fork spring length

How critical is the at rest length of the fork springs? Technical data says 18.687". The pair I am working on has one that is 18.625" and the other is 18.500". Does anyone know if that would create any issues?
The bike was disassembled when I received it so I can't say how they worked before.

NickZ said:

How critical is the at rest length of the fork springs? Technical data says 18.687". The pair I am working on has one that is 18.625" and the other is 18.500". Does anyone know if that would create any issues?
The bike was disassembled when I received it so I can't say how they worked before.

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Seems fine to me . A bigger issue is if they are rusted or not . Or they fit on the rods somewhat snakey or straight .

Torontonian said:

Seems fine to me . A bigger issue is if they are rusted or not . Or they fit on the rods somewhat snakey or straight .

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I doubt that a street rider will notice any problems from the OP's length difference.

I bought a set of progressive wound springs from RGM, but I found thatthey were very noisy in use, I think because they were buckling under compression. Apart from the annoying noise, this will also produce metal particles to mix in the oil, resulting in a grinding paste.
I notice that the Marzocchi forks in my Ducati use 2 tubes on each end of the spring to precent this - perhaps it's worth trying on the Norton fork?

I've been experimenting with dampers and springs for a while. I've been reading a lot and trying to apply what I've learned to my commando. It's been challenging. You should just add a spacer to the short spring to make them the same length. I make spacers out of delrin, but a few washers will work too. If they are a bit shorter than the stock commando springs, that would be OK because the commando doesn't have enough static sag anyway, so a little extra sag won't hurt at all.

I make top and bottom bushings to center the spring out of delrin and I also cut a polycarbonate plastic sleeve (6.5" shorter than the spring so it doesn't interfere with compression) to slide between the spring and the damper rod to keep the spring straight and quiet by keeping it from rubbing the damper rod or the inner fork tube.... The rubbing of the metal spring on metal damper rod and inner fork tube is one of the ways your fork oil looks like metalic soup very quickly.

If you are going with the stock commando dampers,.... JSmotorsports makes a kit to improve the horrifying stock damper parts.

o0norton0o said:

I've been experimenting with dampers and springs for a while. I've been reading a lot and trying to apply what I've learned to my commando. It's been challenging. You should just add a spacer to the short spring to make them the same length. I make spacers out of delrin, but a few washers will work too. If they are a bit shorter than the stock commando springs, that would be OK because the commando doesn't have enough static sag anyway, so a little extra sag won't hurt at all.

I make top and bottom bushings to center the spring out of delrin and I also cut a polycarbonate plastic sleeve (6.5" shorter than the spring so it doesn't interfere with compression) to slide between the spring and the damper rod to keep the spring straight and quiet by keeping it from rubbing the damper rod or the inner fork tube.... The rubbing of the metal spring on metal damper rod and inner fork tube is one of the ways your fork oil looks like metalic soup very quickly.

If you are going with the stock commando dampers,.... JSmotorsports makes a kit to improve the horrifying stock damper parts.

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Thanks for the suggestions. I was having the same thought...... symmetry is more important than the exact length. But the fact that Norton specified the length to 3 decimal places made me wonder.

I have been looking at the JSMotorsports damper parts.

NickZ said:

Thanks for the suggestions. I was having the same thought...... symmetry is more important than the exact length. But the fact that Norton specified the length to 3 decimal places made me wonder.

I have been looking at the JSMotorsports damper parts.

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Maybe the springs were originally specified as 18 and 7/16 inch back in the days when some people still used fractional measurements, and somewhere along the way Norton switched to decimal notation, so it became 18.6875, but someone thought it was silly to spec it to four decimal places and decided three would be just right

NickZ said:

but the fact that Norton specified the length to 3 decimal places made me wonder.

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The development lab in charge of the process for supplying inert gas to the cooling portion of the oven had the Nitrogen spec as 99.99% purity. When I was looking for cost reductions when I asked why it was 99.99% they replied, because that is the purity of BOC's tanker supply. So what to does process actually need then ? 97% was the reply. Onsite generation suddenly became cost effective.

Hence the reason to ask the 5 why's to get all the facts.

o0norton0o said:

I make top and bottom bushings to center the spring out of delrin and I also cut a polycarbonate plastic sleeve (6.5" shorter than the spring so it doesn't interfere with compression) to slide between the spring and the damper rod to keep the spring straight and quiet by keeping it from rubbing the damper rod or the inner fork tube.... The rubbing of the metal spring on metal damper rod and inner fork tube is one of the ways your fork oil looks like metalic soup very quickly.

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You don't perhaps happen to have the dimensions handy for the sleeves? I want to do the same, but don't have the forks opened up at the moment.

kommando said:

The development lab in charge of the process for supplying inert gas to the cooling portion of the oven had the Nitrogen spec as 99.99% purity. When I was looking for cost reductions when I asked why it was 99.99% they replied, because that is the purity of BOC's tanker supply. So what to does process actually need then ? 97% was the reply. Onsite generation suddenly became cost effective.

Hence the reason to ask the 5 why's to get all the facts.

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In the early 80's I was doing some draughting work for a small company in North London. They were producing control units for disco lights, and I was doing some sheet metal drawings for them. I found out that drawings dimensioned in mm, Imperial fractions and Imperial decimal units all had different production prices from out suppliers.
As far as I remember, mm was lowest, then fractions and decimal was the most expensive. Since circuit boards are (or were) specified in increments of 0.1", the front panels (which had holes which had to align with components on the circuit boards) were the most expensive.

I always thought that was odd, considering that the specified tolerances on all the parts were similar.

It is amazing how stupid the internal spring design is compared to the previous external spring. The amount of sludge (metal dust) found in the bottom of the fork slider after only one season is noticeable.
Draining the forks and refilling helps but it ain't enough.

SteveBorland said:

You don't perhaps happen to have the dimensions handy for the sleeves? I want to do the same, but don't have the forks opened up at the moment.

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I ordered silicone tube first which had a higher temperature tolerance than the PETG plastic, but it didn't fit. Then I ordered this PETG clear plastic and was skeptical of it's heat tolerance since fork oil does heat up a bit. In the end, I just tried it and it seems to be fine so far. I have installed these tubes on 4 sets of CBR600f4i dampers that I modified to fit my commando. So far the plastic tubing looks uneffected by heat or the chemistry of the fork fluid.

I went out to the shop to check if the PETG tubes fit the stock commando dampers and they fit perfectly. The only other issue with the stock dampers that needs to be addressed to center the spring is the upper damper assembly doesn't have a stepped bushing to center the spring up top, and the plastic sleeve will drop all the way down to the bottom of the spring, so you should make a top bushing.

Here's the link to the tube I bought. It has dimensions so you can search to find a source closer to you: https://www.amazon.com/dp/B00ZC0U4K4?psc=1&ref=ppx_yo2ov_dt_b_product_details

Here's what it looked like when I tried it. As you can see it fits fine.




And here's the stepped delrin bushings I make the center the spring top and bottom on the dampers that I am using. (you only need to make 2 to center the top of each spring)

o0norton0o said:

I ordered silicone tube first which had a higher temperature tolerance than the PETG plastic, but it didn't fit. Then I ordered this PETG clear plastic and was skeptical of it's heat tolerance since fork oil does heat up a bit. In the end, I just tried it and it seems to be fine so far. I have installed these tubes on 4 sets of CBR600f4i dampers that I modified to fit my commando. So far the plastic tubing looks uneffected by heat or the chemistry of the fork fluid.

I went out to the shop to check if the PETG tubes fit the stock commando dampers and they fit perfectly. The only other issue with the stock dampers that needs to be addressed to center the spring is the upper damper assembly doesn't have a stepped bushing to center the spring up top, and the plastic sleeve will drop all the way down to the bottom of the spring, so you should make a top bushing.

Here's the link to the tube I bought. It has dimensions so you can search to find a source closer to you: https://www.amazon.com/dp/B00ZC0U4K4?psc=1&ref=ppx_yo2ov_dt_b_product_details

Here's what it looked like when I tried it. As you can see it fits fine.

View attachment 115185

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o0norton0o said:

McMaster- Carr has 15mm OD 10mm ID polycarbonate tubes. Maybe keeps spring more centered. Rated to 200 degrees F.

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o0norton0o said:

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

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15mm tube doesn't fit inside 14.91mm ID springs last I checked...

Must be a nominal dimension just like when buying timber

o0norton0o said:

15mm tube doesn't fit inside 14.91mm ID springs last I checked...

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My springs are measuring over 15.5mm ID.

Nice, The polycarbonate is definitely a better choice than PETG plastic if it fits I have been using a lot of different springs lately, so I may have chosen the 14mm x 10mm tubes so they would work with all the different springs I was working with....