Need to find some stiffer piston springs for my SUs

[Zedyone_kenobi]

Just like the title says. Who sells the springs that sits inside the SU dome?

I really think this may solve my problem I have been having. Well I least I hope it does.

[Duffman]

Would think that Bruce at ZTherapy is best source to ask.

[black gold man]

Maybe here

SU Carbs.com

Dana Britton

445 N. Broadview

Wichita, KS 67208

(316)258-2140

Maybe this will help.

Dome Spring Rate Calculation

Another method of fine-tuning your SU carbs, especially with a modified motor, is changing the dome spring for a stiffer or softer spring, or a combination of both. Think along the lines of a progressive suspension spring. A softer spring will let the SU piston lift fast, providing more airflow at lower RPM. A stiffer spring will resist the upward movement of the piston, providing a degree of fuel enrichment for acceleration. A combination of the two would allow for more airflow at the low end while providing more careful fuel metering at higher RPMs. Your goal is to find the dome spring that will allow the piston to reach maximum lift only at peak horsepower. This is just another variable along the path of perfecting your SU setup. And you thought SUs were simple...

I have calculated the approximate spring rate for my dome springs. These measurements are based on a dome spring from a set of 38mm flat-top Hitachi-SUs. You may download the Excel spreadsheet if you wish to enter your own information.

Spring Rate for 38mm Flat-Top Springs

Number of coils 28.500

Coil diameter 0.938

Wire O.D. 0.035

Free length (in inches) 4.000

lbs/in= 0.096

Rating a dome spring by pounds per inch seems a little hokey. British dome springs are rated in ounces, so multiply your result by 16 to get a comparable number. 0.096 x 16 = ~1.5 oz/in.

Typical British dome spring rates: This information was gained from MiniMania.

Yellow, 8oz part number AUC1167

Red, 4.5oz part number AUC4387

Blue, 2.5oz part number AUC4587

As you can see, the calculated rate for my dome springs is pretty low. I have a set of new Z dome springs (#16346-12210 @ $8 ea.), and they are identical to my 38mm springs. See the Dome Spring Experiment page to find out what's up with that.

http://www.jetlink.net/~okayfine/su/sutech.html

http://www.google.com/#hl=en&q=make+piston+springs+for+my+SUs+carbaraoters&gs_sm=s&gs_upl=91911l122011l1l124155l26l22l0l13l0l0l100l665l5.2l9l0&um=1&psj=1&ie=UTF-8&sa=N&tab=vw&psj=1&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=bb554c7314cf5cee&biw=1024&bih=659

[clayton 260z]

Bruce would be the man..

Also tell him what problems you think you are having...

[Captain Obvious]

I would sure hope they had appropriate springs in them, especially after being through ZT's shop. But, I understand. Straws it is! :classic:

I measured the spring rates from my round tops and then as an error check, I calculated the rate as well.

Four screw round tops of unknown origin and history, but from everything I've found on the carbs, they look to be pretty much original. One spring is stretched out about an inch longer than the other, but if you do the measurements and calculations correctly, it doesn't matter.

I got the following:

k (measured) = 0.109 pounds/inch

k (calculated) = 0.111 pounds/inch

I'm very happy with how closely the calculated reflected the experimentally determined.

Before you go swapping springs, you need to know where you are to start, right? You should probably determine the spring rates of what you currently have.

[240260280z]

Brit Data

[Captain Obvious]

Blue, Forgot about that... Thanks for the reminder.

Even though I can accurately determine the spring constant, that still doesn't tell me what I would call my springs on the open market. By that, I mean... I now know my spring constant (k), but the issue is that the people who sell springs don't use spring constant. The use paint color and "load at length".

Problem with color is that mine have been through the carb cleaner dip so many times that there's absolutely no color left on them. Can't use that.

And the problem with "load at length" (like what is on that chart) is that it depends on the original at rest length of the spring and since I have no idea if mine are the original length, can't use that.

One of my springs is about an inch longer than the other which makes "load at length" measurements invalid. I can assume that the longer one used to be shorter, but the shorter one? Who knows? Maybe it was stretched in the past as well, just not as much.

I don't like "load at length" measurements. :mad:

[240260280z]

I read that the springs are very long so as to maintain a relatively constant pressure from piston closed to open.

It has to do with the small differential change in compressed spring length relative to the rest length.

[Zedyone_kenobi]

Correct, a spring like the damper spring has MANY coils so the delta movement linearly of each coil is VERY small thus keeping the spring constant practically uniform through the small rise length.

What I am hoping is Z therapy knows what they put into my carbs and then upping it a bit. If they do not know,well then I could order a couple of sets and do a side by side measurement myself easy enough using a kitchen scale that reads grams. As long as the initial length is long enough, the numbers need not match any table, as long as they are comparable to one another, I can choose the stiffer of the two.

[Captain Obvious]

Well I know for a fact that one of my springs is even LONGER than it's supposed to be!

I suppose if the choices are few enough and different enough from each other, I could probably identify my springs through the "load at length" method.

If I'm reading that chart correctly, it appears that there are only four applicable choices for the round tops, and they are different enough that I could probably figure out what I've currently got (even if they have been tweaked some).

[240260280z]

[beermanpete]

Try here:

http://www.sucarb.co.uk/ProductCategory.aspx?ParentId=261