FF: Forks Modified Steering Stem

[Martyn Goodwin]

Some thoughts on Vincent Front Suspension. PART 2

For anyone interested I have constructed an EXCEL model that calculated the performance of up to 3 different front springs plus any combination of those springs. You enter the spring free length, the thickness of any pre load shim and the spring constant (in Lbs/in) for each of the 3 springs. If you know the weight required to achieve the desired 30% static sag point you can quickly see what spring you need. By playing around with values you can then experiment to get a required total force at full compression as well. Remember, the physical design of the Girdraulics is such that springs MUST be in the free length range of 14 to 17 inches. If you want a copy of the spring calculator send me a PM with your email address.


Now let’s look as some spring tables, assuming in all cases that the springs are linear on operation. That is the coil spacing is constant and the wire gauge is constant within any one spring.

Table 1: Original Vincent Springs

The next table was kindly provided by Davis Dunfey for the springs he developed a while back. These are shorter than the factory springs at a whisker over 14 inches and have almost but NOT zero pre load when installing them – which makes spring box fit/remove a doddle.

Table 2: David Dumfry Springs, no pre load spacer

So what happens to these Dunfey springs if we pre load them by putting a spacer into the bottom of the spring box, under the spring? The next 3 tables show the impact of adding a preload spacer on David’s springs, nothing else is changed.

Table 3: Davids springs with a 0.125” thick pre load spacer

Table 4: Davids springs with a 0.25” pre load spacer

Table 5: Davids springs with a 0.5 inch pre load spacer,

 

[Martyn Goodwin]

Some thoughts on Vincent Front Suspension. PART 3


Now let’s look at some springs that have been suggested recently. These springs have a lower spring rate but, with an initial free length of 16 .5 inches, when installed in the spring box, they have 2.5 inches of pre load. While we could increase the pre load further with spacers, there is no way of decreasing preload with these springs.

Table 6: Linear springs with 2 ½ inches pre-load built in – free length 16 ½ inches

Initially I had a set of 16 ½ long, 30 Lb/in springs installed in my bike. My experience was that the front end was topping out with the static sag happening at around 2 to 5% of suspension travel. On the road the suspension was bottoming out with ease. My conclusion is that I need a spring that was much stiffer to prevent bottoming out, but one with less preload so I could realise an acceptable static sag point, around 30% of travel.

My on-road testing of springs has shown me that to prevent bottoming out on any but the most horrid bumps I need a set of springs that provides for about 600 Lb of front end impact force at the limit of suspension travel (compression) . If I used a spring with 2 ½ inches of preload – that’s 16 ½ inches free length, then based on the information presented I would need something like a 55 to 60 Lb/in spring – but the problem with that (as can be seen in table 6) is then my static sag will be almost nil.

With my personal set up I currently have a Red/White set of David’s springs installed and there is static sag of around 30% which is close I want/need. This equates to an equilibrium force from the pair of springs of 153 Lb. Which is just about the downward force, less the unsprung weight of 50 Lb, the front wheel exerts when I am astride the bike. If I were to change these out for springs 16.5 inches long (thus having 2 1/5 inches of preload) to get the equilibrium point the same ( that’s 30% sag) I would need a pair of 24 Lb/in springs BUT at full compression these 24 Lb/in springs will only be providing for an impact load at the 11 inch mark of 264 Lb and they would bottom out with ease.

Using the spring calculator I mentioned earlier I believe the ideal springs for the front of my bike would be a pair of springs each with a spring constant of 90 Lbs/inch and a free length of 14.125”. If I was after a softer overall ride, while maintaining the same static sag, then the front springs would need to be changed to a pair with 75 Lb/inch, each 14.25” long. Even softer ride, still maintaining the 30% static sag point, would be a pair of 60 Lb/in springs, 14.5 inches long.

Remember, in all instances to achieve the desired spring free length you can start off with a 14 inch long spring and add a pre-load spacer to get to the required total free length.

I hope I have not added confusion to the mix. What I have attempted to do is illustrate that springs and shock absorbers perform totally different functions and that if you know just a few simple things about you bike, yourself and the spring options available it can be possible to select front springs that best suit you and your bike.

Once you have sorted your springs, only then can you start to set up your shock absorber, that is IF it has any adjustment capability!

All of this assumes you have done all you can to eliminate possible friction points in your front (Girdraulic) suspension.

 

[timetraveller]

When or where was it decided that one should have 30% of the movement taken up by the static load. Our bikes only have a potential movement of about three inches to start with which would leave us with two inches maximum left. Not disagreeing here, just asking.

 

[Chris Launders]

Hi Martyn
I disagree slightly with the sag point being at around 30%, this may apply to other vehicles but with the restricted travel available on a girdraulic fork this would leave less than 2" travel so I am using about 20%, this puts the links in the horizontal position as original.
Also short stiff springs and long soft springs can give the same sag point, but completely different suspension characteristics.

Chris.

 

[timetraveller]

I wrote the above without seeing the later two parts. I refer back to the suggestion of putting a cable tie around the front inner spring boxes and measuring the actual movement over rough roads.

 

[davidd]

When or where was it decided that one should have 30% of the movement taken up by the static load. Our bikes only have a potential movement of about three inches to start with which would leave us with two inches maximum left. Not disagreeing here, just asking.

Just to supply some reference, Paul Thede in his book Motorcycle Suspension Bible, states: "Our testing shows that the ideal sag is about 1/4 to 1/3 of the suspension's total travel." John Bradley in his book "The Racing Motorcycle" states, 25% (Paul Thede mentions 25% as the racing set up and 33% as the street set up.)

Laney Thornton suggested 50% in his first manual on his suspension, but later changed it to 30% in later printings.

I believe the logic behind these numbers is a concern that the fork should not "top off" during use. There seems to be a fear that if the fork tops off during riding that the front wheel will loose its ability to absorb bumps and that would lead to serious traction problems. Very small bikes (125cc and under can be set up with less sag, mostly because they are so light that the rider provides most of the sag.

David

 

[davidd]

Also short stiff springs and long soft springs can give the same sag point, but completely different suspension characteristics.

Chris.

This is true, but the free sag (just off the stand with no rider) will be different on both set ups. Typically a spring with no free sag is to soft and one with too much free sag is too stiff. It is a little counter intuitive, but the soft spring will have too much preload in order to support the bike and the stiff spring will have too little preload, making the spring feel soft. I believe it is best to have the preload adjustable by either shimming or limiting the spring so you can find the sweet spot.

David

 

[timetraveller]

Before we all get bogged down in theory can we wait for feedback from users? I know that the 33lb springs and 36 lb spring are now with Greg, who will have to send them on to other people. If two 33s do not do the job on a Comet then two 36s should do. Two 36s with 3" of preload are exerting a force of 206 lbs. Another half inch of movement moves that up to 252 lbs. A further inch of compression increases that to 324 lbs. Use the whole 3" of travel and one is up to a force of 432 lbs, just about the total weight of the bike without a rider. Now start off with 90 lbs/inch springs. In order to get the initial required force of 206 lbs there would have to be a compression of 1.14". (Remember there are two springs). Compress a further one inch and the force is now up to 360 lbs and with a further one inch the total force is up to 540 lbs. That seems like a pretty stiff front end to me but let us see what users find. If that is what is needed than all the springs cost about the same to be made so there is no cost implication, only a comfort one.

 

[greg brillus]

It is still quite surprising to me how few of these are actually on the road yet, and this ultimately is limiting the amount of feedback. The spring issue will sort itself out very soon once a bit more testing is carried out. People need to realize that the forks "Will definitely be more active than before" not in an excessive way but more than stock Girdraulics, that was the purpose of going to softer springs, with the new location of the lower link this becomes possible. Theory's about spring rates are only a guide, as changing the link bushes to sealed bearings will "Free up" the forks very much, as Chris discovered.

 

[vibrac]

The spring is coming to the north that will sort the springs