fork sag on 69 T350 with external springs

[jabcb]

Any tips on fixing the fork spring sag on a 69 T350?
Its the model year for the T350 & has external springs.

[Coyote]

At my age there's a sag in my forking spring 24/7 Seriously I don't know anything about your bike. Is there a way to install some sort of collars?. I know on internal springs that plugs are used. You need the opposite. Something to pre-load them. My Dennis Kirk catalog doesn't show anything by Progressive. You might check with them though.

[silverstone]

change springs, oil, seals, and O rings

[jabcb]

The springs are also used on the T305.
Looks also to be the same as the T20 / X6 springs except those are chrome plated.

Per the T20 parts manual, the springs are ID 39 mm, OD 48 mm & length 178.5 mm.
Don't know if the spring length is correct.


The T20 doesn't have the boots.
Above the spring it has part #17 (see diagram) to locate the spring but doesn't have part #16.

Part #16 locates the boot.

I'm thinking about using a PVC spacer & changing the order of parts, starting from the top:
1) part #16 to locate the boot
2) PVC spacer
3) part #17 to locate the spring
4) spring

The boots conceal the PVC spacer with this ordering of parts.

Any thoughts or suggestions?

Parts diagram:

[jabcb]

Interesting possibilities that might also apply to the T500?

Some of the early Triumphs & BSAs also used external fork springs.
I've noticed listings for Triumph springs that are 49 mm OD & 248 mm long.
Also noticed shorter springs for a lower ride height that are 203 mm long.

49 mm OD is almost the same as the Suzuki 48 MM OD.
But the Triumph springs are longer.

So could new Triumph springs work in a T350 or T500?
How much would they need to be shortened?
What are the pros/cons for shortening fork springs?

[oldjapanesebikes]

Been wondering the same thing - looking forward to hearing from any experts out there !

[akendall1966]

I would be reluctant to declare my self an expert.... but have played a little with spring rate and pre-loading on a dirt bike.

Increasing the spring rate will stiffen up the ride throughout the suspension travel and may become too harsh. Damping although not very sophisticated on these old bike can become compromised particularly rebound damping and the springs push back harder. Upside you will retain all your suspension travel. You could increase the weight of the fork oil to help rebound, but this will probably make the compression even harsher.

Pre-loading loads up the spring so you have to input higher initial force before the spring starts to compress than without pre-load. once the spring start to compress further it behaves the same as before so the ride should be closer to that you experience without preload. Rebound damping is less compromised as the spring is push back with similar forces as stock. Down side is because you have compressed the spring with pre-load you have reduced the amount of travel before the spring becomes coil-bound ( coils touching each other and cannot compress further) so you may make your forks bottom out sooner (reduced travel). Of course if the bike is sagging too much your have reduced travel to start with so you still might be better off.

If you like the way the bike rides now at the front, then add some pre-load just to hold the front of the bike up more is probably OK. If it too soft and wallowy perhaps its worth getting a pair of springs wound to a higher rate. Most spring makers will have a tester so they could measure the rate of your current springs and make something a bit stiffer. Of course pre-loading can be done cheaply and changed quickly. I know a lot use PVC tube for spacers on internal springs but as there fork tube to contain the PVC and stop it barrelling, I would be inclined to make some spacers in aluminium tube for external springs, but I am cautious on these things.

Suspension is about 20% science / 50% black art / 30% personal preference IMHO, and I understand about 10% of all of it so feel free to dismiss this as a load tosh if you wish.

[jabcb]

Thanks. I hadn't thought about the PVC barrelling.

The fork springs on the T350 can sag so much that most of the travel is used when you just sit on the bike.


Have done a little more research.
Triumph did have a part that would easily work as a metal spacer that looks to be about an inch long.
But it has a 33 MM ID & the T350/T500 fork tubes are 34 mm OD.


The Triumph Tiger Cub is a lightweight 200c single-cylinder.
It has external fork springs that are 8.5" long.
Spring rate is 4.285 kg/cm.
If those springs are shortened to 7", you get a spring rate of about 5.2 kg/cm or about 29 lb/in.

The Triumph 500/650s had 10" long springs.
Lowering springs that are 8" long & have the same spring rate are available on eBay.
Haven't found the sparing rate yet.

[akendall1966]

Small correction if I may - spring rate is constant so shortening the spring won't change it rate. Unless its a progressive or multirate spring and the bit you chopped out was a different rate to what is left.

Another source of springs may be from modern bikes as the ID of the internal springs will be approaching what you need I have some from some white power (KTM) forks in the garage that came up at 32.5mm ID and they were from 42mm upsidedown forks some of the bigger 50mm might just get you the ~2mm extra.

[jabcb]

The T350 fork tube is 34 mm OD.
In addition to that, the spring needs to fit over parts # 13 & 17 in the diagram.
The stock spring has an ID of 39 mm (per a parts manual).

I checked the Progressive Suspension website & didn't find any of their springs that would fit.
Please let me know if you are aware of any that might fit.

----

The formula for spring load is: P = (G * d^4 * F) / (8 * N * D^3)
where:
P = load in pounds
F = deflection of all of the coils
N = number of active coils

spring rate = P / F = (G * d^4) / (8 * N * D^3)

So if you shorten a spring by cutting off 1/2 of the active coils, you double the spring rate.


In my calcs I took a bit of liberty in that I used a ratio of lengths rather than the ratio of the number of active coils.

[akendall1966]

Ooops - yes sorry I was thinking spring index.

It was just speculation that some of ths bigger upside downs might be in the ball park of id you are looking for.

[Powderfinger]

I would cut a few sets of spacers in 2.5mm increments from PVC, a power miter saw works really well for a clean straight cut. Do some testing to see what works best. Then get some made from aluminum. They will make big difference and should put your ride height where it belongs.

[jabcb]

I would cut a few sets of spacers in 2.5mm increments from PVC, a power miter saw works really well for a clean straight cut. Do some testing to see what works best. Then get some made from aluminum. They will make big difference and should put your ride height where it belongs.

I think that might be the best way to go.

An aluminum spacer with the same ID & OD as part # 17 should work fine.
I don't have machine shop contacts.
So I think I'll take the bike to a local bike shop that does a good bit of work for me as they will have the contacts.


I did find out a bit more about the Triumphs.
The 500/650s had different spring options -- the solo version has a spring rate of 26.5 lb/in.
If you start from the 8"-long lowering spring & shorten it to 7", then you get a spring rate of about 30 lb/in.


There apparently are some companies that will make custom springs.
I wonder how expensive that option would be.

[jabcb]

Found the specs for the fork springs. The 69 T350 has pretty much the same forks as the T305.

Per the T305 service manual:
http://www.oldjapanesebikes.com/mraxl_G ... index.html
111 kg / 114.5 mm = .969 kg/mm = 54.3 lb/in

That's a lot of spring for such a little bike.

Would the the 14.5 mm in the diagram be the preload when new?

[yeadon_m]

I believe the 'negative X-axis' value is how much HIGHER / upwards out of the forks the front end can rise as its unloaded.
A guess though - I've not seen such a diagram before.
Mike