Fine Tuning My 78 250CR Suspension

[Tahitian_Red]

I've been slowly tweaking my suspension on my 1978 250CR. I've got the Ohlins piggybacks on the rear (stock length, setup for my wieght and riding ability) and they are SWEET! The front is giving me the challenge though. I'm taking it in small increments. First I switched to .44kg springs with zero preload (gives 2 1/2 inches of sag). Then I raised the oil level (10 wt) to 5 1/2 inches from the top with the the springs out and forks fully collapsed. They still bottomed a little too easily over jumps. I then added Goki replica air caps and put in 7 psi. I still feel they need some work. This weekend at the Glen Helen AVDRA race I'll run 15 wt oil, 5 1/2 inches from the top and go up to 10 psi. If after practice they need more I'll go to 15 psi, but if that doesn't do it I'll rethink my options.

This is a slow process. What are my options if this doesn't work out? Progessive springs and Race Tech emulators? Simons forks? 1983 white leg 40mm forks with spacers to reduce travel?

 

[Leftcoast leftkicker]

I've heard great things about Race Tech springs & Gold Valves but they require damper rod mods. Simons forks are vey expensive and harder to tune because of a lack of parts plus you have the whole Euro front brake stay to worry bout. Any of the Husky 40mm are better because they're more rigid, no need for spacers if you can find the WR damper rods. Definitely don't run air in any of your forks.

 

[2whlrcr]

You don't say how much you weigh and what your riding ability is.

 

[pcnsd]

I have a some questions, an observation and some generic advice.

Is this one of the early full travel Husky's with 35mm tubes and about 12" of front susp? If so, any of the later 40mm forks and trees are a bolt on and much superior to the earlier "spaghetti" forks. Spaghetti? Because that is about how stiff they are.

What do you weigh? Are you running spring spacers? Use your spacers to set your pre-sag to about 1 " (+/- .25) Your race sag should be 1.5" -2.25" .44kg/mm springs are on the stiff side (24.64 lbs per inch) for most riders. I weigh 190 lbs and run .39kg/mm (21.84 lbs per inch) front unloaded sag is .75 and 1.75 inches at race sag. (1981 430 with 40mm tubes)

Air - Don't use any, IF you can tune to the pre-sag, race-sag numbers without. (Last resort)

Front oil viscosity? Start at 15 wt and work up. Most people think the best compromise is somewhere between 15wt and 20 wt. (I use 15wt.) Use real suspension fluid not motor oil. Suspension fluids have anti-foaming and anti-corrosion agents and are more consistent in their SUS-VI index. (viscosity)

Front oil level - After you have setup your front susp based on pre-sag/race sage settings, then set your oil level to allow you to occasionally bottom the suspension lightly on the biggest stuff you ride at speed (jumps, whoops, braking bumps, etc.) You can use a panduit clamp around the fork tube to check travel.

Regards,
PC

 

[Tahitian_Red]

You don't say how much you weigh and what your riding ability is.

Fat and none.

Fast novice (is that possible?) and 205lbs.

 

[Tahitian_Red]

PC,

The 78 has 35mm forks with only 9.5 inches of travel, so they are not quite as bad as the long travel 35's. I have never run air in any of my forks except my 77 Suzuki RM125, which work teriffic with 12 psi! I'm using the air caps as a fine tuning device and will replace the air volume with oil when I find a setting that feels right. I'm beginning to think that the .44kg springs I was sold may be softer than that. I'll give a report and talk more about the 40mm option after this weekends testing.


Thanks for all the input so far guys****************************************

 

[ruwfo]

Tahitian_Red,
I have a bone stock 78 250CR, so you know the stock rear shock
length is 15" c-c, non piggy back shocks.

If your using longer length shock then that you need to adjust the
forks up or down to match,so the bike turns like it was made too.

I use the same zip tye method to determine fork travel as PC
discribed, & like he said oil amount & weight is all determined by
your speed & weight.

If you go the 40mm route be careful to get forks that are close to
the same travel as you currently have. Later model Husky had much
longer travel shocks closer to 17-19"c-c so they made
the forks longer too.

For example if you used forks of a 81 250CR there close to 14" of
fork travel & had 17" c-c rear shocks. So you'd need to pull the forks
up 2" in the triple clamps to have the same balanced handling when
using 15" c-c shocks. This also requires you to be careful with your
front brake cable,as they will get stuck on the portion of the
tube stickin about the triple clamps.
Not a problem you just need to use a cable saver number plate.

I also have a 80 390CR & went the 40mm front route ,
but i felt the 78 turned & jumped fine with the stock forks - set up
right.

Later
John

 

[HuskyT]

John,

Can you post up details of your 1980 CR390 conversion over to the 40MM forks from the black buggy whips? Start a new thread if you wish as a lot of people are interested in the conversion to 40 mm's

I have forks off of a 83 CR500 and am going with an 18 " 1982 rear wheel / remote reservoir Ohlins on my 1980 CR390 ( with long 23 1/4" swingarm)

T

 

[pcnsd]

PC,

The 78 has 35mm forks with only 9.5 inches of travel, so they are not quite as bad as the long travel 35's.

QUOTE]

Shorter travel forks use a stiffer spring. You may be OK with the .44kg/mm springs.
I am unclear if you are using a spring spacer. If not, cut some an appropriate length spacers from 1/2" schedule 40 PVC. (Available from Home depot and others.) so that your forks set 1" into the travel under the bike's weight. This is the basic starting point and defines the pre-sag measurement. Then with all your gear on, you on the bike in a neutral riding position. Have someone measure how far the suspension has compressed. This is the race-sag measurement.

If you set pre-sag at 1", but the race sag measurement is more than 2.25". Your springs are too soft. If the race sag is less than 1.5" your spring are too stiff.
If your springs are soft, you can add air pressure and trim the spacer as needed to achieve the proper settings.
Lastly, work on oil level as described above.

Regards.
PC

 

[Tahitian_Red]

UPDATE:

I went with 15wt Bel-Ray (damping feels closer to the rear shocks now) fork oil 5 1/2'' from the top with the forks compressed and the springs removed. I used 12 psi of air in each leg and had right around 2" of race sag. The Glen Helen REM track was setup pretty mild and there were no big jumps. I ended up using all but 1" of the fork travel, so if I were hitting some decent "Vintage style" jumps I should be just about right.


Now for replacing the air. Should I put in about 10cc of oil at a time till I get the same feel or would I be safe to go to 5" from the top to start off and suck some back out if needed?

Thanks for all the advice****************************************

Now on to the rear tire issue.

 

[pcnsd]

Much of suspension setup is empirical, but sometimes, especially with old bikes and a budget, it has to be trail and error. I can only suggest what I would do and I tend to be cheap.
There appears to me to be two issues.
The first is related to spring force. At 12 psi air pressure, your springs are likely too soft. The other is oil level, 1" of spare travel to is much too much.
I will suggest some options. Each is to itself.
1. Leaving everything else the same, reduce the oil level by 1/2" initially and test again until you bottom at least once a track day (Ride hard) Continue to reduce the oil levels in 1/4" increments until the above is achieved. If need be add a few CC's back in to finalize the set up. A syringe works well for this.

2. Stiffen the springs, reduce air pressure (0 is best, but I have run up to 7 psi and been happy) Then pursue final oil level. There are only two options for increasing spring rate. (OK three, but I don't like progressive springs types) Buy new springs or shorten the existing. If you choose to buy new, I will recommend both Race Tech and CR Performance as having decent products that will work in your bike.
Before cutting your springs measure the total spring length. Measure the wire diameter in thousands of an inch and count the total number of free coils.
Multiple the number of free coils by the wire diameter and multiply that sum by 1.1 (Fudge factor, you can use another number if desired 1.15, etc.) Subtract this number from the total length measurement. This is the available travel in the spring. You need at least 10" of spring travel in order not to coil bind the spring. (9.5" of actual travel)
Based on what is available, begin to cut 1" at a time from your spring, reform the end (Torch, hammer grind) and test. You can use the pre-sag/race sage setup numbers to determine the best compromise. Each time you cut the spring you have to recalculate estimated spring travel before cutting again and you need to make new spacers that put you at about 1" of pre-sag for testing.
The bad news with cutting your existing springs. Spring life is shorten due to the increase compression work of the coils. My spring handbook says the mean time between failure for properly designed chrome silicone and chrome vanadium springs (suspension springs) is about 1 million full cycles. With old bikes how much is left?

Hope this helps.
Regards,
PC

 

[Tahitian_Red]

PC,

I much prefer buying new springs to cutting my current ones (I have cut springs before and it really does raise the rate.), but thanks for the formula.

The track I was on basically had no jumps except two small table tops coming out of corners, so it really wasn't a test of the "G-out" bottoming performance. I was happy with 1 inch left on this type of track. I will get the oil level up and eliminate some of the air pressure for sure and perhaps even try some .45-.46kg springs. Maybe losing 10-15 pounds would also be a cheaper option. LOL

Thanks again for the excellent advice!

 

[Leftcoast leftkicker]

T_R: with 1" left of travel you're not fully compressing in the turn thereby making the bike harder to turn (and they're already one of the hardest)

 

[Tahitian_Red]

T_R: with 1" left of travel you're not fully compressing in the turn thereby making the bike harder to turn (and they're already one of the hardest)

I guess I'm more of a rookie than I thought. I'm supposed to use up all of the fork travel in the corners? I'm a Novice racer and the track was pretty smooth except for some rough straightaways. I thought having that extra inch left over for big jumps and really rough sections was a good thing. Maybe that's why I'm not very fast in the corners. (could be lack of talent also. LOL)

 

[Leftcoast leftkicker]

Sorry, not clear enough. It's OK to have some travel left just don't confuse that with setting up the big correctly in the first place: static sag, race sag, pre-load. Then you can fine tune for the conditions. My comment was more about the bike's inherent inability to turn on tight or flat & off-camber corners.

 

[Tahitian_Red]

Thanks Leftcoast!

I am having some trouble adapting to the Husky cornering technique.

The bike feels so solid and stable everywhere else that I'm willing to put the time in to try and improve the bike's setup and my technique. I will race it a few more times this year as a Post-Vintage/Evo MXer and probably use it for Vintage Cross-Country races in 2010, where I will most likely not even notice any cornering deficit.