Calculating/measuring Fork Spring Rates

[HispanicSlammer]

Measuring coil length on the short side of the spring

Let me begin by addressing a few items first as to why post this in the first place. One reason is to do what I always do, (being the cheap bastid I am) - why spend good money on a spring you can make yourself for a fraction of the cost! Assuming of course you need a stiffer spring! You know you do because you changed your fork oil, its spring like weather outside and it is warm, you set your sag to 1/4 - 1/3 of the total fork travel to the optimum sag for a vfr (I split the difference) and of cours to answer the question below!

Howdy,

I finally sent my forks in to Aftershocks. They've told me a few things that I'm having trouble understanding and thought to get forum advice.

1. Rather than putting in new springs, they can modify the progressive springs I'm currently using (by cutting off the "progressive" section) and this will give the same desired end result.

($50 to cut off part of old spring vs $110 to put in new springs)

2. Age of the spring does not matter, springs actually get better with age (my current set of progressive springs have something like 17k miles on them. My old stock springs have ~25k miles on them)

3. Stock springs are typically better than aftermarket springs b/c the stock springs are "heat-wound" (or something like that) while most aftermarket springs are not.

4. Cutting off coils on the stock spring (~0.74?) can somehow modify them to a 0.95 spring rate and this will somehow be better than using new aftermarket springs that are speced at 0.95.

How can cutting coils get a better spring rate? Wouldn't this mean they would cut the coils and put spacers in? Wouldn't this reduce my fork travel? If springs get "better" with age (assuming better means consistently maintaining their spring rate), then why do I need to crank my pre-load tighter and tighter as they age?

So, if anyone has had their springs modified and can attest to successful results, I'd love to hear about it.

Cheers,

Zohar

Los Angeles

98 VFR

06 Triumph S3

total for travel 120 mm / 3.5 = 35mm

Wow so you checked it like I show you here and you end up screwing the preload all the way too the top! Something is wrong here! I guess your too darned fat for the vfr :blink: heh just like me! What do you do ? You have kids to feed! ~ and a wife who needs money for groceries! Here is what you do, you cut the stock springs down to size - thats what! You will need some tools but your a guy and guys have tools right! (well so do women who ride)

Now how do you figure out what the spring rate really is? You can look it up in your shop manual, according to everybody at vfrd your springs should be rated at 0.74 kg/mm! There is a simple way to measure it yourself! Vfr springs are steel! Go get your wifes bathroom scale, without pissing her off about it - or buy your own! And put it on the floor, clean up the fork spring first (not oily or rusty!) and get a tool you can push down on it with and keep it in line, like a shaft of a large screw driver! Thats how I did it!

First measure the total free length of the spring!

Measuring the Spring Free Length Amazing its exactly 15 inches pretty darned close anyway.

You would think they got them from the USA?

Pushing down on the spring using a large screwdriver I stop at 10 lbs tape now reads 14 3/4 inches or so

I use this as my baseline measurement and then I push down on it more to add more weight to the scale!

Pushing down on the spring more I pushed on the spring till the tape moved 1 inch from the free length reading 14 inches

It was a difficult to take a picture and do all this so I just took the photo to demonstrate then did the actual measurement after I put the camera down, the spring will want to twist up on you which is why you need a long shaft to hold it. I ended up pushing in the spring one inch and getting the scale to read 40 lbs!

Take the difference in weight

40 minus 10 = 30 lbs

Take the difference in length

14.74 minus 14 = 0.74 inches

OK simple calculation gives you the spring rate in empirical scale

30.0 divided by 0.74 = 40.54 lbs/in spring rate

convert that to metric using conversion constant (looked up on the internet)

40.54 divided by 56 = 0.72 kg/mm

wow thats pretty close to the book! And too weak for my fat a$$! I found this chart from Race Tech (vfrd members 15% off BTW)

Race Tech Diagram spring rate recommendations

According to that chart I should be using a spring rate of 1.1 kg/mm?? Jebus I already bought the 1.0 and those are stiff enough for me! - But this writeup is about you not me! I think race tech is too stiff for road applications, great for riding at the track but the street its too stiff. Me I would go with 0.9 kg/mm if I had the chance to do it again! Been there done that see here! If I could I would do it again!

How do you do that with stock springs! YOU CUT THEM SHORTER! But not without calculating how much! to get from .72 kg/mm to 0.9 kg/mm the calculation is rather strait forward surprisingly! Just figure out the percentages!

0.9 minus .72 divided that by .72 = 24% holy moly!!!

you need to cut off 24% of the coils to get a 0.9 spring rate? Or you could just settle for 0.8 kg/mm or 10% cut off! Is that even doable what if you cut it off too short? How do you know! Me I read Mark Lawrence's page on doing it and and he shows the calculations, sort of like doing your taxes but its not too difficult!

http://www.calsci.com/motorcycleinfo/Suspe...easeForkSprings

There is also a page for calculating any steel spring rate! besides breaking out the bathroom scale!

I took the measurements 26 coils, diameter .2 in outside diameter 1.2 inches = 52 lb/in divide by 56 = .928 kg/mm WRONG! doesn't account for pitch!

http://faq.f650.com/FAQs/ShocksSpringRateF...te%20Calculator

Calculating how many coils to cut

Back to cutting the springs! Now we need to figure out how many coils we can safely cut using Marks calculations it goes like this!

(a) Spring wire diameter - 5 mm

(b )Number of coils - 26

© Total free length of the spring 375 mm (13 inches)

(d) Spring binding length a * b = 130 mm

(e) Available spring travel c - d = 245 mm

(f) Travel per coil (average) e / b = 9.423 mm

(g) Fork tube travel (manual says) 120 mm

(h) Excess spring travel e-g = 125 mm

(i) Excess spring coils h / f = 13.26

(j) Number of widely spaced coils: count 18 (dual rate spring)

(k) Number of coils to cut j * 24% (from above %) = 4.376

(l) Length of widely spaced coil measured 16.61 mm

Measuring coil length center to center on long side 16.61 mm or so

(m) Length of stock spacers measured: 50 mm

Measuring stock spacer 2 inches

(n) Length of new spacer (k * l) + m + plus a safety margin 10 mm = 138 mm

add 10 mm in case you cut the springs too short and you cant achieve sag, you can always cut the spacer shorter!

(o) Desired Dynamic Sag g / 3.5 = 34 mm

(p) Saftey margin (excess spring - length to cut off) h - (k *l) = 53mm

Now to actually cut off 4.4 coils off! Me I would use my cutoff wheel then grind it off at the end - heating up the coil with my torch to flatten the edge. and your done, some use PVC 1 1/4 inch pipe for spacer material, I use aluminum pipe, or brass pipe even would work.

Attached is a microsoft excell spreadsheet I made with all the calulations above - easy cheesy

forkspring.xls

[HispanicSlammer]

I think Mark refers to Dynamic sag as Static sag - Dynamic meaning the bike with the rider on top, static no rider. Think about it Dynamic is power, moving Static means not moving. Riders are Dynamic!

[tyrroneous]

Cool. So did you check the spring rate after you cut it? You know, just to see how close you got to the desired result.

[TommyHoffman]

Won't the spring bind up at some point.

Don't heavery springs have thinker wire and/or more coils?

Either way the write up is pretty cool. Somebody has been doing there math homework.

[skuuter]

Can make coil springs heavier rate by using larger wire or more coils.......progressive rates (heavier the more they're compressed) are achieved by tighter wound coils on one end....................... :thumbsup:

[Baileyrock]

Cool stuff HS, I was playing with this on my F4i a little, but gave up for now. I was cutting off the tight wound end of the spring, I guess that was wrong! :goofy:

[skuuter]

Cool stuff HS, I was playing with this on my F4i a little, but gave up for now. I was cutting off the tight wound end of the spring, I guess that was wrong! :unsure:

On progressive wound springs, cut coils off "regular" part of spring.....NOT the tight-wound end.................. :thumbsup:

[HispanicSlammer]

Won't the spring bind up at some point.

Don't heavery springs have thinker wire and/or more coils?

Either way the write up is pretty cool. Somebody has been doing there math homework.

Well yea! we calculated that to be 130mm for a stock spring! Its just the thickness of the wire diameter times the number of coils - easy!

.5mm * 26 coils = 130mm

If you cut off 4 coils as in the exorcise to increase the spring rate you get

.5mm * 22 coils = 110mm binding length.

I found a better formula for calculating spring rates - you can only increase a springs rate by a finite amount till you run out of coils, or fork travel.

http://www.bluecoilspring.com/rate2.htm

[V4 Rosso]

Won't the spring bind up at some point.

Look at: (h) Excess spring travel

No they won't

(heavier the more they're compressed) are achieved by tighter wound coils on one end.

Larger pitch means higher spring rate.

Great write-up Slammer :thumbsup: The hardest part will be flattening the spring where the coils were cutt off. Or make a cutout in the (thick walled) spacer at the same pitch of the spring. Where did you find the spring rate of the factory springs, it's not in either the service & owners manuals.

Edited January 10, 2007 by V4 Rosso

[HispanicSlammer]

Larger pitch means higher spring rate.

Here is another missunderstanding. Spring rate is constant and has little to do with the pitch angle of the spring!

Spring RATE is determined by the number of coils, the diameter of the wire, and the mean diameter of the coil (center to center looking top down) not the pitch.

Meaning the pitch angle or distance of the coils together does not effect the rate of the spring per sae - UNTILL they bind! In a dual rate spring such as the vfr the closer wound springs will bind first - they simply have a shorter distance to travel before binding - once a spring binds the number of effective coils is reduced by the number of coils touching each other! So the close wound springs at the bottom of the fork will bind before the longer wound springs and be removed from the number of effective coils increasing the spring rate at that point....

Pitch does effect Spring LOAD - which is determined by how much load a spring can support at a given height. The rate only tells how much height will change as load is changed. A spring can lose its load height over time if steel is not heat treated properly. When a spring sags, its rate is still the same as when it was new.

In example a VFR spring has 6 closely wound springs and 20 longer wound springs. The total free length of the spring is 375mm, the free length of the close wound coils is 50mm. 6 x .5mm = 30mm, you have 20mm of spring travel before you lose 6 effective coils then the spring rate is increased because you have less coils now in the spring - that is to say once the short wound coils bind it is no longer part of the effective spring. That is how you get a dual rate spring, the shorter coils bind and you get fewer coils from that point on with a new higher rate. Here is the forumla

G * WD⁴ / 8 * N * MD³ = Spring rate lbs/in

G = Torsional Modulus for Steel = 11250000

WD = Wire Diameter in Inches

N = Number of Active Coils

MD = Mean Coil Diameter in Inches. Mean Diameter is: Looking top down center to center

I.D. = 1 Wire plus inside Diameter

O.D. = 1 Wire minus outside Diameter

8 = A Constant for all Coil Springs

Calculating the spring rate for a vfr spring at no compression and after 20mm of compression

26 coils at total free length

G = 11250000

WD = .19 in

N = 26 coils

MD = 1.2 in

lb/in divided by 56 = kg/mm

spring rate = 40.79 lb/in or 73 kg/mm

now with N = 20 coils after compressing the spring 20mm (all 6 of the short coils binded)

spring rate = 53.03 lb/in or .95 kb/mm after 20mm of spring compression!

the attacment file in the first post is the updated excell worksheet with the above formula

[skuuter]

In a dual rate spring such as the vfr the closer wound springs will bind first - they simply have a shorter distance to travel before binding - once a spring binds the number of effective coils is reduced by the number of coils touching each other! So the close wound springs at the bottom of the fork will bind before the longer wound springs and be removed from the number of effective coils increasing the spring rate at that point....

YES.....Progressive Springs or Progressive Rate Springs

[magellan]

Threatening people with all this math is a fantastic way to encourage weight-loss among VFRD folk. Thanks for always looking out for us, HS! :P

[vfrjim2002]

spring rate = 40.79 lb/in or 73 kg/mm

now with N = 20 coils after compressing the spring 20mm (all 6 of the short coils binded)

spring rate = 53.03 lb/in or .95 kb/mm after 20mm of spring compression!

the attacment file in the first post is the updated excell worksheet with the above formula

HS,

This would seem to suggest that if you remove all the close wound (low rate) coils, you wind up with exactly what RaceTech should have sold you, a .95kg/mm straight rate spring. If you do that and add the length of the low rate section to your current preload spacer, wouldn't that put you pretty close to spring nirvana?

Perhaps not, it seems too simple.

Edited January 12, 2007 by vfrjim2002

[HispanicSlammer]

Cool stuff HS, I was playing with this on my F4i a little, but gave up for now. I was cutting off the tight wound end of the spring, I guess that was wrong! :unsure:

That depends - see above two posts, might have made the best spring of all!

[skuuter]

Lots of people prefer to race (cars & bikes) with straight rate springs because of consistency on the smoother better traction surfaces. Also if you race "seriously" it is easier to change the set-up and detect differences. Helps to have several various rates at the track. Variable rate springs work well for "street" riding/ driving because of surfaces and varying speeds/ operator skill levels. Yes, Kevin, you may like your springs at the track with all the "progressive" end loped off................................... :thumbsup:

[Guest mgtx73]

I'm overwhelmed. I'm buying new ones when it comes time.

[Alan]

I'm overwhelmed. I'm buying new ones when it comes time.

Man, I'm with you on that one!!!!

[Guest bsd43]

Just found this incredibly informative thread. Thanks!

One question though -- how much weight is put on the spring before the ".74 coils" bind up and the ".95 coils" take over? It's probably not a simple linear calculation, but I'd thought I'd ask.

[Guest Viper6]

I'm overwhelmed. I'm buying new ones when it comes time.

Man, I'm with you on that one!!!!

Me too.

Can someone tell me to cut off all the tight wind coils and add a spacer of XXMM so I can start thrashing my bikes front end cheap to work with my Ohlins shock I just ordered? I have ADD... :joystick:

[HispanicSlammer]

you have ms excel? dl the spread sheet and do the calculations there. its on the orginal post bottom of the text

[JZH]

Don't heavery springs have thinker wire and/or more coils?

(Excellent work, HS.)

One way to think about length of, and strength of, a coil spring is to remember that a coil spring is really just a torsion bar that's been coiled up... If you grew up with VWs and air-cooled Porsches like I did, you may be familiar with torsion bars, which those cars used instead of coil springs (at the front, anyway). One end is held fast in the chassis, and the other is held fast in the suspension arm. When the suspension arm rotates, the steel torsion bar is twisted, which it resists, thus the car's weight is sprung. Thicker torsion bars twist less than thinner torsion bars, but they also weigh more. However, you can make the same diameter steel bar "stronger" by reducing its length. A shorter torsion bar weighs less than a longer one, and is stiffer to boot! (It would probably fatigue more quickly, too, but I'll let any resident metallurgists take care of that issue...)

A motorcycle fork coil spring is like a very long, very thin torsion bar that's been coiled up. One end is held fast in the stationary part of the fork, and the other in the movable part of the fork. When you compress the fork, you are effectively twisting the spring similarly to the way you twist a torsion bar. So, given the way torsion bars work, a stronger coil spring could have larger diameter coils, but that would add weight and reduce the distance before the coils contacted one another. The alternative, as above, is to make the coil spring shorter and make up for the lost height with a spacer, which forces the weight of the bike to be sprung on a shorter "torsion bar". That's the way I tend to think of it, anyway! HTH.

Ciao,

[Guest v4bluey]

Thanks HS for this information. It caused me much time to get my tiny mind around the calculations but I eventually got there.

One very interesting thing I did discover (as I suspected) the Vtec springs are not .74 kg/mm but according to the calculations .86kg/mm.

I pulled a spring out and measured it up, dimensions are:-

Mine is a dual rate 334mm long (workshop manual says minimum length 327), 22 coils, 16 open coils, 6 tight coils, wire .19" or 4.8mm, C/C diameter 1.2".

After the first compression, approximately 26mm or 1" when the tight primary coils are bound the remaining 16 coil section of spring is 1.18kg/mm.

One other thing I am curious about is the spring is tapered at the bottom tight wound end to a smaller diameter, only the last 3 or 4 coils. This would have a bearing on the overall calculation and would make the spring slightly lighter in terms of my calculated figures. Why does Honda do this? Does anyone know?

These calculations are all loosely based on rough measurements so are not accurate.

Thanks again HS top post.

Edited January 1, 2008 by v4bluey

[Guest v4bluey]

Asphalt,

Don't know if you're still interested, I used the formula on my Vtec spring measurements and it calculated on overall spring coils.

If you cut one coil off it increases it to .90kg/mm, 2 coils .95kg/mm and 3 coils 1.0kg/mm.

BTW the spring sleeve in my VTEC is 100mm long. I think I saw that the early VFR800's were 50mm long.

[Guest v4bluey]

I pulled the spring out again just to confirm and it seems I miss calculated a little. There are only 21 coils 15 wide, 6 thigh and the diameter is closer to 1.26" not 1.2". This changes the spring weight considerably. Now don't place too much credence on my information as I obviously don't go to a lot of trouble and I don't have fancy measuring equipment. New figures I came up with .78kg/mm full spring and 1.09kg/mm 15 tight coils.

Edited January 2, 2008 by v4bluey

[Auspanglish]

Wow... just dusting off the cobwebs on this one...

Nice thread!!