Actually stiffer fork springs

kawabunga

kawabunga

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I've been searching all over for info on stiffer fork springs, but everything I've been finding is in regards to replacing worn out stock springs with new stock springs to stiffen up the front. I'm looking for fork springs that are stiffer than stock. I ride spirited, and off-road, and I want something stiffer with better feel. The stock fork springs are mush. I've raised the oil level and used heavier weight oil, all with the preload maxed. They're just too soft...
Aftermarket options don't seem abundant, or very clear on whether they're actually stiffer or if they're just stock replacements. Is there a spring from another bike that drops into the '79 XS650 fork and is more stout? Somebody has had to try to stiffen the front end of these things.
Thanks, kawabunga
 
In order to keep the tire in contact, the forks need to comply with the pavement, responding to bumps that differ in size and shape. The older and cheaper way to accomplish this is to use dual rate springs. The better and more expensive way to maintain suspension compliance is to use valves which enable damping to vary in response to the road surface. Straight rate springs should be used with damper valves, because dual rate springs would create a false signal. Mike's XS offers heavy duty dual rate fork springs rated at 35 and 50 pounds/inch.
 
Have used big twin straight rate 35mm sportster springs on 2 XS650's now, by my measurements about 15% stiffer than decent stock springs. Like Superjet they are a bit long stiff so starting the caps can be a challenge. Would have to check records but one fork has emulators, other has minton mods done. Don't offroad but some wisconsin roads are that in name only.
 
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Poor man's fix - you can add 1" preload spacers to your stock springs to stiffen them up. The adjuster cap gives you 2 steps of 10mm each, or 20mm total. An inch is about 25mm so these spacers will start you out about a half step stiffer than the stiffest stock setting. Then you have 2 more 10mm steps you could take to make them even stiffer.

Njud69o.jpg
 
No, but 25mm is pretty much the limit you can go. I did a bunch of measuring and calculating a few years back. I fully compressed some fork springs and measured them. I know you can supposedly calculate full compression spring length by multiplying the wire diameter by the number of coils. I did that too but those results never matched my actual measurements exactly .....

1fvZ4nJ.jpg


z4QB951.jpg


From all this I deduced that a 1" spacer is what you want. Less than that is pointless because you can achieve that amount of preload already with the adjustable cap. Any more and you risk coil bind at or near full compression.
 
I like your science experiment there, 5T. Big springs under tension are always fun!

I don't think adding spacers or air caps is going to achieve what I want. Both of those options are increasing preload, which will raise my ride height but not have that big of an impact on actual compression rate. True, preloading the spring will make it stiffer, but it's limiting travel and definitely affecting rider sag. The front already sits higher than I want, so increasing preload is going backwards there. A stiffer spring rate is what I'm after. Honesty, if I could get the forks to sit an inch lower AND be stiffer, I'd be really happy.

So, the fork springs from MikesXS are stiffer than stock and not just OEM replacements? The product description doesn't list spring rates or say they're actually stiffer then stock. Just says they "improve handling".
https://www.mikesxs.net/yamaha-xs650-fork-springs-progressive-type-74-76-oem-256-23141-62-00.html

@gggGary, your link shows part number 45408-75 on the page. @Superjet, the post you linked to has a photo attached and the package also has 45408-75 on it, so I'm assuming you guys are running the same spring... Have either of you tried the MikesXS springs? Thoughts between the two?

For reference, I'm running Progressive Suspension 12-Series shocks, 14.25" tall, with progressive rate springs, 75/120 on the rear. They make the front end feel like mush in comparison.
 
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I had the progressives from XS 650 Direct and to me were not worth it. I wanted to try something different and went with the HD springs which were cheaper. I have it set at 1st preload setting on my adjuster caps. I also went with recommendations from the guys here. I try not to buy XS 650 Direct unless I have to. J-C
 
Allisons period piece has stock front springs, hate it. I wuz guessing SJ and I had bought the same springs. I think on resto I have the mikes emulators and non adjustable 77 caps. Whilst messing around in the shed I found that 34mm and 35mm fork tubes have the same thread for caps so you can interchange caps.
 
The springs you linked to from Mike's are for the '74-'76 34mm forks. '77 and later 35mm fork springs would be different. I have the Mike's progressive springs in my '78 and they're OK, but I bought them years ago when they were very reasonably priced. I would never buy them today. They jacked their price so much you can get real PS springs for about the same now.

I like the stock springs but they're just too soft. They are a nice progressive spring and give a nice ride, if only they were stiffer.
 
Looks like Drag Specialties has that HD fork spring for $50, the same one that Superjet linked to:
https://www.dragspecialties.com/products/?productId=469833&partNumber=04050329

Using the model applications on that page, I went to Progressive Suspension and put one of the bikes it fits into their by-bike filter. Part number 11-1120 comes up. I then went to their technical data sheet to see info on it, two listings come up with that number but one has an "S" on it.

The info in the listing are as follows:
Part number: 11-1120 / 11-1120S
Free Length - In.: 19.75 / 18.75
Free Length - MM: 502 / 476
O.D. - In.: 0.90 / 0.90
O.D. - MM: 23 / 23
Rates lbs/in: 35/50 / 35/50
Rates kg/mm: .63/.89 / .63/.89
Spacer: 2.12 / 2.12

So, only difference is free length by one inch between non-S and S part numbers.

grizld1 noted that MikesXS springs are 35/50 lbs/in, so they should be the same as these springs...in theory. I don't know about in real life.
 
The original 35mm fork springs are just under 19" in length, hence the added difficulty cramming 19 3/4" springs in. My tool of choice for installing fork caps is a 3/8" drive T handle. It allows you to push down really good while you turn. This $10 tool is something I think everyone who works on bikes should have. It turns your whole socket set into T handles, lol .....

zqQVDt6.jpg
 
I put A 35 mm fork on my 75. Didn't like the sagged out springs. The spring I replaced them with I got from J+P. They were for older Harley XL's and some Superglides.
The J+P part number was 230/584. And cost $49.95. The specs in kawabunga's post are pretty close to what I remember measuring.
I have the adjustable caps. On the setting giving the least tension I get about 25mm sag, on the middle setting I get about 12 mm sag, can't get to third setting.
I have did the Minton mods to the dampers, ones that apply anyway.
Tried many oils. Straight weight, multi viscosity, motor oils, forks oils, Dexron tranny fluid. I tried variations in oil level.
Best ride and handling seems to be with 6 inches down fill with the tranny fluid.
Very firm, Almost a bit harsh. Been thinking about the cartridge emulators.
Leo
 
5twins said:
The original 35mm fork springs are just under 19" in length, hence the added difficulty cramming 19 3/4" springs in. My tool of choice for installing fork caps is a 3/8" drive T handle. It allows you to push down really good while you turn. This $10 tool is something I think everyone who works on bikes should have. It turns your whole socket set into T handles, lol .....

zqQVDt6.jpg
Click to expand...

S4.jpg
Or this type of socket crank handle.
 
I found that Progressive Suspension does offer a stock replacement fork spring
Just putting the technical info they have listed here for anyone interested.

Part number: 11-1119
Free Length - In.: 19.00
Free Length - MM: 483
O.D. - In.: 0.95
O.D. - MM: 24
Rates lbs/in: 25/35
Rates kg/mm: .45/.63
Spacer: n/a
 

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