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Discussion Starter #1
Now that I have a couple hundred miles on my restored 72 Ive noticed how lame the front springs are. I assume they are just worn out. Do the new replacement, progressive springs make a big difference or am I expecting too much from an old setup? I just cleaned everything up, put in new seals and new ATF. My rear shocks are new and perform really well. Any tips or suggestions much appreciated.
 

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Before I go with the Progressives, I would first measure stock spring length to make sure it is up to spec.
I would then get pre-load set correctly...you might have to make up some spacers.
Next I would dump the ATF and try tuning it with real fork oil. 10 wt or 15 wt is a good starting point. I am not sure, but I think ATF is between 5 wt and 10 wt, which may be too light.
Lastly, make sure your front end is properly aligned with no binding in the forks.

Progressive springs are ok and a lot of people like them. Others like me prefer straight wound, especially for aggressive cornoring. I would use them if no alternative was readily available.
 

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Good advice from motobob.
I fitted progressive springs to my T140D and found them too stiff tbh.The forks would crash into bumps and potholes them 'top out' badly.I ended up refitting the origional springs and using a 10w fork oil.
 

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Definitely NOT. They are too stiff. My friend was having problems getting his T140D off the centre stand until we removed the progressive springs last weekend. He also said that the bike handles much better now with the standard springs.
 

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I'll admit to being insensitive to the subtleties of suspension performance but when I needed to replace the fork tubes on my T140E I did install progressively wound springs.

I didn't ride the bike without the progressives so I can't offer a comparison, but I'm satisfied with the way my front end feels and the way the bike handles. I don't know if that's helpful information, but it's all I have to offer.
 

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I'll admit to being insensitive to the subtleties of suspension performance but when I needed to replace the fork tubes on my T140E I did install progressively wound springs.

I didn't ride the bike without the progressives so I can't offer a comparison, but I'm satisfied with the way my front end feels and the way the bike handles. I don't know if that's helpful information, but it's all I have to offer.
Haha ! Looking at your Avatar pic I can see why you cannot notice any difference at the front end :D
 

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Progressive springs just get harder,the more they're compressed (increasing spring rate).
You'll get that effect,to some extent,because of compressed air trapped above the oil in the fork leg.

Personally,I go to great length to minimize that effect and keep the spring rate more constant.This involves bleeding most of the air out of the staunchion,while holding the fork fully compressed,and then sealing the cap nut.If the ride height is then too low,I fit spacers on the springs.
I'm not the only one who does this.Air makes a lousy spring,and it's temperature sensitive.Springs make better springs,because they aren't affected by temperature.
 

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Discussion Starter #8
thanks for the great replies. I definately think mine are a preload issue. So how do I do the spacer thing? Is that an inch and a half of PVC pipe below the spring? I will try heavier oil when I go in as well. The springs look great and are of correct length so will stay with them
 

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I had progressive front springs installed last year on my 68 T120R with 20w fork oil. I have ridden over 5,000 miles since and find the front end very stable over the road and in the twisties. I don't find them too stiff by any means. Under hard front breaking I don't get the severe nose dive...for me they were definately worth installing...

Bob
 

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Progressive springs for the T140 are a wee bit on the stiff side,depends on how you like your bike set up.
Personally I like them as they are good for all the pot holed roads in the UK,as the forks are less inclined to bottom out when you hit one of the many holes in the road.
 

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Sounds like the fat boys like the Progressives and the skinny ones don't. As you can see from the replys getting suspension setup to your taste has a lot of variables.

I never saw the point of progressive springs since you could use a tad more fork oil and increase the air spring if you wanted a progressive effect. Or you could actually bump the air pressure. A bunch of Jap bikes from the ealy 80s had an air filler in top of the forks for this purpose. You could even buy an aftermarket set of top caps with the fillers.

But as Mr. Pete mentioned, this is not the best way to set up suspension. That's probably why you don't see it on modern bikes.

I do like the Progressive external springs better then the side car springs on my 67 TR6C.
 

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A couple of thoughts on fork springs and oil weights.

First, the spring whether progressive or not, should be sized to the weight of the rider. One size does not fit all.

The intention of a progressive spring is to give you more response to small irregularities and still have a higher spring rate for the heavy inputs. If your new progressive springs bottom out, then your springs are not the correct rate for your weight and/or the application.

You determine the preload by setting the sag on the bike. You can do a search for setting sag on the web and get a better explanation than I can give you. It normally takes a couple of guys because you have to pick the bike up as part of the process.

Oils made for different applications and identified by "weight" are not necessarily related. By that I mean a 10W ATF is not the same as a 10W motor oil or a 10W gear oil or 10W fork oil. This is another topic you can search on the web and get good detailed and accurate info.

Lastly, the damping systems in our forks are based on oil flowing thru orifices. It is an okay system, but does not do well across a broad range of road and riding differences. The basic problem is (I'm reaching here to remember the actual relationship) the pressure drop across an orifice increases as a square of the velocity thru it (I think I got that right). So, that means when you sit on the bike and push the front end up and down, it feels nice and cushy because that is relatively slow velocity. But that same front end hitting a speed bump at 40 mph becomes pretty much a rigid front end because the velocity of the oil trying to pass thru those same passages is orders of magnitude higher.

This could be why some have reported "bottoming" or harsher rides when installing progressive springs. Assuming the initial rate of a progressive spring is lower than standard, non-progressive springs, this would allow the fork to compress at a higher velocity than a std spring for the same bump and hence reduce the ability of the fork hydraulics to absorb the impact.

This is why cartridge forks have replaced them in all racing applications and most street bikes. The cartridge system can react to the increase in pressure and make the oil passage larger to flow more oil and absorb the impact.

Now couple our Flintstone forks design with heavier viscosity fork oil, which helps in low velocity/high load situations like high speed cornering and the front end gets even stiffer when it encounters a bad bump or pothole. If you take our style forks and a modern cartridge fork mounted on similar bikes thru the same corner at the same speed and enounter the same bump while in the corner, the cartridge fork will react to the bump, absorb it and keep the bike stable. Our fork will absorb little to nothing and jolt the front end, upsetting the bike while in the corner.

So the point is your forks are a system, and the parts work together to maximize their ability to react to a broad range of inputs. It's more complicated than just replacing springs or oil weight.

I've not looked into this so I'll just toss this out. Are their cartridge emulators offered for our forks? I can seem them being offered for the later model damper rod forks, but probably not the earlier designs. Installing a set of emulators and adjusting them for your weight and riding style would probably make the most noticeable difference to ride and handling you could ever make on one of these old girls.

regards,
Rob
 
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