fork sag on 69 T350 with external springs

[jabcb]

Had a bit of luck & found NOS fork springs.
Spring OD & length match the numbers in the T20 parts manual.

Am extra busy at work so it looks like this will be a project for next spring.

[Ivan]

I installed 69 & up T500 springs in both of my 305's.... they work excellent.

I can't remember if I used 10-40 oil or 20-50... you'll have to try it and see.


Ivan

[jabcb]

I installed 69 & up T500 springs in both of my 305's.... they work excellent.

I can't remember if I used 10-40 oil or 20-50... you'll have to try it and see.


Ivan

Interesting. That option hadn't occurred to me.
I had looked for but couldn't find the specs for the T500 springs.

Haven't had my T350 forks apart yet.
But having looked at the NOS springs & online pics, I think the springs are at or near solid height when the forks are fully compressed. So using a spacer to set sag would reduce spring travel.

The old Triumphs with external springs used much longer springs (10 to 11 inches vs. the T350's 7 inches).
So they could use softer springs with more preload.

For the cafe racer bikes that don't have fork boots, another option would be to used cut down Triumph springs.
They have a slightly larger OD (2" vs. the T350's 1.9")
They might even produce a better ride if they are cut down to produce the correct sag, but one would need to do the math to make sure that's the case.

[jabcb]

Interesting find….
All-Rite Spring has an easy-to-use tool for designing springs.
http://www.allrite.com/inventory/design ... calculator

And they sell standard springs close to what is needed by they T350 & T500.
http://www.allrite.com/inventory/compre ... 012-detail
standard spring CO16550807012 has
OD of 1.962" & free length of 8.38"
wire diameter of .218" , 7 active coils , 87 #/in & solid height 1.74"

Looks like its easy to get a better-than-stock spring if you know what you are doing.

[jabcb]

Found an easier-to-use spring calculator that isn't tied to a vendor's database: http://www.springulator.com

Using approx. dimensions for the 69 T350 fork springs of:
free length = 7"
active coils = 10
mean diameter = 1.75"
wire diameter = .177"
ends closed & ground

I got:
total coils = 12
spring rate = 26.33 lb/in
stress at solid height slightly exceeds the limit
solid height 2.1"

Now the T305 service manual spec of 54.3 lb/in makes more sense -- 54.3 lb/in is for both fork springs!

Its interesting that the T500 fork springs work ok.

Does anyone have the spring dimensions for the T500 springs?

Has anyone measured the spring preload for the T350/T350 or T500 (inches of preload without any load)?
(Will be a while till I get to measure it on my T350.)

[jabcb]

Looks like I found an alternative external fork spring for the T305/T350.

They are for a Triumph bobber and only $37 for a pair including shipping.
http://www.ebay.com/itm/261270650412?item=261270650412

I bought a pair & this is the result:
free length = 8.125"
active coils = 12
mean diameter = 1.776"
wire diameter = .194"
ends closed & ground

I got:
OD: 1.97" (50.0 mm)
ID: 1.582" (40.2 mm)
total coils = 14
spring rate = 30.29 lb/in
stress at solid height slightly exceeds the limit
solid height 2.72"

Look like the Triumph springs will fit.
They are marginally stiffer: 30 lb/in vs 26 lb/in
They provide an additional 1.125" of preload
They are 0.6" taller at solid height but this might not make any difference: 2.72" vs. 2.12"
The stress at solid height is a smidgen lower but it still exceeds limits per the spring app

I think the additional preload might actually improve the sag.