could happen when braking hard going into a corner.
There are several other things wrong with what you have written but I have no wish to go through an item by item refutation in public. Rather, I would warn others that some of what you have written is incorrect and should not be taken as an indication of what really happens at the front end.

 

[fogrider]

Sorry Mr fogrider, Sir, but some of that is not right. When one brakes really hard with the standard set up then the bike rises up over the front forks until the damper is fully extended and the wheel is as far back as it can go, giving about 5" of trail and the lower mudguard stay very near the mag cowl. There is no longer any movement in the front suspension and neither the trail nor the suspension movement can be recovered unless the brake is released. Gentle braking does not produce this effect which is why so many of us never realised just what could happen when braking hard going into a corner.
There are several other things wrong with what you have written but I have no wish to go through an item by item refutation in public. Rather, I would warn others that some of what you have written is incorrect and should not be taken as an indication of what really happens at the front end.

I agree with you about the forks rising, this is one problem with the big variation in trail Girdraulics give. I find by having the lower link JUST lower at its rear end lets the forks work better when braking. It's the compromise position where, so far, trail and damper travel give me the best ride/brake effect/fork travel.
You're very polite, I expected much more flack than that, thank-you ! I continue to resolve a best set up with the original parts. I will add she's a Touring twin, not the best steerer and somewhat slower steering with that wide (and heavy) front tyre and mudguard, hence the need to be particular about trail.
Regards, Terry.

 

[davidd]

I know that there are others that feel that the trail is the key to the bad handling. I can only say that in none of my experiments on the road in inducing wobbles or seized forks under braking did trail have any affect at all. I conducted experiments with trail at maximum solo and half that figure. The bike does not care. Changing trail does not create the rather large mechanical forces needed to induce the front end to reach full extension or compression...weight transfer under braking does. Additionally, all telescopic forks have wildly changing trail figures. I could easily be convinced that trail is a factor, but to date I have not heard any hypothesis or experimentation that supports trail as the culprit.

David

 

[timetraveller]

It is also worth mentioning that when I had my speed wobble and crashed at about 100 mph while riding at Cadwell I was not braking. I was flat out on the top straight, laid down as far as I could. There was a diagonal joint in the tarmac about half way down the straight and the bike had twitched each time I had gone over it. The final time the twitch turned into a flat out tank slapper and there was nothing to do but let go of the handlebars. The only thing that was wrong was that the front damper had leaked and I don't pretend to know what the mechanism is that leads to a tank slapper in these circumstances.

 

[fogrider]

It is also worth mentioning that when I had my speed wobble and crashed at about 100 mph while riding at Cadwell I was not braking. I was flat out on the top straight, laid down as far as I could. There was a diagonal joint in the tarmac about half way down the straight and the bike had twitched each time I had gone over it. The final time the twitch turned into a flat out tank slapper and there was nothing to do but let go of the handlebars. The only thing that was wrong was that the front damper had leaked and I don't pretend to know what the mechanism is that leads to a tank slapper in these circumstances.

Dry roads this morning so I thought I would go and try her out with the lower link slightly down at the rear. Timetravellers note about the forks fully extending had me realize I've never really watched what the forks do on the road. A 40 mile blast around the B roads saw the Maughans/Vincent damper smoothly allowing the forks to nod up and down. I tried a sudden stop at 30mph: the forks appeared to rise 1/4" then went firm. I tried many times at speeds up to 70mph with the same result. It just will not top out. I had thought that with the lower link like that, there would be insufficient trail, but its' fine. I deliberately went over the white lines and cats eyes at 70mph to provoke it, she did'nt care.
I still think the new/modified steering stem is how the Girdraulics should have been in the first place but I'm now satisfied how she rides, it would be nice though to have the amount of travel CL's bike had when I looked at it at Bawtry, luxury !
Regards, Terry.

 

[davidd]

I have always said from way back that the geometry problem on the Vincent was not the cause of all of the speed wobbles. I hope we all know that motorcycles can be inherently prone to wobbles. Someone contacted me recently who was riding with one hand and went into a dip and the Vincent went down quick and hard. I explained the Girdraulic geometry problem to him, but ended by saying "You fell down because you were riding one handed into a dip and forced the front end hard to the left." However, I also said that I believe it is best to set up the fork to work as well as possible in all conditions.

I think the modified steering stem goes a long way to making the Vincent safer in most conditions. It certainly improves the braking. There will be a point where someone will have a bad wobble and "get off" on their Vincent with a new steering stem. I am afraid that I won't be surprised, because you cannot take the risk of a wobble out of motorcycling. Making a wobble low risk does not mean no risk.

David

 

[greg brillus]

To make matters worse, most people can fall off their Vincent without the bike even moving...................