Short throttle springs

[Steve Swan]

Mark, Honda Parts Bulletin dated March 23, 1970 states "original" springs are 128 mm long; 1.15 pounds tension.  "Modified" springs 145 mm long; 1.54 pounds tension.  I cannot find any documentation about the number of coils.

[kmb69]

Bravo Steve! Excellent detective work! Didn't think to check the bulletins. We were also missing the tension rate just measuring old springs. Very important info right there.

Based on measuring 40+ year old springs, the "28" springs are definitely relaxing or taking a "set". The K0's I measured appear to be holding their length pretty well.

Obviously, Honda changed them for a good reason. I'm guessing the "28" springs were relaxing in a fairly short timeframe.

Also makes me wonder if they changed the material since the K0's seem to be holding up pretty well for this long.

[kmb69]

Plus, excluding any additional friction implications, the throttle "pull" went from 4.6 pounds to 6.16 pounds. Bet you could feel that after a few miles.

[Erling]

Found the long springs. Was fitted for say the 5 minutes it took me to decide they were too strong for my weak arms. They now measure 141mm 143mm and twice 144mm!! took one original spring out of carburettor and it was 111mm long! 0.9 thick 18.5 outer diameter same as new ones and had 13 1/2 turn to the 19 3/4 of new ones of 1mm thickness and still 18.5mm outer diameter. With the higher carburettor slides and thicker cables never again had any troubles of cables nibles "jumping" out of the bottom. Of cause the thing has not been used since fall 78.
       Youers Erling.

[markb]

Mark, Honda Parts Bulletin dated March 23, 1970 states "original" springs are 128 mm long; 1.15 pounds tension.  "Modified" springs 145 mm long; 1.54 pounds tension.  I cannot find any documentation about the number of coils.

Thanks Steve!  That is excellent information.  It coincides with my guess of 125mm.  I'm assuming the 1.15 lbs. is when the spring is fully compressed.  That info will be very helpful for the spring winder.  I noticed my coil count is a little higher than others.  In fact my count ranged from 15 to 15 1/2 but that included the closed ends.  If I don't count them then I am very close to 13 1/2 active coils.  Can anyone else confirm this?  Thank you all for your help.  I'm very close to placing the order and feel that these repros will be very close to original.  By the way, the price should come in about $6 each.

[Steve Swan]

the other thought i have, after i read Keith's email.... is the tension in pounds ?  or is in in kg ?  the Service Bulletin does not denote what "1.15" measurment scale.  i wonder if there is somethign in the Honda Service Manual addressing spring length, seems i remember there is not..... ?

[markb]

Hmmm.  Another good point.  I would guess the units are kg.  That said if we have the wire diameter, spring diameter, free length and number of coils we should be about as close as we can get.  I will give all the information I have to the manufacturer included a cap and slide just to make sure everything fits up.  If anyone else has anything to add it will probably a few days yet before I place the order.  Thanks again.

[kmb69]

My guess and it is a guess would be pounds because:

4 x 1.15kg = 4.60kg = 10.14lb

4 x 1.54kg = 6.16kg = 13.58lb

Either one sounds like an extremely heavy throttle pull to me.
Going to do a little research.

[markb]

I've been thinking about this too.  Not meaning to get too anal about this but the force would be compression not tension.  That number 1.15 that Steve cited may be the spring rate which would be in units of force per length (example lbs./in. or kg/cm) so one would have to measure how much the spring is compressed to determine the force.  Either way it does seem that kg would be quite high but it seems odd that Honda would mix units.  Steve, what is the number of the Service Bulletin?  Again, as I said earlier, if all the dimensions match, the spring rate or force should come out in the wash.      

[kmb69]

Don't sweat the anal part. We need to get it right. The rate or force is also effected by the modulus of elasticity of a given material. These are relatively small springs, so the material selection will probably have minimal impact other than holding their size.

Yes, the spring is in compression but when you are twisting the throttle, you are experiencing the resulting tension from compressing the springs and the numbers I was citing doesn't take into account any friction in the cables, slides, etc. Nor does it take into account any leverage benefit that exists from the spool diameter on the throttle pipe. You may be correct about the number mixing but the kg force is very high. And the bulletins were written by American Honda. I have seen them mix and match before. I plan to take some "used" springs to work today to measure their force and see if there is any correlation.

I think the bulletin is #15. Unfortunately, my copy is the Rev 1/31/75 which does not include that info.

Steve, sending PM for you guessed it!

[markb]

I plan to take some "used" springs to work today to measure their force and see if there is any correlation.

I think that is a great idea.  I agree that kg seems way too high.

[Marcello Tha]

I plan to take some "used" springs to work today to measure their force and see if there is any correlation.

I think that is a great idea.  I agree that kg seems way too high.

I am not 100% convinced but in my opinion must be 1.15N (newton), International System (SI) not kg or lb.

HOOKE LAW

F = k * x

F is the force that the spring exerts when the spring is either stretched or compressed by a distance x , unit N (Newton).
x < 0, means compression, x=(a-b) , unit m (meter).
k is the so-called "spring constant." K = - F/x  , unit N/m.

so,

F= 1.15N
b= 0.128 m (extended measure)
a= ____m (compressed measure applying 1.15 N)

k= -F/(a-b)


1.15 N = 0.117267 kgf = 0.258530 lbf

The power applyied by hands to accelerate must be about 4x1.15N = 4.60 N = 0.4690 kgf = 1.03412 lbf , plus friction.

Seems plausible?


Marcello

[kmb69]

I measured 1 of each, a "28" and a K0, spring compressed to 1/2 of their free length respectively. Results as follows:
28 - at 58mm = 1.20 lbf
K0 - at 73mm = 1.90 lbf

The "28" spring came in pretty close to the numbers Steve posted while the K0 came in on the heavy side.
This is a very small sampling size and I would assess the accuracy of these measurements at +/-10%.
Also, these were both used springs that have been sitting in carb sets for who knows how many years.
The compressed installed heighth of the 28 is ~55.00mm. Pretty close, 3mm more than my measured heighth.
The compressed installed heighth of the K0 is ~60.75mm. Hum, 12.25mm more than my measured heighth.
I had not measured the installed heighth prior to reinstalling the springs tonight. You can definitely feel the difference in force.

Marcello, Based on this test, I think it is lbf. The "28" is significantly stronger than 0.258530 lbf.

I am speculating that American Honda, who published the info, used metric calipers and an "American" spring tester that would have been most available at the time.
And if they were using a "standard" valve spring tester, it would have probably been more accurate at much higher force, maybe 90-150lbf. Just guessing??? 

[markb]

Sorry for being anal again, I just want to make sure I understand.  I'm assuming lbf is lbs per foot (the spring rate or constant).  So did you measure 1.20 lbs at the compressed height of 58mm or calculate 1.20 lbs per foot based on your measurement at 58mm?

[Marcello Tha]

Dear Mark;

1.20 lbf seems plausible (kmb69 measure).

Assuming gravitational force  g=9.806 m/s²   

1lb=4.448N=lbf     1kg=9.806N=1kgf

lb is used for mass and lbf for force, but the module is the same.


Marcello