The Passenger rides on a rigid frame

[Monkeypants]

A portion of the passenger's overall weight is transferred to the fully sprung part of the bike thru the rear footrests. This amount is more than I guessed it would be-44 lbs for me as a passenger or about 28 pounds with my wife as passenger. This loading makes the rear suspension more active which will improve comfort for the rider on a rough road. Unfortunately, it makes things worse for the passenger, at least for the passenger's tailbone. The more active the rear suspension is , the bigger the jolt received by the passenger as their part of the seat hammers solidly into their backside. In the same way it is quite likely that stiffer rear springs, while giving less comfort to the rider, will quiet things down for the passenger. I can see now that this effect (less give in the rear suspension with heavy springs) is actually an improvement for the passenger.
The difference in ride between the front part of the seat and the rear is night and day.

 

[hadronuk]

..... In the same way it is quite likely that stiffer rear springs, while giving less comfort to the rider, will quiet things down for the passenger. I can see now that this effect (less give in the rear suspension with heavy springs) is actually an improvement for the passenger.......

Glen,
I think you are right. As I decided fairly quickly to stick with fully sprung, I haven't thought about the original set up so much.
Also as you said earlier, the tyre absorbs more with a passenger, because whilst a low unsprung weight is the Holy Grail for roadholding, a higher unsprung weight is actually good for comfort, as it forces the tyre to absorb more road irregularities.

.....If this is the case would this be "falling rate" suspension rather than the "rising rate" suspension that is found on such bikes as my Ducati, etc?......;-)

Eddy,
A bit of lateral thinking! I was only thinking about comfort and spring rates, but now you mention it, I suppose if the proportion of passenger weight carried by the springs rises as the suspension compresses, that is effectively the same as a falling spring rate. I think it is only a small effect though.
Incidentally, I think I read somewhere that modern Ducatis have "top out springs" in the forks, so you might have degressive at the front and progressive at the rear!

 

[Monkeypants]

From riding and comparing the two bikes I have here, it seems that the stock setup provides good comfort to a solo rider. For solo riders or riders who rarely carry a passenger, there is no great advantage in converting to full suspension, although there is some. If you really prefer the look of the original seat stays, then it likely isn't worth making the change.
For someone who often carries a passenger and luggage great distances over less than perfect road surfaces, the improvement in passenger comfort with the full suspension setup is huge.
If the bike is used in this way then the difference in appearance shouldn't be much of a factor. The fact that the orginal seat stays have been removed and replaced with struts is barely noticable because the area where the seat stays sit is covered by the panniers.
The sporty look is gone with the luggage added. With the bike setup this way, it is ideal for comfortable two up riding over large distances and also offers a good amount luggage space. I suppose that makes it attractive in it's own right.

Glen

 

[ray vinmad]

For an explanation of the standard B/C setup, see the opening paragraphs of Know Thy Beast, Chapter 3.
The theory is, as the suspension compresses, the pivot of the lower seat stay rotates forwards & thus the rear of the seat lowers.... Unfortunately, the theory is much more comfortable than reality!

Ray

 

[Monkeypants]

It would appear that Eddie Stevens never rode pillion on a Vincent!
The "A"drawing provided by A-BC at the top of the second page of this thread looks to be correct. It shows the rear of the seat moving upwards as the suspension is compressed.

on edit- I've just reread that section of "Know Thy Beast" and I don't see where he is stating that the rear of the seat goes down.
Here is the excerpt:
"The makers quite correctly pointed out that the addition of the passenger's weight has only a minor effect on the main springs and that consequently, there is no need to change springs to accomodate varying weights of crew, within limits. But the fact is that the seat mounting system is so designed that 3"of movement in the rear frame(ie the wheel) imparts 11/2" movement to the seat at the extreme rear end. In other words, the passengers rear end, poised on the thinnest part of the seat squab, is only half sprung. No wonder some of them complain.....Some cries of anguish must have reached the factory for the powers that were eventually reacted by fitting a fully sprung seat on SeriesD models."

The only thing I would disagree with in the statement above is the mention that the seat at rear is "half sprung". This seems to be a common misconception tied to the fact that there is a mechanical 2 to1 ratio of movement between the rear axle and the rear of the seat. This is not in any way the same as having the seat rear riding on suspension, partially or otherwise.The quote from the manufacturers says it all " the passenger's weight has only a minor effect on the main spring"

 

[hadronuk]

It would appear that Eddie Stevens never rode pillion on a Vincent!
The "A"drawing provided by A-BC at the top of the second page of this thread looks to be correct. It shows the rear of the seat moving upwards as the suspension is compressed.

Glen

I can't make sense of the quoted amount of suspension travel in that drawing. With a standard Vincent damper having a stroke of about 3" and the C series lever ratio being 1.5 to 1, that gives 4.5" of travel, which is close to what I measure on my bike. I know B series have a lower lever ratio, (1.4?) but not enough to reduce travel to 3.5"!

Then near the end of the article it says 6" of travel. Impossible on a standard B or C.

I suppose the "3.5 inches of travel" could refer to bump travel from the static ride position, in which case the figure seems very high for a bike with standard length springs, unless the rider was near weightless.

From memory (dangerous!) the standard spring length is 6.75". It is solid at about 4.7", but the standard damper bottoms out a bit before that. (Other dampers bottom a lot before that.) So the total available spring compression is 2" at most, which is 3.0" at the wheel of a C.
But that allows nothing for the weight of the bike or rider.

 

[Monkeypants]

Rob, 4.5" is also the max travel which I measured here on these two Rapides. That was measuring from the fully extended wheel hanging position to fully compressed with damper against stop. As you say, add in the weight of bike with rider and the effective travel is quite a lot lower, at least from the resting postion to full compression. On a really big whoopdeewoo where the rear wheel leaves the ground then crashes back down , the full 4.5 inches could come into play.

Bikes that have tired springs don't have much travel space at all from rest to full compression. You often see Vincents with only an inch or so of inner spring box showing at rest. These bikes must bottom out very easily, epsecially if a large rider and fully suspended luggage system is added to the mix. If the fully suspended passenger is added as well, then a spring change would be a must.
Glen

 

[davidd]

This stop animation video may help. The scales move a bit, but it is useful in seeing the motion of the seat. The video was done by Laney Thornton. David http://www.youtube.com/watch?v=sqQEbOL8Fak&feature=youtu.be

 

[hadronuk]

This stop animation video may help. The scales move a bit, but it is useful in seeing the motion of the seat. The video was done by Laney Thornton. David http://www.youtube.com/watch?v=sqQEbOL8Fak&feature=youtu.be

A (moving) picture is worth a thousand words, (and a lot of maths).

Well done Laney Thornton and David.

If you look closely at the motion of the front wheel spindle, you can see that initially, it is moving forward so much that it compresses the springs very little.

At approaching full bump, it is moving back towards the springs, as well as up, compressing the springs more quickly. This is why the effective spring rate at the wheel is non linear.

 

[Monkeypants]

You can see why the passenger's bum gets such a paddling on a rough road . Bad passenger!!;-)
Looks like about 4" total movement in both the front and rear.When fiddling about with measurements before building the Glenli rfm, I measured 4.5 inches non progressive movement on the rear of my Rapide. Perhaps the Thornton hits the stops a little bit earlier.
The relationship between spring movement and axle movement in the front end is ideal, to my way of thinking. As Rob mentions, there is progression in the movement, which gives the front end the ability to soak up the millions of small irregularities like a softly sprung setup while retaining the ability to handle a big jarring bump like a firmly sprung setup.

Glen