Spring rate?

Had an emergency stop the other day and the forks pretty well bottomed out. I also note there is an awful lot of static sag. Can anyone tell me the standard spring rates for the Commando so I can compare them with my Ceriani springs. I've got a feeling they're just a bit on the soft side!

IIRC Cdo springs rate ~ 36 lb per inch travel. I've read of other brands fork spring being tried and liked but if your on the heffty side best simple way might be cut off ~1.5" of spring and stick a stronger progressive spring in place, like a valve spring. Shortening a spring stiffens it up. Also might try up to 175 ml of heavier grade fluid but any more risks hydrolock, bad ju ju. I hadn't practiced hard braking for a month so made myself today to be rather pleased how little fork dive and rather pensive how easy to squeal the new tire, so easy I thought something binding at first. I've the opposite issue most have, having to be careful of factory brake effectiveness. My forks with the valve spring inside act almost like a big dog dragged into Vet clinic. I also remember how much work and fear to brake on restrictive stock assembly of master cylinder and nose diving too boot.

Thanks Steve,

yeah, I thought of trying a heavier oil instead of the 10-weight oil that was supplied when the forks were overhauled - ATF perhaps? Also noticed that a (3mm) spacer had been fitted at overhaul time. I'm pretty sure the forks were intended for a much lighter bike than the Norton (at less than 80kg I'm not adding a lot of burden to the whole show!) Need to prove this conjecture by measuring the rate of the existing ones and proceeding in a systematic fashion from there. About 6.5kg per cm... ok.

Unless forks modified inside to tighten up clearances or different damper device, 10 grade or ATF is too thin even for me @ 160 lb. I like the 20-50 grade on factory Roadholders but like power steering fluid in my modified ones. ATF was a bit too thin so forks jarred on THE Gravel and more dive on braking. That size spacer would help a bit in the resting sag but not much else.

Hi Dave,

My Norton is very similar to yours. I have 35mm Ceriani forks also and they're the same way. Lots of sag and very easy to bottom out. Sorting my suspension is my next project. I really like the dampening on these forks they are just way too soft.
I weigh about 220 lbs. so you can imagine how much sag they have now...
I'd love to know the spring rate too. This place has quite a few different rates for reasonable prices.
http://durelleracing.com/store/index.ph ... Path=33_38

davamb said:

I thought of trying a heavier oil instead of the 10-weight oil that was supplied when the forks were overhauled - ATF perhaps?

Click to expand...

http://www.classicbike.biz/Norton/Repai ... mmando.pdf
Section G8, 10

150cc of SAE 20

Click to expand...

Thanks Jeff and LAB. Investigations will continue.

Hey Dave,
I'm no suspension expert but haveing had a MX, Dirttrack, Enduro background have played with suspension a lot.
Obviuosly Road is different to long travel dirt, but I think the setup principals would be similar.
Perhaps try more preload on the springs untill you are happy with the ride and the amount of sag that siuts your riding.
Then work on the dampening, especially the rebound, to assist the extra pressure created by the now stronger springs.
If the the springs don't get coil bound with the extra preload, they might be ok, but if they do then you can take them
to your local spring maker, (the Dirt bike shops will know the good ones), and show him what you need.
If you change the front, you'll probably have to change the back as well.
I think you might get away with just adding thicker oil, but as I see it, it's placeing more pressure on the fork
internals, than on the springs. Like adding heavy duty shockers to your car but not doing the springs, the shockers
take up the extra load, not the springs which are designed to carry the load.
Ceriani make fantastic forks so I imagine that they are adjustable. (dampening)
With some luck this might be a real easy process Dave, with great returns.
If it's not,... send the bike over and I'll set it up for the next Dirt track event. (1/2 mile at Kapunda)
Hope it's helpfull Dave.
AC.

Seems to be a bit of confusion here. Lots of good replies on oil to use in stock Commando forks, but it sounds like davamb has Ceriani forks, not the stock ones. Is that right, dav? If so, 10W fork oil, or ATF, which is usually close to 10W, might be the right grade to use. Pretty good chance that the Cerianis are just too softly sprung.

As hobot mentioned, stock early Commando spring rate is 36.5 lb/in.

Ken

Just to repeat what most of us know -

When suspension bottoms that means that the energy has to go into the tire and thence the contact patch and then, if there is too much energy, traction is lost.

So, I think that bottoming trumps "sag." I don't start with sag.

That is I set sag with pre-load after I have enough spring for my weight and the travel available to head off bottoming, and that means that I usually set stuff up a little stiffer than most.

The main thing that 30% sag does is allow the wheels to follow the road when the bike is rising, coming up, due to acceleration or jumps and humps. While that mode is important, I am more concerned about the bottoming induced squish and slide.

I agree with AC.

I don't like ATF as the weight or grade isn't as accurate as fork oil and you can get lost with what weight oil it is.

Ceriani are good forks, what are they off? You could try Pantah springs.

Yep pre-load spring adjusters do same thing as a solid spacer under spring, lifts bike not compresses spring. Can't coil bind factory Roadholders as Norton bean cutters made fork bushes collide just before springs do. Only way w/o a whole new spring is trim old spring and put in spring spacer with higher rate so original spring gives the main ride compliance over regular road texture while the high rate spacer handles the brake dive and hump hits. Don't forget thicker oil and more of it - short of hydro lock air pump down in Roadholders. On your own on other brands fluid levels.

Thanks for the input guys. First thing to address is the spring rate, I've got to measure what the ones I've got are. Just pogoing the front end from the bars, they sure seem soft compared to Pantah and TRX, but that's just subjective, I'll be measuring the deflection in a manner similar to what Ludwig describes to be sure. Keep in mind the linear spring F = k * x and you can see why preload is not going to achieve what I need here.

I've no idea what they were off Graeme, just what was in the box when I bought it off the DPO, same deal as the Morris wheels. Things weren't so easy back in the 70s when he built the bike, you took what you could get, not what you wanted. I will give your idea of using the Pantah springs a go, if I can research what rate they are.
Thanks for the link there Jeff, I had a look. Nice to know there are bits available.

So, once the spring rate is corrected and I don't freak myself out in a panic stop with forks bottomed out, I'll start looking at the damping!

(I'll give serious thought to riding over to SA once it's sorted AC, I've some rellys that I owe a visit in Adelaide. Seriously thinking of riding it instead of the TRX to Bathurst for the annual mid-winter BSHTFU if I can get it sorted and reliable in time.)

Hey Ludwig,
Hopefully I can explain what I mean with out upsetting anyone.
No disrespect intended.

The word "stronger' was the wrong term to use.
Obviously the spring can't become stronger by adding a few washers or spacers (preloading them).
Perhaps I should of said "seemingly stronger".
Preloading the springs will "only raise the ride hieght" if they are excessively preloaded.
Sometimes just a little more is enough, that's why I said,, "perhaps try"

To use riders weight and bike weight etc, etc, is fine, and infact very technical, the only
problem is, that not all riders of the same weight, actually ride the same.
Some riders brake heavily and accelerate hard, others seem to get around the corners very smoothly,
some like to slide,... I reakon there's 3 different setups right there.
I,ve only had a dozen or so sets of springs made, you may have had a hundred,
but I know this, (without reading a book), that if I take a pair of fork springs into our local
spring maker with the internal measurement of the compressed fork staunchen and leg ,
along with the overall uncompressed length plus the amount (in length) of preload added,
they seem to be able to make a set of springs that work. I did not imagine this.
When I take a pillion passenger, I don't fit new stronger springs, I preload the existing ones.
If I was takeing a pillion every time, I would get stronger springs, etc.

Not trying to be stubborn Ludwig, or declare war, we Australians don't like that, hehehe.
But it works for me.
Thank you for your time and understanding.
AC.


I

You can add hydraulic assistance to the springs compresssive rate, the spring will not become stronger..But the with "added hydraulic's " the compression rate[speed of compression] can be adjusted.
LAB and Reggie found that out with the Lansdowne Kit! ...

Norton forks have very little compression damping,with light fork oils the effect is almost Zero..notice i said almost Zero. Thick gear oil will produce some compression resistance. Duncan's manx had 140 gear oil in the forks, before a race Dave [Duncans spannerman] warmed the forks with an air drier. he told me the other riders frowned at this ! I have seen tyre warmers...but fork warmers :?: :lol:
Also adding more oil will reduce the dive rate as the pnumatic effect increase's .NOT RECOMMENDED! to much oil will produce a solid state after a small travel, 300ml will produce about 2 in of travel than a solid state!
http://www.lansdowne-engineering.com

I think the problem lies with Italian springs.
Although my Ducatis run Marzocchi forks, I'm going to assume the principle is the same.
These forks are legendary for bottoming out - it resulted in the ex-Hailwood bike getting half destroyed on the Isle of Man some years ago.
They may be fine when factory fresh, but I suspect they lose tensile strength very early.

Both my bikes suffered from the same problem. I just don't, er, *ahem*... 'rate' the standard springs at all.
I replaced mine with German Wirth Federn progressive jobbies, which not only sorted out the bottoming, but are also infinitely superior in terms of ride quality.

http://www.wirth-federn.de/products/en/ ... riani.html

It may be worth establishing exactly which bike your forks were specced for, but I hope this is a start

B+Bogus said:

I think the problem lies with Italian springs.
Although my Ducatis run Marzocchi forks, I'm going to assume the principle is the same.
These forks are legendary for bottoming out - it resulted in the ex-Hailwood bike getting half destroyed on the Isle of Man some years ago.
They may be fine when factory fresh, but I suspect they lose tensile strength very early.

Both my bikes suffered from the same problem. I just don't, er, *ahem*... 'rate' the standard springs at all.
I replaced mine with German Wirth Federn progressive jobbies, which not only sorted out the bottoming, but are also infinitely superior in terms of ride quality.

http://www.wirth-federn.de/products/en/ ... riani.html

It may be worth establishing exactly which bike your forks were specced for, but I hope this is a start

Click to expand...

Andy ,You are quite right .
But in all fairness to our "Beloved" Norton engineers RIP.
Their original design was back in 1953 for drum braked machines,modern equiped twin disc Commando's would require something differant .
The Norton stock spring is a creature that as restraints, Firstly it as to fit into the stanchion with enough room to allow buckling, this first diamension sets the rest, static sag and full bound length are determined by wire dia,material and amount of coils.
Springs from a "all alloy" 350 single are just the same as a MK 111 commando...Nortons Brief "One spring fits all"

I personaly think the fork spring buckle's to much,with considerable binding, on the stanchions bore. well check out the side wear! I have noticed cheap indian made stanchions have a rougher finnish inside which produce's a grateing sound ,
But i dont want to raise the Genuine parts issue again!

I have made a" built up " spring with short springs and nylon connectors, But the cost is a none-starter!

Dowty Oilomatic got it right, spring less forks. i am just building a set with hydralic dampers...Imagin that Air forks with oil controlled dampers. Pump up your desired ride lads :!: YES i have considered them for commandos!!!! NEXT PROJECT?