rebuild '81 forks

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warmshed
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rebuild '81 forks

Postby warmshed » Mon Jul 08, 2019 9:16 am

Just getting round to rebuilding my 1981 RS forks, thinking of using the progressive springs, anyone used them? I believe the original springs had open coil ends and has plastic ends to engage with the open coils of the spring. The progressive springs are listed as 1970-1984, the pre 81 ones had flat ground ends on the springs so the plastic ends on mine wont fit so what do I use?
Any advice would be welcome.

It seems the 81-84 forks were updated by bmw several times and some parts no longer available, can all the updates be used on my forks?

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Rob Frankhamr
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Re: rebuild '81 forks

Postby Rob Frankhamr » Mon Jul 08, 2019 10:00 am

I think first we need to clarify what we mean by 'progressive'. In this context, there are two possible meanings. If using the word 'progressive' as a description, it means springs were the pitch of the coils gets progressively smaller throughout the length of the spring. This means that the springs get progressively harder and the spring rate progressively shorter as the suspension is compressed. Most motorbike springs and all 247 series fork springs are progressive. If, however, you go to a dealer and buy Progressive (note capital) springs, you should get springs manufactured by 'Progressive Suspension Inc.' in the USA. These are, indeed, progressive (note lower case) but have a different spring rate to the stock BMW springs. You do, however, have to be sure what you are buying because not all springs advertised as progressive are 'Progressive' and almost all bike springs are progressive.

Not everyone is happy with the feel of 'Progressive' springs which tend to be harsher and less compliant than the stock springs. This might be mitigated by varying the weight of the fork oil used. You pays yer money and takes yer choice.

There were, indeed, several detail variations in forks over the period. Hopefully, you won't have to replace too many of the parts. Most of the wear occurs to the stanchions and sliders and if they're beyond use, you are probably looking at new forks anyway. If you do have to replace parts with a later variant, it is obviously best to do both forks the same.

I think this is a case where you need to dismantle and assess before making a decision on what you are going to do. If the springs are sagging and you do need to replace them then it's up to you whether you use stock springs or aftermarket replacements.

One thing worth mentioning is that 'Rs and 'RT machines were fitted with a spacer on to of the top spring seat to compensate for the additional weight of the fairing. If this has been omitted, the springs may seem to be sagging when, in fact, they are serviceable.

Rob
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warmshed
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Re: rebuild '81 forks

Postby warmshed » Mon Jul 08, 2019 2:49 pm

Thanks Rob I understand I may well have to wait and see whats what when dismantled. I asked about the "progressive springs" as that is how they are listed in motorworks. Certainly from the picture in Motorworks they look closer wound at each end.

These forks have an unknown history and clatter on potholes especially on full extension. Are the spacers fitted the ones that pick up the last curl of the springs on the 81-84 forks as I know they were not finished by grinding the ends flat like earlier and later fork springs. I think they called them "guide supports" and the were fitted to top an bottom of the springs, could not find any reference to spacers though I could turn some up if I can find the length. I am more concerned with the clatter than the need for changing the springs.

If the ends of the springs turn out to be ground flat (which would also be the case if I buy new ones) these guide supports would not fit and so is something else used instead?

Thanks, Dave.

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Re: rebuild '81 forks

Postby barryh » Mon Jul 08, 2019 6:32 pm

Noise on full extension was what many of the mods to the valve body were designed to overcome. The valve body became sprung and the valve washer clearances were reduced to increase rebound damping. I spent years experimenting with mine and did eliminate the noise eventually. Rather than spell it all out you might want to look at Mike Fishwick's document - IMPROVE YOUR LEGS! Damper Valve Modification for 1981 Model Forks.

Google search that title and it should come up with a link.

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Re: rebuild '81 forks

Postby Roy Gavin » Tue Jul 09, 2019 12:09 am

Springs only become progressive when the tightly wound sections become coil bound, and a simple observation will reveal that very little does, and what little does quickly sags out and stays that way.
Worst are the Wirth springs which have around 250mm of tightly wound coils which are a solid spacer within 2000 km.
In the normal working range of a spring, preloaded and with rider sag, no one who has measured has ever detected any progressiveness whatsoever in a so called progressive spring.
One reason for uneven winding might be to dampen resonance in the spring - some valve springs are wound that way for that reason.
FWIW I found the Progressive brand springs work OK with Konis, if you like firm suspension, but my preference is for the softest factory spring I can fit coupled with 5 wt fluid, and the rear tuned to match.
Air is progressive, so a little more fluid will reduce the air volume and give a little measure of progressiveness at little cost.

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Re: rebuild '81 forks

Postby Rob Frankhamr » Tue Jul 09, 2019 1:48 am

Are the spacers fitted the ones that pick up the last curl of the springs on the 81-84 forks as I know they were not finished by grinding the ends flat like earlier and later fork springs. I think they called them "guide supports" and the were fitted to top an bottom of the springs, could not find any reference to spacers though I could turn some up if I can find the length.

Thanks, Dave.


No, the spacers are short lengths of alloy tube that are fitted on top of the spring seat.

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warmshed
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Re: rebuild '81 forks

Postby warmshed » Tue Jul 09, 2019 8:36 am

Do you know the spacer length, Rob? cant seem to find them on parts site.

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Re: rebuild '81 forks

Postby warmshed » Wed Jul 10, 2019 9:58 pm

Found the Mike Fishwick document, covers both pre an post 81 forks, very informative, here is the link :-
http://www.vintage-register.co.uk/libra ... 0Forks.pdf
hope to strip them down so will report back and no doubt ask for help.
Dave.

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Re: rebuild '81 forks

Postby Jon K » Thu Jul 11, 2019 6:52 pm

I put Motorworks progressive springs in when I rebuilt my bike a few years back. They do make the bike feel better planted when braking and cornering, and greatly reduce the sagging feeling - previously my bike would dip at the front even as you took it off the side stand - but I’m not sure I’d make the same decision again.

I probably compounded the problem by putting on lower bars, not any of the stock ones but some Motorworks found laying about and sold me for about 20 quid. With my weight now forward and on my wrists a lot of the time, and the rather firm springs, every bump in the road jars and starts to hurt quite quickly.

I won’t replace the springs until I’ve tried putting the original bars back on. Truth is though, I rather like the riding position and if I use my cunningly hidden behind flab stomach muscles to keep the weight off my wrists, it all works rather well. Even small potholes are best avoided though.

Naked R100 by the way. A faired bike may have less firmness issues with the springs.

If you ever want to pop over to my workplace in Basildon (again) and take my bike for a spin to try them out, be my guest.

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warmshed
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Re: rebuild '81 forks

Postby warmshed » Thu Jul 11, 2019 7:25 pm

Hi Jon, thanks for your post. May well pop over some time if only for your experience and advice. Other tan my old Velocette Clubman, these are the first forks I've played with,
The 81-84 forks seem to have been changed every year of production and only some of the parts are available. i measured the sag on the forks and came out around 2.7". I will turn up a 1" spacer to get it back to the prefered 1.5"-2". Have removed the lowers but need to release the big Fork nuts to see what springs are fitted and if any spacers or caps are fitted to the ends of the springs.

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Re: rebuild '81 forks

Postby barryh » Thu Jul 11, 2019 9:14 pm

Sag is certainly the right way to go about choosing springs and pre-load. Laden sag is supposed to be approx. 30 percent of maximum travel. How does that compare with your preferred range of 1.5" to 2" as I think your 2.7" is only marginally too much. Bear in mind that stronger springs or more pre-load are liable to make the topping out clunk worse unless rebound damping is increased proportionally.

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Re: rebuild '81 forks

Postby warmshed » Thu Jul 11, 2019 9:36 pm

I fancy trying to get things correct, its easy to reduce the preload if it does not work

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Re: rebuild '81 forks

Postby Roy Gavin » Fri Jul 12, 2019 5:25 am

Barryh, you have the cart before the horse, in an ideal world when you have found your perfect rider sag it will be at or around 1/3 of the fork travel, but the converse does not necessarily apply.

On a road bike around 50 mm rider sag at the front and 10% less at the rear works well for most.
On a soft roader (G/S etc ) around 20% more works well, but needs better matching of the spring and damper rates and good set up is much more difficult.
While minor adjustment to preload works OK there is always a point where you have to admit you have the wrong spring and fit something more suitable.
Advice as to what this point actually is is hard to find, but as a guide Ohlins seem to supply most shocks with springs set at 16/18 mm preload with the instructions not to change it by more than +- 3mm
That is for optimum performance, of course, another 3/4 mm each way may still be acceptable and make an improvement to what you have!
Information on forks is even harder to find, most forks will be preloaded 16/18 mm just screwing the caps on, and most need a bit of a push to get the thread started, so it appears that around 30% more preload at the front than at the rear will work OK.

Getting suspension working well is as much a matter of setting the balance correct , front to rear, as any other setting so advice on setting one end without stating what it works with at the other is of questionable value.
One clue you are close to a good setting is when a small change at one end makes a noticeable improvement at both ends, and sometimes that is as little as one turn on the fork preload.
FWIW forks at/around the optimum setting will dive on braking, so lack of dive is generally an indicator that the springs are less than optimum and are on the stiff side.

As is advice from someone who believes in progressive springs, they simply do not exist and their main use seems to be to warn you of someone who is giving advice which he has not personally validated.
Most of this is directed at Mike Fishwick's fork article, which while it may be accurate regarding the technicalities of the forks shows little appreciation of the dynamics of suspension set up.
But he is not an orphan there -------!

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Re: rebuild '81 forks

Postby warmshed » Fri Jul 12, 2019 3:05 pm

Removed the forks and internal dampers. The dampers parts look good with not visible wear. The stanchions have a few chrome defects from flaking so I'm getting re-hard chromed ones from Motorworks.

Springs, here I am unsure, the length when measured are 52.75cm long, they have consistent spacing along 3/4 of their length then the final coils are Slightly closer together. I was expecting open coil ends as listed for a 1981 bike that should have nylon spacers that pick up the open spring end. The springs on mine are flat ground ends and no spacers.

I believe BMW did do some "heavy duty" springs for the RS/RT bikes so I'm told. These spring are supposed to be shorter by about 20cm than the standard springs. The BMW standard springs are listed as 55cm long so this all adds up.

My springs,being shorter and no spacers do not need compressing to screw in the fork nuts and I think this is where my clanking is coming from. So I am of a mind to add a spacer around 2.5 cm on top of the springs to get overall "spring" length right, this will also give me the correct Sag according to my calculations.

So any comments or advice would be more than appreciated. I do not want a track spec bike just a good touring machine.
Thanks, Dave

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Re: rebuild '81 forks

Postby Jon K » Fri Jul 12, 2019 3:38 pm

I spent ages trying to work out how to fit the spacers onto my Motorworks progressive springs. In the end I rang them and was told to omit the spacers, that they’re not needed with the progressive springs.

My old springs didn’t look that different, still coiled tighter at the ends.

Jon


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warmshed
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Re: rebuild '81 forks

Postby warmshed » Fri Jul 12, 2019 4:24 pm

motorworks say they make their springs longer so should not need spacers, so many choices.

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Re: rebuild '81 forks

Postby Roy Gavin » Sat Jul 13, 2019 1:12 am

Warmshed, springs need to be peloaded to work properly, so your idea to add add 25 mm sounds good.
I would aim for 25 mm preload as a starting point, and see how your sag works out at that.
You don't say what shocks you have, I have found fully adjustable budget priced piggy back shocks like those sold by TEC, Banggood and others work very well and make it easier to get a balanced set up. Better than well worn 20 year old Konis, anyway!
Last edited by Roy Gavin on Mon Jul 15, 2019 2:43 am, edited 1 time in total.

warmshed
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Re: rebuild '81 forks

Postby warmshed » Sun Jul 14, 2019 7:29 pm

Roy, I measured the required sag spacing before dismantling and it is 25mm. My rear shocks are new Icon adjustarides,

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Re: rebuild '81 forks

Postby Roy Gavin » Mon Jul 15, 2019 3:09 am

I am not certain what you mean by required sag spacing, but if you have Icons on the rear set them at the position you intend to use for solo riding and measure the rider sag,
When you have that it is generally found that around 10% more sag at the front is a good starting point and close to optimum.
Remember, you are balancing front to rear!
"Progressive" brand springs will be close if you have the rear firm and they come with fitting instructions and spacer lengths.
Hagon springs are OK too and usually include a bit more information on fluid level if you are looking for a bit progression which a spring alone will not provide.
Setting front sag can be tricky without a air bleed valve, so there is the possibility that immediately after assembly the initial air pressure will be higher than it will be after a few weeks use, and create a false impression.
Not a problem with forks that can be filled / topped up without removing the top cap, but worth remembering, as the long term air gap will be the space above the oil with the forks at static sag.


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