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Discussion Starter · #1 ·
My '68' T120 seems to have a bit too much fork sag. I measured the springs when I had the forks apart and they were in spec length wise. I have no way of measuring the rate. I'm getting over two inches of sag from static (frontwheel off the ground) to weighted (me sitting on the bike). I weigh around 140 lbs. Does that sound to be in the normal range for these oldsters or is it a bit too much. I feel like I'm riding down hill and would like to raise it up a bit.
 

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Progressive Springs

Your bike is a 1968 model that makes your springs over 40 years old. The metal has fatigued. If it was a car you'd have swapped them out a long time ago. You could try and do all kinds of things with them but the real solution is to put in a set of progressive fork springs. They aren't expensive and they perform much better than the originals even when they were new.

Garanteed, you'll be asking yourself, why didn't I do this as long time ago?
 

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Discussion Starter · #3 ·
It is a '68' but I don't know how old the springs are. The PO had installed internal springs to supplement the external springs the bike came with because he weighs over 300lbs. I think he put new externals on then too I but can't confirm it. I'm not disputing you, just mulling the possibilities here but it seems that if the metal had fatigued, they would have collapsed some and wouldn't measure the correct length. Can a spring loose its spring rate (stiffness) an maintain its rebound length? I don't know. Progressive springs could be the cure I'm looking for though. Do you know of any sources that do the progressive external springs for these old bikes?
 

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Many, if not the majority of bikes are ridden with too hard front springs. Many run topped out, which means when you ride over a dip in the road, the wheel has to fly because there is no movement left for the forks to track the surface. Not only that , hard springs give a harsh ride. I do not say your springs are not worn, but they have to have the ability to track the ground so do not be tempted to load up the spring rate so your are topping out. The other thing to watch is again, many people over damp the forks, thus preventing them again from tracking the road surface.
You want your suspension to be able to move quickly or you will have a lot less contact with the road.

Re the downhill feel, just check your rear suspension has not also been 'hardened'.
 

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Discussion Starter · #5 ·
Panda, the rear shocks are new (after market but new. stock length.) and set to the lightest setting. I was wondering if 2" was the "normal" sag for stock suspension with a lightweight such as myself aboard. I am running 190cc's of 20 wt. Belray in each leg so it shouldn't be 'over dampened'. I was considering adding some oil or going to the next heavier oil to stiffen it a bit as it nearly bottoms out when hard braking is applied.
 

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It does sound as if your springs are weak, but try a standard set, and only go to stronger ones if you are absolutely sure you need it.
Also look over the forks, I found one 650 running on filed down big washers for 'bearings'. It can be scary seeing what some folks will do. (this was done by a 'professional garage').

I fitted new standard springs to my BMW, and to be truthfull, they are too hard and will top quite easily, yet many on the bmw forum advocate using stronger springs ?? I am not light either being about 90 kilos.
 

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Jimmy bush I don't think your forks are nearly the same as mine, But I would look at three things.
Firstly the original fork springs can be re tensioned by a spring smith even though they measure the same. If you are satisfied they are okay, reuse them.
Secondly what type of oil have you got in them? There may have been an oil leak or when you filled them you may not have put the correct quantity or grade of oil in. You can of course use thicker grade oil. I know many of the older guys do this to overcome the problem.
Thirdly increase the volume by say 20% then try it. If it works great, if it's too stiff using a syringe at the drain plug to measure and draw off the oil in small equal amounts from each fork and then tested.

Your problem is most likely the quantity and oil grade. Also use a syringe to fill the forks with oil. You may also have slightly different fork. Check the year models around you bike to see what volume and grade oil these bikes have. I.e. 1967 -1969 there may have been a small change.
 

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If you want to measure your spring rate, make up a rig with a rod attached to a base to keep the spring straight but will allow it to slide as it collapses. A piece of threaded pipe and a flange screwed to a board will do the trick. Then rig a lever that will allow you to compress the spring. Now, put your base/rod rig on a bathroom scale, slide the spring over the rod and slowly compress the the spring until you just get the coils to all touch. It is just at the point of touching that you read the scale and this is the spring rate. Helps to have two people to do this. You can only do this on for a fixed rate spring. If you keep pressing down after the coils touch the scale will continue to climb giving you a false reading.
regards,
Rob
 

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Discussion Starter · #9 ·
I changed nearly the complete front fork assemblys after I took them apart because they were trashed. I have new stanchions, bushings, restricters, shuttle valves, seals, damping sleeves, gaiters, and l/h lower. The only thing not new is the r/h lower, the dust excluder sleeve nuts, spring abutments, cork washers, and Springs. As per my shop manual, I put 190 cc's of Belray 20 wt in each leg. I thought fork oil viscosity and volume only affected the damping. Changing it won't change the sag will it? I know it needs some sag so it doesn't top out, I still haven't found out what sag it should have. It just seems like 2" is a bit much.
 

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jimmy bush at second thoughts I had a similar problem you may have to search for the post with photos. Did the new stanchions’ have fairly big holes at the bottom or was there a small hole slightly further up? Point being even though they look the same they are sometimes different (you might have to close the holes). Check what your old ones had also check if they are the same length as the old ones. With the bigger holes the restricter rods will force the oil out quicker especially if you use thinner oil.

Really try thicker oil 80 or so with increased volume.

End of the day if it’s sagging from the normal position it may be something else. How are the shocks normally if bounced up and down?
 

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2" is (I believe) about 30% of the total travel (if not more); so, yes, it's significant. Heavier oil will keep them from bottomng out, but won't affect the sag; so if that's your main issue, change the oil.
 

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Sagging Forks

Hope this may help.

Try placing spring on the ground end on and press down as hard as you can. You should be able to generate approx 100lbs downward force. Spring will compress. Measure distance compressed. Believe t120 fork springs to be about 60lb/inch so you should be able to compress springs about 1.66 inches. This is a bit crude and you may have to do it with springs in the stanchion but it is a guide. I cannot find a figure for the spring rate but will look further when I get a chance. I have a load of factory data here just a question of finding the right source.

As for the oil, factory manual states SAE 20 for temps below 60F and SAE 30 for temps above. SAE 30 is not the same as 30wt as found in modern fork oils.. Equivalent rating in a modern fork oil would be about 50wt if you could get it. This is the cause of bottoming out but not the fork "sag". SAE rated oils are available from specialist oils suppliers, usually industrial oil suppliers or classic car suppliers.
 

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Discussion Starter · #13 ·
Thanks for the input and suggestions. I think I going to try a spacer under the abutment to prelad the spring a bit more. I have some old forks in the shed and I'll borrow some of the washers or ever some of the abutments from them and try it like that. If that cures it, maybe get some new springs. I have to borrow the stanchion installation tool again so it may be awhile before I get to it.
 

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Hi Jimmy Bush.

Do you or any of the other members have an exploded view and parts list of the front forks for your '68 Bonneville from your parts book or manual that they could post on the forum please. I would like to compare them to what I got.

BB
 

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Discussion Starter · #15 ·
I'm going to open up the forks again tomorrow to install some new springs. I'm am trying to get a definitive answer as to the orientation of the plastic damper sleeves. Nothing in the manual. They have a built in ring about 3/16"-1/4" wide on one end that fits snugly around the stanchion. The other end fits looser around the stanchion. I installed them with the snug ring at the top and my forks top out on full extension, like there is no damping. I pulled a set of 70 forks apart the other day and the tight ring was to the bottom. In this configuration, at full extension of the fork, it would really restrict the oil flow through the 8 holes in the stanchion just above the lower bush. It doesn't look right to me but if it is, maybe this could be my topping out problem. If not, then I have to figure a way to plug an extra hole higher up the stanchion. I asked this before and got an answer but I can't find the post so I apologize.
Also, I've seen posts that 30wt fork oil isn't the same as regular 30wt oil so what oil is appropriate. I weigh 140lbs.
Thanks,
Jim
 

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Your bike is a 1968 model that makes your springs over 40 years old. The metal has fatigued. If it was a car you'd have swapped them out a long time ago. You could try and do all kinds of things with them but the real solution is to put in a set of progressive fork springs. They aren't expensive and they perform much better than the originals even when they were new.

Garanteed, you'll be asking yourself, why didn't I do this as long time ago?
Still no answer on the progressive springs. Where can you get these?
Cheers.
 

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Discussion Starter · #17 ·
I installed some new standard springs. It helped the sagging problem. Never looked for a progressive spring. I installed the damper sleeves with the tight ring to the top and increased the quantity of oil by 20cc in each leg. Still tops out when lifting up on center stand but not as bad. I'm going to road test it NOW so see ya.
 

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Damper Sleeve

While I am certainly not an authority on this issue, I did just rebuild the forks on my 68 last week. The tight end of the plastic dampers were oriented down when I opened them up, and that is how they went back together.
 

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Discussion Starter · #19 ·
jrector; I believe you are correct. I tried them both ways and it doesn't seem to make much difference. I was rereading the shop manual and came across a note I had previously missed stating: "From engine number DU.13375 the damper sleeve is stepped and when reassembling, the sleeve is pressed in with the thick end pointing down". Unfortunately, the last iteration I used was with the thick part to the top. It seems to be working better but that might be because I increased the mount of oil. At any rate, the front end seems better balanced with the back end now so I'm going to ride it like it is for awhile. Thanks for the input.
Jim.
 

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The thick part of the spacer goes to the bottom of the stanchion. I just recently went through this. Evidently, it is meant to restrict flow when fully compressed (or, do I have that backward?) ... Anyway, for whatever reason, that's the way it's supposed to go ... I doubt it makes an appreciable difference in feel.
 
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