Centre of flywheels and lateral centre of weight for a twin engine.

[Monkeypants]

This GSXR spring shows 31.5 pounds per .10 compression and was linear over the 1" I compressed it.

The shock unit has a total travel of 2" . Working travel would be around 1.5- 1.7 inches with spring preload. Looks like an overall motion ratio of approximately 3 to one is needed for this unit to work properly and give about 4.5 to 5" travel.

 

[BigEd]

Frame Building Website

My friend Tony has been following this thread with great interest. He has built his own "V" twin engine that he has squeezed into a long suffering Featherbed frame. He has put a few thousand miles on over the last year or so to find the bugs.
His second engine will incorporate several improvements and he is going to build his own Egli-style frame. This will make installation and the run of intakes and exhausts much simpler.
He has found a website that has information that may give a good starting point for anyone contemplating building their own frame. The link is below:

http://www.denoonsp.com/serv011.htm

 

[Monkeypants]

I've been ruminating over rear suspension and have a few thoughts, please feel free to comment or correct.
One thought is that it is in many ways much simpler to restore a basketcase bike to original specifications than it is to come up with a special. At least with the basketcase you generally have all of the big parts and you know what the finished product is supposed to be. If new parts are readily available as they are for a Vincent, Norton, BSA etc. this makes it even easier.

On the other hand there is no limitation with a special, you can use almost any component and design it just about any way you like. The downside to that is it may turn out to be a complete POS!

The GSXR or Suzuki SV type rear suspension is interesting because it is a modern progressive linkage system that is known to work well. The problem with it is that is uses a longish swingarm mounted onto a fairly short motor. On top of that, it requires the creation of strong mounting points (for the rocker and for the top shock mount) where there is nothing on the Vincent engine or on an Egli style frame.
The mounting points could be created, but would require a lot of fabrication and some additional weight.
I think I have ruled out the use of that style of rear suspension.

I'm back to using something similar to what Terry Prince uses on a monoshock frame or , perhaps even better, the setup that Horners used on the Daytona OZ flag bike.
If the shock is mounted in such a way that the bottom connection to the swing arm comes into line with the travel of the upper arm of the swing arm near full compression, then there will be progression built into the setup. I think this is what Horners have done with their Flag bike. Some of their earlier bikes used geometry straight from a D Vincent, which, if I'm not mistaken, actually progresses the wrong way on full compression, that is, the rear wheel gains leverage on the suspension as compression increases.

The simplest thing might be to fab up a mock swing arm. A single piece of scrap steel tubing of almost any dimension will work as the main swing arm beam. I can drill a hole in it and let it pivot off the Vincent swing arm bolt and weld an upright which can have the Suzuki shock (spring removed) attached so that the full amount of shock travel can be viewed, rear wheel travel can be measured etc.
It will be easy enough to move the dimensions around until I get the desired geometry.

 

[Howard]

Can you get different springs for the shock? If not, you may also have to move the shock to a position where the spring strength required and movement is the same as the Suzuki.

I think this is another justification of your ruminatings. Take a Norton, BSA etc or even a twin shock Egli, and you're spoilt for choice on springs and dampers - I've never tried to get alternative springs for a Jap bike.

H

 

[bmetcalf]

At http://old.racetech.com/evalving/Sp...R750&formuse=form1&SpringType=Fork&bikeid=466

I found this:

Part NumberDescriptionPrice Select to Buy
SRSP S6319075 RT SK SPR 63.5x62.6x195 7.5 kg$109.99
SRSP S6319080 RT SK SPR 63.5x62.6x195 8.0kg $109.99
SRSP S6319085 RT SK SPR 63.5x62.6x195 8.5kg $109.99
SRSP S6319090 RT SK SPR 63.5x62.6x195 9.0kg $109.99
SRSP S6319095 RT SK SPR 63.5x62.6x195 9.5kg $109.99
SRSP S6319100 RT SK SPR 63.5x62.6x195 10.0kg$109.99
SRSP S6319105 RT SK SPR 63.5x62.6x195 10.5kg$109.99

 

[Monkeypants]

Talked to Murray Neibel, Suzuki dealer and Vincent owner yesterday. He has quite a collection of springs for me if needed. He also suggested belt sanding the existing spring if it is too stiff. He has done this in the past with good results. As he suggested, if you want the spring made 5% softer then remove 5% of the metal by letting the spring roll around on a large belt sander. This puts a flat on the outside of the coils as the material is removed.

I've got a good idea of what the trial RFM will look like. I'll make all of the variables variable, if that makes sense.
For example ,swing arm length is not a variable, it should be same as standard Vincent or Egli to maintain a 56" wheelbase.
Once I get the numbers, I'll run them by Timetraveller who has offered to use his CAD program for this.

 

[ET43]

A few years ago there was an article by Royce Creasey in Classic Mechanics magazine of which I have a copy. I could possibly scan it and send it to interested parties. Likewise, in 1978 S&W produced a suspension engineering handbook written by a chap called Tim Witham, and this goes into the subject in depth. Somebody on your side of the pond must have these articles, and I have to say that they served me well when building the outfit. I can be contacted via the Wylye Valley Section e-mail address. Cheers, ET43

 

[timetraveller]

Lowinger, you are a hard task master. Not only measurement but also graphs eh! Well here you are. The springs were all used and I cannot guarantee that they are as new. The mill only has a vertical scale with millimetres and a pointer, i.e. no vernier or digital read out so distance measurements could easily be out by one or two tenths of a millimetre. The scales did not like to go above 230 lbs so that is why the end measurements tend to be at different spacing from the middle ones.

I measured three springs, Series ‘D’ solo, Series ‘C’ solo and Petteford solo. The graphs will be at the bottom of the page. Gradients are 270 lbs/inch for the ‘D’, 202 lbs/inch for the ‘C’ and 169 lbs/inch for the Petteford. Note that these measurements are for single springs so with a ‘B’ or ‘C’ set up with two springs the relevant spring rates, rounded off, would be ‘D’: 270. ‘C’: 400 and Petteford : 340. I have no idea how these figures compare with what was originally intended but it does explain why the ex-Cecil Mills ‘C’ solo that I ride with a fully sprung rear end is not as comfortable as my ‘D’

 

[Tom Gaynor]

Did you happen to do front springs?

 

[timetraveller]

Hi Tom, my own milling machine is only one size up from a model maker's size and it was a bit of a struggle to get the 'D' spring in there. I do have old front springs about here and could determine their rate in a different way but would struggle to make a graph. If you really need the data then let me know and I will see if I can scrounge the use of a larger mill. However if you want the values for different types of front springs then I probably only have access to old standard 'C' and some 'D's I do not have Thornton springs. One thing to note from the graphs is the discussion which was taking place re varying spring rates with the 'barrel' shape of 'C' and Pettiford springs. The lines shown on the graphs were best fit linear fits but I did try both second and third order polynomial fits, just to see if was possible to detect any non linearity. The polynomial lines were not significantly different from the linear fits.

I have also just drawn up a Vin rear end to see if I can understand Monkeypant's point about progressive springing. I have to confess that the swing of the rear end pivot point attacking the springs at different angles and this giving effectively different rates had never occurred to me so this was a bit of education. I drew up a 'D' rear end with a 10" long damper unit and assumed that one might want to have 6" of movement at the rear spindle which requires 4" of movement on the spring/damper unit. The reduction in rate when the spring is fully extended compared with when it has compressed by 4" is only 0.96, so most of us are not going to notice that. I will send the drawings and details of this to Monkeypants separately as he might see a flaw in my calculations. It has been known!!