Fork spring preload calculations?

[testrider]

Hi all,

I'm going to order a set of RaceTech fork spring (0.95kg/mm) for

my weight. What outside diameter of PVC should I get to use as spacer?

- what do you use to cut the PVC straight?

- To calculate how long a spacer I should cut, are my calculations below correct:

To have a front sag of 30mm and having 0.95 kg/mm springs, assuming that my XX + me weight about 720 lbs and the XX has about 50% front and 50% rear weight distribution:

720 lbs / 2 = 360 lbs = 164 kg on the front end

164 kg / (0.95 kg x 2 forks) = 86mm <-- without any preload

So to have 30mm sag, I will have to have a spacer length of 86-30 = 56mm + whatever length to make it flush to the fork top.

Is this right?

-Mike-

[Redbird]

No need to buy PVC, a length of metal spacer should be included with your springs. Use a miter box to cut it if you don't have access to some sort of chop saw.

Not sure on the math, but I figure it's gonna be hard to figure the spacer length without knowing the spring length first, no? And perhaps the length of the space they'll reside in.

[testrider]

I'm guessing here: the spring length shouldn't matter in my calculation because I know it's rated at 0.95kg/mm. The calculation gives 86mm-30mm = 56mm and this is supposed to be the preload that we need to apply. In other words, the spacer should stick out of the top of the fork by 56mm minus the length of the fork cap. So if the spring is longer or shorter, we still use 56mm of preload. What's a XX's wet weight? 540 lbs? Is the weight distribution really 50/50?

-Mike-

[Redbird]

Motorcyclist puts the XX at 556 fully fueled, and yes, it's pretty much 50/50 on the weight distribution.

[testrider]

I'm going to order a set of RaceTech 0.95kg fork springs from Partmonster (Louie) here. What weight of fork oil should I use? THe stock was Honda SS8, which is 5w. I changed it to SS7 (10w) a while ago with my stock fork springs and found that it was slightly better. When I put in the Racetech springs, should I go back to 5w or should I keep 10w? I assume any fork oil should be ok? (Honda/BelRay etc...?)

Oh, will I still use the same fork oil level (154mm for my 97) even thought

the Racetech springs is longer and therefore occupy more space?

Thank you again!

-Mike-

PS: I'm very tempted to get an Ohlins shock but Louie said there is 3-6 weeks wait.

[Redbird]

Stock is indeed SS-8, but that's 10w. SS-7 is 5w.

Any fork specific oil of the weight you want is fine, I chose to stick with 10w until I do something with the valving, but that's just me.

And yes, set your fork oil level at 154mm if that's what the manual recommends, unless RaceTech says different.

[testrider]

Thanks, Redbird. I think I got it reversed. The stock was 10w, I replaced with 5w. I guess I'll go back to 10w.

When measuring fork oil level, I'm supposed to have the fork fully compressed and no fork springs inside the fork, right?

-Mike

[Northman]

When measuring fork oil level, I'm supposed to have the fork fully compressed and no fork springs inside the fork, right?

Correct.

You can also play with your oil levels to fine tune your suspension's resistance to bottoming.

Mine is 10mm above stock, FWIW to limit diving when braking hard.

[blackhawkxx]

Mine is 10mm above stock, FWIW to limit diving when braking hard.

How important is the oil level? I mean, will a 1/16 of a inch make any difference? A 1/8"?

[tvking]

I switched to 5w oil in the stock fork. The stock setup has too much rebound damping for me. Since this affect compression damping too, I raised the oil level 10mm. Seemed better to me, but maybe I was just hoping it was so that my work was not for nothing. :cool:

[Northman]

How important is the oil level? I mean, will a 1/16 of a inch make any difference? A 1/8"?

Probably not very noticeable at all.

[G2]

1 liter of oil will do it for both forks and put you just a couple of millimeters above the stock recommended height. (even Honda fork oil @ 946ml) Divide equally and pour.:wink: Adding oil helps the forks to resist bottoming and diving too much under heavy braking by making less air in the fork to compress.

I've measured sag and cut spacers for 4 different brands of springs for 4 different riders. All XXers. Spring lengths and of course rider weight with gear vary and will affect the spacer length. 1" I.D. schedule 40 PVC works great and cuts easily and square on a decent miter or chop saw. Save the aluminum tubing for when you've found the sag that works best for you.

If your around 200 lbs less gear and you are buying Race Tech springs, start out with a spacer @ 130mm. This should put your sag in the mid 30mm range. Measure your sag with the stock springs first to have a baseline. I'll bet it will be >40mm. Remember to factor in fork stiction when measuring. :???: RaceTech should have the procedure somewhere on their website.

Finally, with the stock valving on '99 and later 'birds, 5wt oil is an improvement over the 10 wt.

Greg

[testrider]

The 99 and later XX have a different fork than my 97? I thought they all have the same one?

-Mike-

[Redbird]

The 99 and later XX have a different fork than my 97?

Different internals, yes.

I thought they all have the same one?  

No, the internals are different. :razz:

PS- Hiya Greg

[G2]

Hi Mike

The '97 has, in my opinion, the better valve set. It is more like the RaceTech valves in that the bypass orifice on both the compression and rebound valves is in the valve body instead of the "hollow" assemby that the valves mount on like the '99 and later bikes. The shimstack is also different. Makes for a more compliant ride. I'll bet your bike doesn't hammer you over those little washboard bumps. I don't know why they changed.

Because of the differences you may notice an audible "whooshing" sound after switching to the 5wt. oil. Some people found it to be a distraction.

I set up a buddies '97. With .95 springs, 10wt. oil (5wt. was too light) 36mm of sag and some shims removed, his bike rides better than any other XX I've been on. Including my '01 with aftermarket valves. He's around 150 lbs and also has a Penske shock. He now rides with the front boys at SeXXt. Faster he is. :-o

[testrider]

THanks Redbird and G2,

I really didn't know until you explaned it! Thank you!

Actually my 97 had the same problem when riding over washboard section of the freeway here in SoCal. I changed the fork oil from Honda's 10w (SS8) to SS7 (5w) and it improved a lot (still stock springs and everything). I honestly didn't notice any "whoosing" sound. May be I wear eye plugs when I ride that I couldn't hear :-)

I'm ordering a set of Racetech 0.95 kg springs (just the springs. No valves) I weight 170 lbs without riding gear. Sounds like I should change my fork oil back to 10w from your experience?

What are the recommended front and rear sag? My front has 41mm of sag and 40mm at the rear with me sitting on it.

Thanks again.

-Mike-

[G2]

I'd leave the 5wt. in until after you've installed the .95 springs. Ride it for a while and then determine if you want it a bit more damped.

PVC is inexpensive so try a few different spacer lengths. Start at 130mm. Changing the fork sag is really just moving the fork travel stroke either up or down on the fork tube. Effectively moving the front of the bike up or down which influences a number of things. 30mm is just a standard baseline starting point. I tried a 32mm setting and it felt like riding a chopper. :shock:

Let me know what you settle on.

Greg

[jcrich]

 

I set up a buddies '97. With .95 springs, 10wt. oil (5wt. was too light) 36mm of sag and some shims removed, his bike rides better than any other XX I've been on.

The springs are actually 1.0 Kg., I believe. Traxxion Dynamics. Greg is right, it is like riding a completely different bike now with the improvements.

:worship: :worship:

:

[G2]

Thanks for the correction Rich. :???:

The Penske 8981 has arrived. :cool:

And no, I didn't get the shock sock. :razz:

[jcrich]

Thanks for the correction Rich. :???:

Not often I get to do that :razz:

The Penske 8981 has arrived. :cool:  

Nice!!!!!

And no, I didn't get the shock sock.

I would have thought you would have, since you have the upgraded model :razz:

[testrider]

I'd leave the 5wt. in until after you've installed the .95 springs. Ride it for a while and then determine if you want it a bit more damped.

PVC is inexpensive so try a few different spacer lengths. Start at 130mm. Changing the fork sag is really just moving the fork travel stroke either up or down on the fork tube. Effectively moving the front of the bike up or down which influences a number of things. 30mm is just a standard baseline starting point. I tried a 32mm setting and it felt like riding a chopper. :shock:  

Let me know what you settle on.

Greg

Thanks G2, I think I'm going to do what you suggested: leave the 5w in there and see how it goes. I'm still waiting for the springs.

Will keep you posted.

-Mike-

[testrider]

I'd leave the 5wt. in until after you've installed the .95 springs. Ride it for a while and then determine if you want it a bit more damped.

PVC is inexpensive so try a few different spacer lengths. Start at 130mm. Changing the fork sag is really just moving the fork travel stroke either up or down on the fork tube. Effectively moving the front of the bike up or down which influences a number of things. 30mm is just a standard baseline starting point. I tried a 32mm setting and it felt like riding a chopper. :shock:  

Let me know what you settle on.

Greg

Greg,

I received the Ractech 0.95 kg (from Louie here, louie@partmonster.com). THe packaging was incredible (he put it inside a huge box full of peanut foam).

Here are some numbers I measured when I had things apart:

- OEM inside fork diamater : 37 mm

- OEM spacer OD = 34 mm

- OEM fork cap length (from where it presses against the spacer to where it touches the fork tube) : 19 mm

- OEM spacer length : 200 mm

- With fork fully extended, the total inside length : 445 mm

The stock OEM springs was 237mm with an OEM spacer of 200 mm. The RaceTech springs are 315 mm long so I cut the Racetech spacer to 130mm (the RaceTech springs came with a single aluminum tube of about 600mm long), but ended up with 129mm as I cut with a reciprocal saw.

I measured the static sag with me (165 lbs) and my gear for a total weight of 180 lbs of about 33mm (can't really accurately measured it. I used a zip tie). For now, I'm keeping the same Honda SS7 (5w) oil in there. Took a short ride around the block and it's definitely firmer. Will going back to 10w make it even firmer? I don't think I want it to be any firmer.

THanks again.

-Mike-