Fork spring recommendations

[Tom M]

When I started this thread last October I fully intended to order some springs soon. I finally got around to it!

 

Here you go Skeeve...

 

Traxxion Dynamics 1.1Kg/mm straight-rate springs:

wire dia = .195" = 5mm

length = 10.68" = 271mm

distance between coils = .330" = 8.4mm

 

'02 LM stock straight-rate springs with 11k miles (40mm Marzocchi fork):

wire dia = .187" = 4.75mm

length = 11.5" = 292mm

distance between coils = .290" = 7.36mm

 

stock preload spacer = 4.75" = 120mm long, 1.38" = 35mm OD, 1.09" = 27.7mm ID

 

By my (shaky) calculations the TD 1.1 Kg/mm springs are about 27% stiffer than stock so the stockers must be about .8Kg/mm.

 

Spring calculator HERE

 

TD recommended these springs for my weight. I suspect they might be too stiff based on what I've read here. If so they said they'll gladly do an exchange.

[Skeeve]

 

'02 LM stock straight-rate springs with 11k miles (40mm Marzocchi fork):

wire dia = .187" = 4.75mm

length = 11.5" = 292mm

distance between coils = .290" = 7.36mm

 

stock preload spacer = 4.75" = 120mm long, 1.38" = 35mm OD, 1.09" = 27.7mm ID

 

By my (shaky) calculations the TD 1.1 Kg/mm springs are about 27% stiffer than stock so the stockers must be about .8Kg/mm.

 

Spring calculator HERE

 

Well, there's really only two pieces of data missing, which is the number of free coils[1] and c-c dia. of the coils[2]. The nifty html spring rate calculator is on Guzzitech, here. Using it, I was able to determine that the spring rate of my stock SuziQ SV650 springs was 39#/in, which compares favorably w/ the empirical guesstimates of 40#/in. [This works out to .7kg/mm, which also compares favorably w/ reports by various suspension-fixing companies that the SV comes w/ .70kg springs & needs about .85kg springs for someone of my wt.]

 

What's so nifty about the spring rate calculator at the link above is that you can easily compute how many coils you would need to cut off to reach your target poundage by trial & error. Then all you have to do is determine the minimum number of free coils you'll need to prevent spring bind by dividing the fork travel you'll need[3] by the intercoil distance, ie: 6" divided by .280" = 21.4 coils. My SV springs have 26 (exactly), so I can only cut off 4 coils [allowing for some added slop to mash down the spring end & grid it flat] This would raise the spring rate to 48# - a 23% increase! In reality, the SV has less than 6" of fork travel; I forget how much right now, but it's closer to 5 or maybe 5.5", which would give a little more room for spring length reduction w/ an according increase in resistance.

 

From the free length of the stock Marzocchi springs you list above, there's not enough there to make any substantial reductions [my SV springs are about 12.5" long, which works out to about 1.75 coils removed before they're even as short as the Marz's. That's a substantial portion of the available reduction. Ouch.]

 

So what good would the Marz springs be if you can't cut them down enough? Well, you can always keep them for another, lighter bike [much as I replaced the SV's springs w/ the stock springs from a heavier, also undersprung bike that another owner had upgraded.] Or you could use them to make your own progressives[4] by cutting them to the spacer length. Or, take up amateur blacksmithing & use'em for quality steel scrap & forge your own water blade out of'em...

 

In my case, I should have enough combined spring material from the Guzzi & SV springs to make an interesting pair of progressives. IIRC, the Showa's 41mm OD stanchion is the same as the Marzocchi's, despite their difference in design [standard vs. upside-down] so the spring ODs should be roughly identical. We'll see...

 

Ride on!

 

[1] - This includes fractions of a coil. Best way to count'em is to start where the spring takes off from the flattened end coil & count coils to the far end minus the flattened end coil, plus any portion (rounded to the nearest 1/8th coil is fine for our purposes) to where the flattened end coil meets the full dimension spring wire.

[2] - Center-to-center dimension is most easily determined by taking the outside diameter & subtracting one spring wire diameter. IE - OD of the spring being 1.350" and the spring wire being .185"dia, you end up with a c-c coil dia. of 1.165". Sorry 'bout the SAE units, but the html calculator at the link above is for us Yanks, who still resist the whole metric system idea, despite its official adoption by our country...after more than a century and a half!

[3] - This is = to the entire range of travel of the fork, not just the remaining travel when at rest. Best to use the official suspension numbers of (for instance) 6.5", not just the 4.25" of actual travel you can measure when holding the front brake & bottoming the forks by bouncing...

[4] - The reality being that most progressive springs are just wound at 2 rates, not a continuously variable one. As the two sections compress at their combined rate, the shorter, closer wound section coil binds and the spring rate undergoes a rapid rise. That's all the "progression" most people need: softer for the initial hit, stronger to prevent hard bottoming.

[docc]

using the Guzzi spec Ohlins springs I hope to put in the sport's Marz forks I counted 14.5 free coils, 0.1855 wire diameter and 1.09" coil diameter. This calculates to 92 #/", or 1.65 Kg/mm.

 

Not likely , so what gives?

 

EDIT: OK, sorry , I used inside diameter. Center-to-center I get 1.26 ", 0.1855 wire diameter and 15 free coils yielding a Kg per mm rating of 1.04 for the Guzzi spec Ohlins spring.

 

Otherwise, is it the free coils in the spring when uncompressed (free length)?

[docc]

Yo, Ratch, you have some old early Marz springs. What do they measure and how does their rate come up on the 'calculator?'

[robbiekb]

my suspension upgrades will be easy. Todd tells me what i need and i give him the money to order it and fit it, then i go ride the bike

[Guest ratchethack]

Yo, Ratch, you have some old early Marz springs. What do they measure and how does their rate come up on the 'calculator?'

Hmm. By rough bench comparison with my new replacement Wilbers progressives, I'd estimated the stock fork springs at .6 kg./mm.

 

Not sure I buy the following. With springs and dial caliper in hand, using the GuzziTech calculator at the link, I get:

 

36 lbs./in. for the stock fork springs.

 

503 lbs./in. for the stock Sachs-Boge shock spring.

 

This seems out o' whack to me.

 

I'm too tired and not motivated enough now to do the conversions.....

 

Wot d'you make of it, Chief?!

[dlaing]

 

503 lbs./in. for the stock Sachs-Boge shock spring.

 

This seems out o' whack to me.

 

Out'o'whack indeed!

I just got back from garage to find you post happilly confirming my scepticism of the conversion tool at Guzzitech.

Here are the numbers in inches from my plastic caliper:

Sachs

EDIT I struck some mis-calculated numbers and the result is even worse!!!

 

Wire diameter .44

Coil diameter 2.89 2.65

Number of free coils 4.5

#/in 517 657!!!!That can't be right!!!!

 

HyperCo spring sold as 475#/in and which helped reduce my sag compared to the Sachs spring above.

Wire diameter .42

Coil diameter 2.93 2.72

Number of free coils 4.0

#/in 454 568!!!!That can't be right!!!!!

 

I am now using a 550#/in HyperCo spring sold as a 550#/in and the sag is finally "correct" (if there is such a thing)

 

 

 

 

FWIW the HyperCoils are claimed to be top quality and about as light as you can get without using Titanium.

My guess is that the HyperCoils are made from a stronger metal that reduces weight by using a thinner coil...making the conversion invalid. <_>

[docc]

36lbs/in translates to 0.64 Kg/mm which is right in the zone of suspicion.

 

What are the actual wire, diameter and free coil counts?

[Tom M]

Well, there's really only two pieces of data missing, which is the number of free coils[1] and c-c dia. of the coils[2]. The nifty html spring rate calculator is on Guzzitech, here.

 

If you're not a geek stop reading this right now and go find a new thread!

 

I think this is how I did my shaky calculations Skeeve...

 

To get the c-c dimension add the space between coils to the wire dia. For the stock spring it's .29 + .187= .477"

 

I calculated free coils as follows :

total spring length minus 3 times the wire dia (assuming 3 non-active coils here, 1.5 to & 1.5 bottom) = 10.939" "active length".

 

10.939 active length divided by .477 (wire thickness plus coil spacing) = 22.933 active coils.

 

I'm sure this isn't totally accurate but it should be in the ball park.

[Guest ratchethack]

36lbs/in translates to 0.64 Kg/mm which is right in the zone of suspicion.

 

What are the actual wire, diameter and free coil counts?

Cross-posted from "How To":

 

Docc, the Marz 040 USD fork springs fitted by Guzzi measure as follows:

 

free length = 29.5 cm., or 11.614"

 

OD = ~3.5 cm., or (as measured) 1.369"

 

wire dia. = ~4.5 mm, or (as measured) .176"

 

free coils = 22.6

[docc]

The Ohlins springs are a lot shorter ( 9 5/8") and only 0.007" bigger in diameter.

 

I wonder if they can still be made to fit . . .

[Guest ratchethack]

Docc, I sincerely doubt if .007" larger dia. would be any limitation wotsoever.

 

It's a no-risk, low effort proposition.

 

I'd knock out a couple o' ~2" longer spacers and give 'em a try.

[Skeeve]

36 lbs./in. for the stock fork springs.

 

503 lbs./in. for the stock Sachs-Boge shock spring.

 

This seems out o' whack to me.

 

This is undoubtedly out of whack for the rear spring. You'll notice how close the computation for the bare metal springs from the forks are vs. the "high" reading for the rear spring. What you overlooked is that the "wire dia." you measured includes paint, which has (for our purposes) a Young's Modulus of zero, & hence, adds nothing to the poundage of the spring. Figure the paint on the rear shock spring is pretty thick, maybe .005-.010", which is doubled for the reading of the wire diameter. For those who have a spring of "known" poundage, ie - Hypercoil purchased aftermarket, it should be easy to establish a rough guide for how thick the paint layer is, which factor can then be applied as a rough estimate to find the stock spring rate.

 

Also, the words "plastic caliper" fill me w/ skepticism when we're discussing measurements in which .005" variations can have significant f/x due to the 3rd & 4th powers values are raised to in the course of computation.

[Guest ratchethack]

This is undoubtedly out of whack for the rear spring. You'll notice how close the computation for the bare metal springs from the forks are vs. the "high" reading for the rear spring. What you overlooked is that the "wire dia." you measured includes paint, which has (for our purposes) a Young's Modulus of zero, & hence, adds nothing to the poundage of the spring. Figure the paint on the rear shock spring is pretty thick, maybe .005-.010", which is doubled for the reading of the wire diameter. For those who have a spring of "known" poundage, ie - Hypercoil purchased aftermarket, it should be easy to establish a rough guide for how thick the paint layer is, which factor can then be applied as a rough estimate to find the stock spring rate.

 

Also, the words "plastic caliper" fill me w/ skepticism when we're discussing measurements in which .005" variations can have significant f/x due to the 3rd & 4th powers values are raised to in the course of computation.

Astute observation, Skeeve. Just took another swag at the GuzziTech handy-dandy spring calculator. All my original measurements confirmed on the Sachs-Boge spring. No offense meant, but I'm using an "adult" dial caliper, get significantly different measurements than Dave, and I also count # free coils differently:

 

Wire dia. = .430"

 

center-to-center dia. = 2.710"

 

Number free coils = 5.0

 

GuzziTech spring calculator rate = 505 lbs./in. Now how it was 503 first pass remains a mystery?!

 

If you back the wire dia. down by .010", all else left the same, the spring rate goes down to 459 lbs./in.

 

That seems closer to expectations, alright. Might even be acceptable.

 

Wot d'you make of it Chief? (Part II)

[dlaing]

Wot d'you make of it Chief? (Part II)

You talkin' to me?

Want me to post pictures of my "Adult" caliper?

It is not nearly as accurate for measuring small things, unlike yours.

So, I stick to the plastic caliper.

It is more than accurate enough to within a hundredth of an inch, but to within a thousandth it is only an estimate.

Good point about paint thickness, that could reflect the difference.

Do you really think spring material does not make a difference?

To confirm, I'll let Ratchet use his Adult caliper on my....never mind, bad idea.

HyperCo boasts about how powdercoating is the ideal choice because it is thinner than paint