The Passenger rides on a rigid frame
- Thread starter Monkeypants
- Start date
So can anyone explain why for sixty years we have been putting heavy rear springs in for the extra load of a passenger when in fact the passenger does not ride on the spring? They ride on those cute little solid seat stays, unless I have the wrong end of the stick, which is possible. That did happen once in 1979 or ....maybe it was 82....
The drawing gives ratio of movement, that is a different consideration, I believe. Nothing in the drawing above shows that weight from the passenger on the seat is transferred to the springs and damper. Give it a try, it is easy to do, just try to make the spring bounce from the passenger position. At 263 lbs with gear on, I cannot with my standard Rapide.
Correction, tried again with and there is tiny movement in the springs with a full on bounce from a 263 lb passenger, perhaps 1/8" max. From the rider position the same bounce nearly bottoms the springs , over ten times the movement.
Glen
The drawing gives ratio of movement, that is a different consideration, I believe. Nothing in the drawing above shows that weight from the passenger on the seat is transferred to the springs and damper. Give it a try, it is easy to do, just try to make the spring bounce from the passenger position. At 263 lbs with gear on, I cannot with my standard Rapide.
Correction, tried again with and there is tiny movement in the springs with a full on bounce from a 263 lb passenger, perhaps 1/8" max. From the rider position the same bounce nearly bottoms the springs , over ten times the movement.
Glen
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I've readseveral articles in mph both old and new recommending the use of heavier springs with passenger load and standard seat setup. Ed Stevens has a table that shows the ideal springs for solo, two up, sidecar, etc in chapter 22 of "Know Thy Beast"
So I do think the common practice has been to go a heavier spring for two up riding.
Glen
So I do think the common practice has been to go a heavier spring for two up riding.
Glen
Why stiffer springs are recommended for riding two up.
My guess is it’s the legacy of the B series rear spring specifications being carried over to the C series.
(Probably best to stop reading here unless you need help to get off to sleep.)
In the absence of a hydraulic damper, the B springs had to be short and stiff, so that as the suspension extended they went into tension and limited how far the wheel dropped.
When the C series came along, the hydraulic damper now limited the extension of the suspension, but worsens the thump from the bump. The opportunity to use longer, softer springs was not taken, presumably for cost reasons, or perhaps because leg length was still rationed.
The damper bottoms out before the springs, which previously served as the bump stop. This further reduces the already very limited available “bump travel”. (That is to say the distance from the static ride position to the point where the damper bottoms out.) The suspension may in theory have about 4.5 inches of travel available at the wheel, but in practice only the last half gets used, unless you go for a long flight off a humped back bridge.
As shown in Fig A above, if you remove the springs from a standard bike, then move the suspension through its full range of travel, you will see that the back of the seat rises by appreciably less than the wheel. This directly demonstrates that a proportion of the passenger’s weight is carried by the springs.
With so little bump travel available, even this moderated increase in load on the springs causes the damper to bottom out. Hence if you stick with the original B specified length for the springs, they have to be stiffer..
My guess is it’s the legacy of the B series rear spring specifications being carried over to the C series.
(Probably best to stop reading here unless you need help to get off to sleep.)
In the absence of a hydraulic damper, the B springs had to be short and stiff, so that as the suspension extended they went into tension and limited how far the wheel dropped.
When the C series came along, the hydraulic damper now limited the extension of the suspension, but worsens the thump from the bump. The opportunity to use longer, softer springs was not taken, presumably for cost reasons, or perhaps because leg length was still rationed.
The damper bottoms out before the springs, which previously served as the bump stop. This further reduces the already very limited available “bump travel”. (That is to say the distance from the static ride position to the point where the damper bottoms out.) The suspension may in theory have about 4.5 inches of travel available at the wheel, but in practice only the last half gets used, unless you go for a long flight off a humped back bridge.
As shown in Fig A above, if you remove the springs from a standard bike, then move the suspension through its full range of travel, you will see that the back of the seat rises by appreciably less than the wheel. This directly demonstrates that a proportion of the passenger’s weight is carried by the springs.
With so little bump travel available, even this moderated increase in load on the springs causes the damper to bottom out. Hence if you stick with the original B specified length for the springs, they have to be stiffer..
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Interesting thoughts hadronuk, and I didn't get at all sleepy. If I have some time for it today, I will measure just exactly how much suspension movement there is with 150 lbs passenger weight added to the rear of the seat.
In the meantime, I have had a chance to do some comparisons between the two Rapides here and the drawing above. The first difference noted is the mudguard to seat clearance requirement for fully suspended and as built situations. The drawing shows the seat at a much higher elevation for full suspension.
In fact both my fully suspended bike and the stock bike have the identical seat to mudguard clearance, 3 1/4" with the suspension fully elongated, on centre stand with rear wheel hanging.
It was mentioned to me some time ago that the main drawback of the strut conversion is the high seat requirement. In fact the seat height of the two machines is identical.
One other thought on the drawing is that the method of full suspension shown might not work all that well. With the seat still connected to the moving rfm top member as shown in the drawing, the effect would be quite different from that which occurs with a one piece strut connecting the rear of the seat directly to the rfm pivot bolt.
If a "full suspension " system like the one in the drawing was used, it could be very difficult to get adequate seat to mudguard clearance. It appears with the stays angle forward to that degree and connected to the rfm, the rear of the seat would dip down on bumps rather than bouncing up as it does with the stock setup. The ratio of movement for the rear of the seat to the anchor point would be quite great and in the wrong direction to maintain clearance.
I wonder if this type of seat stay was ever utilized?
With the long struts to the fixed pivot bolt, the seat stays more or less level on bumps.
Glen
In the meantime, I have had a chance to do some comparisons between the two Rapides here and the drawing above. The first difference noted is the mudguard to seat clearance requirement for fully suspended and as built situations. The drawing shows the seat at a much higher elevation for full suspension.
In fact both my fully suspended bike and the stock bike have the identical seat to mudguard clearance, 3 1/4" with the suspension fully elongated, on centre stand with rear wheel hanging.
It was mentioned to me some time ago that the main drawback of the strut conversion is the high seat requirement. In fact the seat height of the two machines is identical.
One other thought on the drawing is that the method of full suspension shown might not work all that well. With the seat still connected to the moving rfm top member as shown in the drawing, the effect would be quite different from that which occurs with a one piece strut connecting the rear of the seat directly to the rfm pivot bolt.
If a "full suspension " system like the one in the drawing was used, it could be very difficult to get adequate seat to mudguard clearance. It appears with the stays angle forward to that degree and connected to the rfm, the rear of the seat would dip down on bumps rather than bouncing up as it does with the stock setup. The ratio of movement for the rear of the seat to the anchor point would be quite great and in the wrong direction to maintain clearance.
I wonder if this type of seat stay was ever utilized?
With the long struts to the fixed pivot bolt, the seat stays more or less level on bumps.
Glen
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I have followed this thread with interest if not complete understanding. It is interesting to compare it with PEI's comments regarding spring frames in pages 2/3 Forty Years on. I have always understood that the obsolete original soft springs fitted to Comets were discontinued owing to their inability to cope with a pillion passenger. If so it suggests that carrying a pillion does have some effect on the springing. Empirically I have always found that the Comet, at least with standard Rapide springs, handles better with two up.
Hugo, I have heard other riders say that their standard type b or c handles better and rides better with a passenger on board. As far as the ride improvement goes, I wonder if the improvement is due the greater flex that occurs in the rear tire with increased load?
As far as the suspension goes, I can't see it helping much there, although I believe a small amount of the passenger's weight is transferred via the seat frame to the forward seat mount and pivot point. This weight does go onto the springs. This only occurs because the centre of the passengers weight is somewhere a bit forward of directly over the stays.
I did some measuring today. With the approximate seat weight of my passenger added ( a difficult number to get an exact fix on!) directly over the seat stays the suspension did not compress a measureable amount. With the weight moved forward to the approximate centre of where the passenger's weight is, the springs compressed 1/32" of an inch, so almost nothing.
As far as the suspension goes, I can't see it helping much there, although I believe a small amount of the passenger's weight is transferred via the seat frame to the forward seat mount and pivot point. This weight does go onto the springs. This only occurs because the centre of the passengers weight is somewhere a bit forward of directly over the stays.
I did some measuring today. With the approximate seat weight of my passenger added ( a difficult number to get an exact fix on!) directly over the seat stays the suspension did not compress a measureable amount. With the weight moved forward to the approximate centre of where the passenger's weight is, the springs compressed 1/32" of an inch, so almost nothing.
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Glen,
that’s a good way of adding a point load. I am surprised the movement is so low.
Two thoughts.
1) Rider and passenger have their legs forward, and usually lean forward, so their centre of gravity may be further forward.
2) Looking at the geometry, I think that as the suspension compresses, a greater proportion of the passenger’s weight is transferred to the springs, because of the change in angle of the struts. So if I am right, adding a passenger to a bike + rider will have a greater effect than adding a passenger to an empty bike.
There’s nothing like directly testing a theory, I would have a go myself, but I have a fully sprung seat. Also my front springs are off at present while I do measurements for my degressive springing project, currently undergoing minor improvements as a result of testing.
Rob
that’s a good way of adding a point load. I am surprised the movement is so low.
Two thoughts.
1) Rider and passenger have their legs forward, and usually lean forward, so their centre of gravity may be further forward.
2) Looking at the geometry, I think that as the suspension compresses, a greater proportion of the passenger’s weight is transferred to the springs, because of the change in angle of the struts. So if I am right, adding a passenger to a bike + rider will have a greater effect than adding a passenger to an empty bike.
There’s nothing like directly testing a theory, I would have a go myself, but I have a fully sprung seat. Also my front springs are off at present while I do measurements for my degressive springing project, currently undergoing minor improvements as a result of testing.
Rob
Dear Rob,
Looking at the drawing "A" in the first post of this thread, the point where the seat strut is attached to the RFM does look to move forward in relation to the point where the RFM pivots. It may take some of your fine calculating skills to confirm if triangulation confirms that the actual forces are changed.
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?
Glen must be fit if he is easily able to lift those weights around.;-)
Looking at the drawing "A" in the first post of this thread, the point where the seat strut is attached to the RFM does look to move forward in relation to the point where the RFM pivots. It may take some of your fine calculating skills to confirm if triangulation confirms that the actual forces are changed.
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?
Glen must be fit if he is easily able to lift those weights around.;-)