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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #1  
Old 2 Weeks Ago
Swamp Biker Swamp Biker is offline
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Default 33.4 mm Fork Springs

I'm in the process of rebuilding the front forks on my original/only owner '71 XLCH. I have new bushings and, after a 5 month wait, tubes from Frank's. I'm fabricating the necessary tools for removal/installation of the bushings. The OEM springs are shorter than the optimal (I think) 19 3/4" by 1/4" & 1/2" respectively. I figured I'd replace the springs and retainer rings at this time. I ordered new springs from Dennis Kirk based on the part# in the '54-'78 parts manual. My OEM springs have 60/61 turns (not sure how to count the turns). The replacement springs have 80/81 turns and are 19 3/4" long (the bag they came in has an Eastern label that says they're the proper springs - it also has a V-Twin label on it). The Eastern springs are ~.030" smaller in overall OD and ~.010" larger in wire dia. My question is: Should I use the Eastern springs and will they allow the same amount of fork travel as the OEM springs? Alternately, could I/should I add spacers to the top of the OEM springs to make up the length difference? Also, the part#'s in the '54-'78 parts manual are incorrect for the slider and damper retaining rings. The slider ring is 11048 and the damper ring is 11047.
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  #2  
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davixlrtt davixlrtt is offline
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Hello Swamp Biker, where did you get the measurement of 19 3/4” for the springs? There was a question posted here lately asking about spring length, maybe you read that. I have a set of springs from “Progressive” in my 33.4mm forks that have the early style dampers and they are 20” long. You could always try the Eastern springs, it’s not that difficult to swap back over to the original you probably won’t notice the 1/4” difference.
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Thanks for the reply davixirtt. I'm aware of the recent discussion about spring lengths however I have a notation in my parts manual that predates that discussion saying spring length is 19 3/4". I don't recall how that number came about. My OEM springs are a bit shorter than I previously stated. Point being they are shorter now than when new. I'm more concerned with compressed length and how it will affect fork travel than relaxed length. I measured the distance between the coils of the OEM and Eastern springs and calculated the OEM springs can be compressed by ~9" and the Eastern springs by ~6". Will that 3" difference have a negative effect on fork travel? Anyone know what max fork travel is?
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davixlrtt davixlrtt is offline
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I think you should still be ok with 6” of compression on the springs, someone else will probably respond but I don’t think there is any where near 6” of travel at the front end.
…..Davi.
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Don't forget the inch or so of pre-load when installing the springs. Still in those days, 4-5" travel was a lot
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What a rabbit hole springs are.

Yes, springs do sag as the get old and used.

The replacements may have a totally different spring rate. You just have to assume the makers worked out a reasonable compromise.

If your looking for the best performance from your springs you also have to take into account the riders weight.

Springs suitable for Jenny weighing in at 56 lbs won't work for Ugly Tom at 357 lbs.

What your looking for is about 1 inch of sag with the rider on the bike. Adding spacers, shortening springs affect the ride height. They don;t change the spring rate, just the ride height.

Use whatever springs you can buy and adjust the ride height with spacers or shortening the springs to suit your weight. That's as good as it will get for these forks.

You can affect the dampening as well by changing viscosity or amount of fork oil but.
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When it comes to riding a '71 XLCH a quarter inch of spring in your forks will be the least of your concerns.
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I dry assembled an old tube & slider (without springs) and measured the maximum tube travel witch was about 8". With the spring installed & pre-loaded I think travel should be less and within the new spring's limit. So, I'll install the Eastern springs and after all is said & done I'll let y'all know how it turned out. Thanks everyone for your responses.
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Quote:
I dry assembled an old tube & slider (without springs) and measured the maximum tube travel witch was about 8".
You'll hit coil bind long before you get 8 inches.
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