1991 FJ1200: rear shock spring

[spacetiger]

Hello,

First time poster to the group; hoping to get some help and perhaps some giveback.

I have the 1991 FJ rear shock but find the oem spring is not quite up to the task.  Just so you all know, I have the oem FJ shock in a 1983 Honda GL650I.  The 91 FJ is quite close in mass to the GL650 and the FJ length is ~ 10mm longer than the GL oem rear shock.

I am looking for a place where I can get an increased rate spring.  I need at least a 700 or 750 lb/in spring; but could use up to a 900 lb/in spring.

A little spring testing background. 
I pulled the spring from the shock and tested it on some weight lifting equipment (pic 1); measuring deflection I can plot out the deflection of the spring for a given weight (max test loading in pic 2).  From this, I can determine the rate of the spring (pic 3).  The rate works out to be ~ 500 lb/in.

I dialed in all the preload of the FJ shock given the 1.5" working range of the shock and the rather long stroke of the swingarm.  Before I reassembled the rear shock, I put the shock body in the GL and moving the swingarm through the range of motion, the 1.5" translates through 5" movement of the rear axle (pic4).  So, I want to bias the working range to be within 1" so the rear does not compress too much (effectively increasing rake and reducing handling crispness).

Riding results.
I got the FJ installed and took the bike for a ride with one up and a weekends worth of crap in the bags.  The good news the spring did a reasonable but is compressing too much using almost all of the 1.5" working range.  You can determine this by reading the position of the rubber stopper (I push it up against the shock body when checking rear sag and after riding to see what range I'm using).  The range of motion of abut 1.4" indicates my riding envelop uses about 700 lbs (1.4 x 500).   I need a stiffer spring to handle two up riding etc.  Can anyone shed light on where I can get a stiffer spring for the oem shock?

Payback.
When I had the spring of the FJ shock, I had a chance to calibrate the rebound damping screw to see what setting would produce what affect.  I wanted to see if I could get a 2.0 or possibly a 2.5 rebound to compression damping ratio.  To check this, I dialed out the rebounding damping setting and put the shock in a jig to time how long it took to compress with a given weight (pic 5).  Timing it 10 times; throwing out the lowest and highest time, I have the remaining 8 trys and get an average time of 1.65 sec (pic 6).  Reversing the setup, I now test how long it take to pull the shock apart from a compressed starting point.  Test 10 times, throw out the highest and lowest time, then average the rest.  I repeat this for several rebound damping settings.  The results (pic 7) is a bit surprising.  Taking the ratio of the times (pic , you can see where you need to dial to get a specific ratio.  The oem damping adjuster goes around 4 full times or 1440 degrees (360 x 4).  you have to dial it out about 3.75 times (or 1360 degrees) to set a ~ 2.0 rebound to compression damping ratio.  You can get as high as 3.o to 1 if you go to 4 full turns. 

I really will will not be able to set the final desired damping ratio until I get the rear spring corrected and front suspension (spring and race tech gold valve dialed in) sorted out.

Thanks in advance
Jerry

[Pat Conlon]

Great first post Tiger! Welcome aboard.

We have in our Suspension Files section (page 2) a post on oem spring rates:
http://www.fjowners.com/index.php?topic=1983.0

700 lbs/in is what we show for the '91-'93 shock. Of course that's a new spring. Your testing shows what spring rate you would end up with after 24 years of service. How many miles are on your bike? Asian spring steel is not renown as being the most durable.
I suspect that replacing your spring with a 700 lb/in spring will get you back to near stock rates...nothing to write home about...
Re: a heavier spring on the oem shock...
I'm not aware that the compression valving on the oem shock can be modified to compensate for a heavier spring....heck, I don't think the oem shocks are even rebuild able.
The Penske recommended spring rate table is interesting, but again, using those recommendations on a shock that does not have the necessary valving to control the spring, would turn your bike into a Pogo stick.

Try the folks at Traxxion Dynamics...good people, very helpful and they know their shit.   Cheers Pat

[movenon]

Great information ! All I can add is that at 25,000 miles my stock spring (1990 FJ) was sacked or at least the over all unloaded length was short.

RPM also has a zeroed in plug and play shock for the FJ that you should take a look at  http://www.rpmracingca.com/proddetail.asp?prod=M%2FC%3ARPMShock%2891-95%29. 

As far as modding another shock you might take look at a newer R6 shock.  Fully rebuildable, all the adjustments you need, almost any spring rate with in reason, RT valving available if needed.  And inexpensive for almost a zero time shock.  BUT it is a mod and you will have to set it up for you not plug and play.  Another member has converted his 92 to one.

Back to the RPM shock.  It is probably the best one on the market for the FJ as it is tuned for the FJ.  Have you worked on the front end?  I note that because my front stock springs were also sacked (unloaded length was short of the factory spec's).
George

[spacetiger]

Thanks guys and the links for the info.

Looks like I'll try a 900 lb/in, 7" free length with 2.25" ID.  This should let use about 740/900 = 0.82" when riding solo + some gear.  That 0.82" will translate to a swing arm movement of about 3.5".    The FJ shock is buried in the GL body, so adding additional preload is impossible, so I'll set it up to give me a slight hedge for 2 up riding without bottoming.

I'll have to see how to dial in the rebound damping once I sort out the front end.  I cannot do this until I get the wheels sorted out.  The GL has a 16" rear wheel, so no radial tire options are available.  I'm trying to get that sorted out first before I decide what I'm going to do with the front suspension.  I tackled the rear shock first because the rear oem shock had a problem.

Thanks again guys.  I'll come back to post how it all sorts out in the end.

Jerry

[Pat Conlon]

....I tackled the rear shock first because the rear oem shock had a problem.

Yep, many of us first started down the path of our FJ upgrades with addressing the limits of oem shock.

Personally, I would be leery about investing any time or money on the oem shock, when there are so many other better options available.

Thanks Jerry, I'm interested in how it turns out for you.  Pat

[TexasDave]

Thanks guys and the links for the info.

Looks like I'll try a 900 lb/in, 7" free length with 2.25" ID.  This should let use about 740/900 = 0.82" when riding solo + some gear.  That 0.82" will translate to a swing arm movement of about 3.5".    The FJ shock is buried in the GL body, so adding additional preload is impossible, so I'll set it up to give me a slight hedge for 2 up riding without bottoming.

I'll have to see how to dial in the rebound damping once I sort out the front end.  I cannot do this until I get the wheels sorted out.  The GL has a 16" rear wheel, so no radial tire options are available.  I'm trying to get that sorted out first before I decide what I'm going to do with the front suspension.  I tackled the rear shock first because the rear oem shock had a problem.

Thanks again guys.  I'll come back to post how it all sorts out in the end.

Jerry

There are a couple of mfgs that make 16 inch radial rear tires. Avon azaros are one.  Dave

[spacetiger]

Thanks guys and the links for the info.

Looks like I'll try a 900 lb/in, 7" free length with 2.25" ID.  This should let use about 740/900 = 0.82" when riding solo + some gear.  That 0.82" will translate to a swing arm movement of about 3.5".    The FJ shock is buried in the GL body, so adding additional preload is impossible, so I'll set it up to give me a slight hedge for 2 up riding without bottoming.

I'll have to see how to dial in the rebound damping once I sort out the front end.  I cannot do this until I get the wheels sorted out.  The GL has a 16" rear wheel, so no radial tire options are available.  I'm trying to get that sorted out first before I decide what I'm going to do with the front suspension.  I tackled the rear shock first because the rear oem shock had a problem.

Thanks again guys.  I'll come back to post how it all sorts out in the end.

Jerry

There are a couple of mfgs that make 16 inch radial rear tires. Avon azaros are one.  Dave

Thanks Dave but the wider cross section (150) is a tad too wide for the swingarm.  The oem tire is a 140/90-16 on a 3" rim.  With that setup, there is less than 0.20" side clearance from tire to swingarm.  I'd have to modify the swingar on the driveshaft side.  I think it is possible and I would consider this if I had a 17" rear wheel as it opens the door for a lot of excellent tire choices.

Pat, I would consider a better rear shock option, but the GL is only worth about $3K.  And with other items to sort out on the GL build, I find for older bikes, you start looking at the oem parts other riders don't use - like a low mileage FJ1200 rear shock.  I was looking for one with the right mounting end points, correct length and at least rebound damping adjustability.  Of course it had to come from a bike close to the same mass.  I wish it had a remote preload adjustability, but you go with what $100 (w/ shipping) gets you.  Besides, I get to hang out with you guys a bit and learn some stuff.

I'm going to try to get a set of Conti Classic Attack tires.  Finally one of the manufactures are making radial tires for the older bikes.  Even so, I will be modifying a CBX rear wheel so I can use a GL1100 hub to match up to the final drive.  The front wheel is a GL1100 19" wheel.  The cross sectional set of these new tires are 100 and 120, so a tad smaller than oem size, but they are radials.  

Its nice you guys let a Honda guy post away on your site.    I will be looking for a smaller bike next year.  If the R3 is everything I keep reading about, it might make it in the fold.  Right now, my knockabout ride is a 84 CB700SC (Nighthawk).   Its a shafty too but came with 16" wheel.  I found the 2010 Honda NT700V wheels would mate up the final drive with no mechanical changes.  That opened up the door on all kinds of 17" radial tires.  I settled on the latest Michelin pilots.  I also swapped in the Superhawk fork with custom stanchion lengths and fixed 4 pot calipers, St1100 316mm discs.  The rear disc was a Blackbird 256mm with twin pot Triumph caliper/bracket.  Rear shock was a oem Kawasaki KYB with rebound and compression damping adjustability.  I got the suspension dialed it and am very pleased to get this guy out on the road on the weekends.

I'll post back when I get it sorted out.

Jerry

[movenon]

Thanks guys and the links for the info.

Looks like I'll try a 900 lb/in, 7" free length with 2.25" ID.  This should let use about 740/900 = 0.82" when riding solo + some gear.  That 0.82" will translate to a swing arm movement of about 3.5".    The FJ shock is buried in the GL body, so adding additional preload is impossible, so I'll set it up to give me a slight hedge for 2 up riding without bottoming.

I'll have to see how to dial in the rebound damping once I sort out the front end.  I cannot do this until I get the wheels sorted out.  The GL has a 16" rear wheel, so no radial tire options are available.  I'm trying to get that sorted out first before I decide what I'm going to do with the front suspension.  I tackled the rear shock first because the rear oem shock had a problem.

Thanks again guys.  I'll come back to post how it all sorts out in the end.

Jerry

Here is a source that a few of have used for springs http://www.eshocks.com/hyp_make.asp.

George

[spacetiger]

Thanks George.  I've bookmarked it as I'm looking for a rear spring for my Can Am Spyder. 

Jerry

[spacetiger]

Ordered the 0700/225/0900 Eibach spring; I thought it had a 900 lb/in rate, it didn't.  But, it is stiffer and may keep me from bottoming out.  I'll use it for a while but may need a 0700/225/1000 or 1100 spring.

Pic 1.  Pulled FJ shock to swap springs.  By the placement of the nylon washer, I can see I was bottoming out solo with some gear.

Pic 2.  oem spring performance.  Note how much preload gets added buy just installing the oem spring.  It has a free length of 7 3/16"  once in the shock, it is compressed to 6 9/16".  The last curve shows what it can hold up almost all preload dialed in (7/8 indent) - ~1,100 lbs.  I had to make this chart for the CX500 crowd as I don't think they really understand this stuff.  I'm posting here so you can see the performance of the oem shock.  The preload indents don't do a lot, so I can see why you'd just dial in all of it.  Using all of it compresses the spring only 10/32", so not much).

Pic 3.  Eibach spring with all the preload dialed in.  The spring has a free length of 7" so it is compressed less than the oem spring.  Oddly, with all preload dialed in, it looks like it can hold up about 1,350 lbs.  So, a 900 lb/ln spring should be able to hold up 1.5 x 900 = 1,350 lbs... so I have close to an effective 900 lb/in spring now.  You can see at 1,100 lb loading the oem FJ spring was maxed out but the new spring should only use 1" of working range.

Pic 4.  Eibach spring installed on FJ shock.

I will have less than 2-% sag in the rear but I am still worried I might bottom out with 2 up and gear on the bike.  But as it sits now, it feels good solo and no gear.  As I ride, I'll be able to see how much more than a inch I need.