Lansdowne Dampers and Progessive Springs

I'm in the process of scheduling the front end rebuild with Lansdowne damper kit from John Robert Bould, probably an early kit. I don't know if the later or Madass units are better, but I bought one from Madass for my other Commando. My question is if the Progressive Suspension springs are not recommended. JRB had posted that they weren't, but from what I've seen, the springs vary. My particular springs are p/n 11-1119 which are not that tightly wound at the ends. More like wound tighter progressively end to end.
Rear shocks are Koni adjustables from the 70's, standard springs, but progessive springs are in house.

So, are mildly progressive front springs any benefit over stock springs (red painted) with Lansdownes?

This is a high performance street ride, that will not be ridden to extreme limits. Well, not the limits of a 74 year old rider anyway

I doubt they are progressive in real life, the RGM ones I bought are wound progressive but when you compress them the close coils did not touch which they must do to change the spring rate. The damping rates when adjusted correctly are matched to the spring rate, if you vary the spring rate and the damping rate is fixed then strange things can happen.

Progessive Suspension has been selling these springs for quite awhile. These are their original type. Not tightly wound at one end like I've seen in current offerings. Long before JRB introduced Lansdowne inserts. There are plenty of threads about both, but little about the combination of the two.
Maybe a constant rate spring works best with Lansdownes. That is probably what he designed them to work with. I never got the detail of what progressive spring make/model JRB said would not work with Lansdownes.
I may just use these because I don't see them as very aggressively wound, just gradually tighter wound throughout the entire length.

There's the theoretical exercise about these components and design of the forks and then there is the real world experience.
That's what I'm looking for.

So only look progressive then, so use them but they will act like the single rate springs they are.

Deckard said:

These are their original type. Not tightly wound at one end like I've seen in current offerings. Long before JRB introduced Lansdowne inserts. There are plenty of threads about both, but little about the combination of the two.
Maybe a constant rate spring works best with Lansdownes. That is probably what he designed them to work with. I never got the detail of what progressive spring make/model JRB said would not work with Lansdownes.
I may just use these because I don't see them as very aggressively wound, just gradually tighter wound throughout the entire length.

Click to expand...

You've got my curiosity peaked, with your early style progressive spring. Could you please post a good quality picture of the spring, as I'm interested to see what it looks like. Thank you.

Here's the most informative link:

Progressive Suspension does have them, but does not list Norton as a make:

The Dennis Kirk site picture more closely depicts what my springs look like. As you can see, the windings are not that close at the end, just gradually tighter.

Deckard said:

Here's the most informative link:

Progressive Suspension Fork Springs - 25/35 Spring Rate (lbs/in) - 11-1119 Dirt Bike Motorcycle - Dennis Kirk

Find the Progressive Suspension Fork Springs - 25/35 Spring Rate (lbs/in) - 11-1119 at Dennis Kirk. Shop our complete selection of Dirt Bike Motorcycle parts and accessories including the Progressive Suspension Fork Springs - 25/35 Spring Rate (lbs/in) - 11-1119.

www.denniskirk.com

Progressive Suspension does have them, but does not list Norton as a make:

Fork Spring Kit | Progressive Suspension

www.progressivesuspension.com

The Dennis Kirk site picture more closely depicts what my springs look like. As you can see, the windings are not that close at the end, just gradually tighter.

Click to expand...

Thanks for providing those spring images. I was intrigued because I happen to have the next numerical neighbor to the 11-1119 (25/35 quoted rates), which is the 11-1120 progressive spring - a significantly heavier spring with quoted rates of 35/55 lb/in. I concur with your comment about the spring appearance looking softer rather than firmer. Below is an image of the 11-1120 next to a fixed rate spring.



Before I started playing with my front end project I incorrectly assumed that as these progressive springs were compressed they provided a continuum of increasing spring rate with compression, when in fact that is not the case. They are basically a bi-rate spring, i.e., the initial rate is the composite of the tight- and loose-wound ends being deforming simultaneously, up to the point that the tight-wound end goes solid (coil to coil contact), at which point any further deformation occurs in the loose-wound end which is the 2nd rate of the spring.

The plot below shows the deformation of the 11-1120 spring where I have added red lines over the blue data to emphasize the rate change - red line intersection shows clearly where the spring transitions from the 1st rate of about 35 lb/in to the 2nd rate of ~ 55 lb/in. Your spring would appear similarly, but the slopes would obviously be lower than shown on this plot (your 2nd rate would be the same as the 1st rate shown here), and of course we’d have no idea at what length your tight-wound end goes solid.



As Kommando noted, until the tight-wound end goes solid it is simply a fixed rate spring. In the alternative, for this specific installation (11-1120), if the combination of installed spring pre-load and sag-to-ride-height together add up to 3 inches of total compression, then the tight-wound end of the spring would be solid and this spring would simply be a fixed rate 55 lb/in spring.

As a final aside to progressive springs, and one that I suspect is of little to no interest to just about everyone here, is the rate of compressions springs in series (which these springs basically are), which I personally found fascinating. If you determine the individual rates of the 11-1120 tight- and loose-wound ends, using wire diameter, ID, and coil count, you come up with the tight-wound end having a rate of just under 98 lb/in and the loose-wound end having rate of just under 56 lb/in. Thus, with a 98 lb/in spring and 56 lb/in spring in series, how can you possibly come up with a 35 lb/in rate which is the factory quoted initial rate that occurs as you begin compressing the spring?

The formula for determining the rate of springs in series is the following. Spring rate = (Rate 1 * Rate 2) / (Rate 1 + Rate 2)

35.6 lb/in = (98 * 56)/(98 + 56)

So there is our initial 35 lb/in rate from 2 significantly stiffer springs in series.

The springs do not only help make you comfortable, The spring rate in the forks, changes the amount the angle on the steering head changes as you brake. If you go into a corner too hot while you are braking, softer springs help you to run wide faster Similarly with the springs on the rear shocks. If you get into a corner too hot, at the point where you stop braking and begin to accelerate, softer rear springs allow the rear to squat more, which changes the steering to help you get around the corner. The bike's steering changes from under-steer to over -steer when you stop braking and begin to accelerate. It depends on the head angle - it only changes very little, but that little makes a big difference.

Thanks WZ507, that was very informational. As I have two Commandos, I will try these in my 850 along with the Lansdownes this year.
The 750 is a project that will follow later. They handle differently due to trail rake, but front end suspension will be the same except springs. Rear shocks will be the same. Koni 76f-1282. So called heavy turn adjustable from the 70's era, but a great shock.
I can experiment with springs on the rears too.

Not to get too far off topic, is there any functional difference in the Lansdownes? ie, early, later and what is currently sold?
I have early and latest.