Easy Fork Fixes You Can Do At Home, Part Ii
Easy Fork Fixes You Can Do At Home, Part II
This Month; Suzuki's 380, 550 and 750; Honda's 400 and 750.
Bob Atkinson
CYCLE WORLD has been dyno testing suspension for about a year now. Our original goal was to analyze the contribution of suspension to overall handling characteristics and then pass that information along to you in our road tests. As we developed our test procedure with Number One Products, curiosity got the best of us. Instead of simply analyzing stock components, we started modifying them in an attempt to correct shortcomings that were being revealed by the routine tests.
We modified street bikes, dirt bikes, anything we could get our hands on. In most cases, we were able to soften up the ride. Occasionally handling improved with changes in fork and/or shock length and spring rates.
Then it dawned on us. Virtually no one was passing this information on to the consumer, especially as it relates to road machines. Our article on the subject, entitled “Easy Fork Fixes You Can Do At Home,” began last month. In it we explained our methodology and modified some popular Yamahas and Kawasakis. This is a continuation of that article and will concern itself with Suzuki and Honda models.
Our intention is to correct the damping rates where necessary and to improve the ride. At anything approaching sane speeds, the softer suspension we are recommending will not affect handling, assuming you have compensated for the increased fork length by sliding the stanchion tubes upward in the triple clamps. For racing, the damper modifications we suggest still apply higher viscosity oil to raise the damper curve, and an increase in spring rate to handle the increased loading in turns and during braking, will be necessary.
CYCLE WORLD'S TESTING PROCEDURE
Before any modifications were begun, all of the machines chosen were ridden and evaluated for both handling and comfort. Next, each machine was taken to Number One Products and disassembled. HD-315 oil was substituted for stock in the forks. Both the shocks and forks were then dyno tested to determine compression and rebound damping. Springs were evaluated on a separate spring tester.
After a brief discussion, forks were modified, spring rates were altered, bikes were reassembled, and réévaluation began.
INTERPRETING THE CHARTS AND DRAWINGS
Dyno charts presented are identical to the ones in CYCLE WORLD road tests. The important considerations are compression and rebound damping. The rate of compression damping is graphically represented by the area above the zero line. Rebound damping is represented by the area below the zero line. This information, as well as spring rate and travel, also appears in the explanation below each graph. For a more detailed explanation, please refer to the initial fork fixes article in the July ’76 cw.
The drawings represent damper rods and/or other fork components that control the rates of compression and rebound damping. With the exception of the Honda 750, altering the diameter of the compression or rebound holes is the method used to alter damping rate. The stock damper rod is represented on the left. The individual components that make up the complete rod are labeled. With the exception of the Honda 750, the modified rod is represented in the center and on the right. The actual alterations are described on this drawing to eliminate any confusion when you modify your own bike. The Honda 750 is an exception because rebound damping is not controlled by the damper rod on that unit, up to and including the K3 model. The right-side drawing pertaining to the Honda 750, then, concerns itself with the stanchion tube and slider which together control rebound damping.
SUSPENSION DYNO TEST SUZUKI380/550
STOCK FORKS
Description: Suzuki 380/550 fork, HD-31 5 oil Fork travel, in.: 4.5 Spring rate, lb./in.: 37 Compression damping force, lb.: 8 Rebound damping force, lb.: 12 Static seal friction, lb.: 12-14
MODIFIED FORKS
Description: Suzuki 380/550 fork, Number One Products damper rod, HD-315 oil Fork travel, in.: 5.5 Spring rate, lb./in.: 26, with 1 in. more preload than stock Compression damping force, lb.: 10 Rebound damping force, lb.: 32 Static seal friction, lb.: 12-14
REAR SHOCKS
Description: Suzuki 380/550 shock Shock travel, in.: 2.5 Spring rate, lb./in.: 133 Compression damping force, lb.: 15 Rebound damping force, lb.: 200
DAMPER ROD REMOVAL AND REINSTALLATION
Before the damper rods can be modified and/or the forks extended, the rods must be removed from both fork legs. The best way to accomplish this is as follows: Place the machine on the centerstand and weight the rear until the front wheel comes off the ground. Remove the axle, front wheel, fender and any brake linkage. With the forks still bolted in the triple clamps, remove the Allen bolts in the bottom of the fork slider. This is located directly above the axle on most machines. With the bolt removed, let the oil drain, then pull down on the slider to remove it from the stanchion tube. Next, loosen the triple-clamp pinch bolts and slide the stanchion tubes downward to remove the upper fork assembly from the motorcycle. Remove the fork cap. Slide out the spring and any spacers. Turn the stanchion tube over and the damper rod should slide out. If it doesn’t, you’ll have to remove a circlip (present on some models) from the bottom of the damper rod.
Reassembly after modification is just the opposite. The only tricky part is getting the Allen bolt tight on the bottom of the slider. To accomplish this, it will be necessary to install the fork spring and cap. This keeps the damper rod from turning while the Allen bolt is tightened. Once tight, remove the cap and spring. Add oil until the rod is just covered. With the rod covered with oil, move the slider up and down. If the oil level drops, repeat the procedure until the oil level remains constant and just covers the top of the rod. Reinstall the spring, cap, wheel, etc., and you’re on your way.
SUZUKI 380 and 550 TRIPLES
Initial Impressions: Except for some cornering clearance problems on early models (the exhaust system was mounted too low), both the Suzuki 380 and 550 are pleasant machines to ride. They have good stability and steer well, so you don’t want to alter the geometry; but the ride is harsh in typical Japanese fashion. Rough roads jar the rider and concrete seams on freeways are annoying because the suspension does not react properly to small surface irregularities.
Damper Rod Modifications: Compression damping on the mid-sized Suzukis is fine, but rebound is way too light. Heavier oil is not the way to go because it would increase compression > damping as well as rebound. Therefore, the best inexpensive fix is as follows: Clean the stock damper rods in soap and water. Braze the upper rebound hole shut on each rod and file smooth. This will increase rebound damping by roughly 50 percent, which is pretty close to what is needed. Travel is marginal, as well. To remedy this, make a 1-in. spacer out of a length of 1/2-in.outer-diameter by 5/16-in.-innerdiameter steel tubing. Insert the spacer in the stock cone and slide the cone back onto the damper rod. Everything is held in place by a l-in.-longer Allen bolt once the slider is reinstalled. It’s a good idea to Loc-Tite the Allen bolt to prevent it from working loose. If price is not a major consideration, a l-in.-longer damper rod is available from Number One Products with the damping modifications already performed. The Number One rod also has a tower and guide bushing design that is superior to the original’s. With this rod, the stock Allen bolt and cone are used.
Fork Springs: The stock 37-lb. spring rate is way too high. On the Suzuki, this is the primary reason the forks do not react properly to small bumps in the road. For solo riding or packing double without much luggage, we found a 26-lb. spring more to our liking for the 380. With this spring, we used a 1-in. spacer under the fork cap to increase preload. On the heavier 550, we recommend a 28to 30-lb. spring.
Suggested Rear Suspension Modifications: The damping curve on the stock shocks is not too bad, but rebound is a little heavy. Travel is minimal, which pretty much prohibits use of a more appropriate, softer spring. Because of the limitations in the stock shocks, we opted for a set of Konis, part number 76F1307. For both machines we chose 90-lb. springs. The weight difference in the bikes can be compensated for by the spring preload adjustment on the shock. Springs can be purchased from Ken Ross, S&W, Koni, or Girling. Boge Mulholland makes a good shock for the 380/550 that is adjustable in terms of compression and rebound damping. The part number is SSI 175. Boge recommends 81/100-lb. progressive springs for the 380. For the 550, use 90-lb. straight-wind units.
Residís And Cost: Altering the damping rate adds marginally to stability, especially in turns on rough roads. Extending the forks for a 1-in. increase in travel also helps the ride, but it is necessary to slide the forks upward 1 in. in the triple clamps because you don’t want to alter the geometry on either the 380 or 5 50. Modifying the stock damper rods should cost you under $ 10, even if you have to have someone else braze the rebound holes shut. If you don’t-want to do the actual modifications yourself, cost of the Number One Products kit is $44.95. The kit includes springs. Part Number is S380/14 or S550/14, depending on which bike you have. Koni shocks for either bike will cost $34 each. Springs should cost around $8.40. Either the Konis or the Boges are worthwhile for their greater oil capacity and travel. Boges cost $59.95 a pair. Springs are another $16.95.
SUSPENSION DYNO TEST
SUZUKI GT750
STOCK FORKS
Description: Suzuki GT750 fork, HD-31 5 oil Fork travel, in.: 4.375 Spring rate, lb./in.: 54 Compression damping force, lb.: 10 Rebound damping force, lb.: 16 Static seal friction, lb.: 12
MODIFIED FORKS
Description: Suzuki GT750 fork, Number One Products damper rod, H 0-315 oil Fork travel, in.: 5.4 Spring rate, lb./in.: 38 Compression damping force, lb.: 10 Rebound damping force, lb.: 40 Static seal friction, lb.: 12
REAR SHOCKS
Description: Suzuki GT750 shock Shock travel, in.: 2.5 Spring rate, lb./in.: 146 Compression damping force, lb.: 10 Rebound damping force, lb.: 190
SUZUKI GT 750 TRIPLE
Initial Impressions: The GT750 is a very smooth, very quiet, very heavy motorcycle. Like other Suzuki Triples, the “waterpumper,” as it is called, steers pretty well, has good stability and reasonable cornering clearance. It could be a Cadillac as far as touring bikes go if it weren’t hampered by harsh suspension with very limited travel.
Damper Rod Modifications: First of all, 4.3 in. of fork travel is not adequate for good control on rough country or mountain roads and it prevents the use of a softer spring, which is necessary to improve the ride. Without adequate travel, a softer spring would let the front end bottom easily. The easiest solution is substitution of a 1-in.-longer damper rod from Number One Products. As with the smaller Suzukis, this rod corrects damping and offers a superior to wer/guide bushing arrangement. If cost is a consideration, fixing the stock damper rods is not difficult. Clean both rods in soap and water. Go to a welding shop and have the top rebound hole brazed shut. File the brazed area down until it blends smoothly with the rest of the rod. Travel can be extended in the same manner as on the other Suzuki Triples. Make a 1-in. spacer out of a length of 1/2 by 5/16-in. steel tubing. Insert the spacer in the stock cone and slide the cone back onto the damper rod. A l-in.-longer Allen bolt holds everything together once the slider is reinstalled. Loc-Tite the Allen bolt.
Fork Springs: We achieved a soft yet controllable ride with a 38-lb. spring. Preload will vary according to rider weight and whether or not your machine is equipped with a fairing, touring bags, etc. If the bike settles more than an inch when you sit on it, you need more preload. The easiest way to achieve it is to insert a spacer under the fork cap. Begin with a 1/2-in. spacer, then increase to an inch, etc., if necessary.
Suggested Rear Suspension Modifications: Damping rates aren’t bad on the stock shocks, but travel is again too limited. We recommend replacement with a set of Koni shocks and 120-lb. springs. Part number for the appropriate Koni is 76F1307. Springs are available from Ken Ross, S&W, Koni or Girling. If you prefer shocks that can be altered with respect to both compression and rebound damping, try a pair of Boge Supersports, part number SSI 175 and a pair of 110-lb. springs.
SUSPENSION DYNO TEST
HONDA CB400
STOCK FORKS
Description: Honda CB400 fork, HD-31 5 oil Fork travel, in.: 4.25 Spring rate, lb./in.: 40 Compression damping force, lb.: 10 Rebound damping force, lb.: 25 Static seal friction, lb.: 10
MODIFIED FORKS
Description: Honda CB400 fork, Number One Products damper rod, HD-31 5 oil Fork travel, in.: 5.25 Spring rate, lb./in.: 24, with 2 in. pre load Compression damping force, lb.: 10 Rebound damping force, lb.: 40 Static seal friction, lb.: 10
REAR SHOCKS
Description: Honda CB400 shock Shock travel, in.: 3.0 Spring rate, tb./in.: 107 Compression damping force, lb.: 6 Rebound damping force, lb.: 156
Results And Cost: Extending the forks and altering the spring rate improved ride a great deal, especially on concrete roads or rough surface streets. We didn’t want to alter the geometry so we slid the forks up an inch in the triple clamps to compensate for the additional length required for the 1-in. gain in travel. With Konis in place, handling improved on rougher streets. There is a little wallow in high-speed turns, but not enough to cause alarm. Nose-dive increased during braking, but this did not affect braking distance. Except under hard braking, cornering clearance remained the same. Modifying the stock damper rods should cost less than $10. The complete Number One Products kit with springs goes for $59.95. Part number for the kit is S750-15. Koni shocks are $34 each. Plan on an additional $8.40 for springs. Boge Supersport shocks are $59 per pair. Springs are another $16.95.
HONDA CB400 SUPERSPORT
Initial Impressions: Honda’s smallest Supersport is a compact machine that isn’t overly comfortable to ride for long distances. In addition, it wiggles some when pushed, particularly when the road surface is not smooth. There are three reasons for this. First, there isn’t enough rebound damping in the front forks. Second, the front forks flex too much because of insufficient stanchiontube diameter. Finally, the rear shocks lack sufficient oil capacity to prevent overheating and subsequent loss of damping control. Correcting the rebound damping and changing rear shocks helps handling a great deal, but does nothing for the harsh ride. Softer spring rates help here but the effect is minimal due to limited fork travel. The forks cannot be extended using the stock damper rod because the standard guide bushing causes the forks to bind as they flex under frontal loading. In other words, this is a difficult bike to fix.
Damper Rod Modifications: Handling can be improved by increasing rebound damping. To do this, braze the upper rebound hole shut in each damper rod. File the damper rods smooth and redrill the top hole in the same location to .060 in. A 1/16-in. drill bit is roughly equivalent to that diameter. If you are not content with improving handling alone, you must exchange the stock damper rods for ones that will not bind in the stanchion tubes as the forks flex. Number One Products has designed a rod with a floating guide bushing that does not bind nearly as easily as the stock rod. With the binding problem lessened, it was possible to use a longer rod that extends travel 1 in. This complete damper rod is available in a kit that includes springs.
Fork Springs: If you choose to modify the stock rods in the interest of handling, use the stock springs. The Number One Products kit includes progressive springs with an average weight in the vicinity of 30 lb. No preload spacers are required with this set-up.
SUSPENSION DYNO TEST
HONDA CB750
STOCK FORKS
REAR SHOCKS
Suggested Rear Suspension Modifications: Without question, the spring rate is too high. Even if the stock shocks are retained, substitute 90-lb. springs the same length as stock. Better yet, replace the stock shock with a Koni, part number 76F1302. Again 90-lb. springs should be used. Springs are available from Ken Ross, S&W, Koni or Girling.
Results And Cost: Koni shocks with 90-lb. springs and an increase in fork rebound damping help handling considerably. There is no wallow in turns and the bike will hold a line when pushed to the limit. Ride, however, remains harsh. Because of fork flexN fork action remains less than perfect, even with a complete damper rod and spring change. Nevertheless, the improvement in ride on rough roads is considerable. On smooth streets there is little difference. If you want to go the handling route, modifying the stock damper rods should cost around $10. Koni shocks are $34 each. Springs again start at $8.40. If you like to ride long distances or commute on rough surface streets, the Number One damper rod kit and springs are worthwhile. Part Number is H400-14. Cost is $44.95.
HONDA CB750 K1 to K3
Initial Impressions: Honda 750s have always been popular, and for good reason. They are smooth, cruise very easily at high speed, and are not effected much by changes in weight distribution resulting from the addition of luggage, fairings and the like. Handling at low speed, especially on the early models, is not very good. They feel top heavy and want to fall over in turns. Once underway, they are fine, even when pushed well in excess of posted speed limits. Ride has improved over the years, but remains harsh when compared to that of the new Yamahas or recent BMWs.
Damper Rod Modifications: There are two types of Honda 750 forks. K1 to K3 models (1969 to 1972/3), have a complicated system in which rebound damping is controlled by the stanchion tube and slider, and compression damping is controlled by the damper rod. Beginning with the K4 model (late 1973), Honda 750s have a conventional damper rod system with compression damping holes at the bottom of the rod and rebound holes at the top. If you have an early model (Kl to K3), the ratio of compression to rebound damping is good. It’s a good thing too because there isn’t any practical method of alteration. As we mentioned earlier, compression damping in this system is controlled by four oil flow holes in the bottom of the damper rod. There are no rebound holes in the rod. Rebound is controlled in the following manner: The upper stanchion tube bushing and the lower stanchion tube bushing form an oil gallery of varying capacity between the outer wall of the stanchion tube and the inner wall of the slider. As the forks rebound toward full extension, the volume of the oil gallery is reduced and oil escapes through two rebound holes in the bottom of the stanchion tube. The diameter of these holes regulates the amount of damping. The damping rate can be altered slightly by changing the viscosity of oil. We found 10-wt. good for touring. Travel cannot be extended because moving the snap ring that limits fork extension would allow the rebound holes in the stanchion tube to pass under the upper guide bushing prematurely and that would hydraulic-lock the forks. Travel can be extended in late model Hondas (K4 on), by installing a longer damper rod available from Number One Products.
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Fork Springs: The standard spring rate is too high. A good replacement is a 28/38-lb. progressive made by S&W. It can be ordered through Webco. The part number is 896. If you have a late model Honda (K4 on), and decide to purchase the damper rod kit from Number One Products, a separate spring purchase is not necessary. They are included in the kit.
Suggested Rear Suspension Modifications: The standard spring rate on the Honda is good. For fast touring or carrying heavy loads, more control can be obtained by fitting a better quality shock with more oil capacity than stock. Two good choices are Koni and Boge Mulholland. The Koni features adjustable rebound damping. The part number is 76F1296. One-hundred to 110-lb. springs should be used. Both compression and rebound damping may be altered on the Boge Supersport shock. The manufacturer recommends use of 110-lb. springs. Part number of the shock is SSI3010.
Results And Cost: The single best thing you can do with early model Honda 750s is to change to the S&W spring. The lower progressive rate improves the ride without any sacrifice in handling. Cost of a pair of springs is $19.95. Touring riders with K4 and later models can benefit from the extra travel and soft ride offered by the Number One Products Kit. Part number is H750-18. Cost including springs is $59.95. With this kit, it is advisable to slide the forks up an inch in the triple clamps. Otherwise steering becomes too heavy at low speed. Handling improves in high-speed turns with either the Koni or Boge shocks. Koni shocks sell for $34 each. Springs should cost approximately $8.40. Boge Supersport shocks run $59.95 per pair. Springs are another $ 16.95 per pair.
None of the modifications we’ve suggested are very difficult. For the most part they require normal hand tools. Welding, when required, takes very little time and should not cost much. If there are any questions, or if you wish to modify a machine not covered here, please contact either CYCLE WORLD magazine or Brian Fabre at Number One Products. We will be glad to offer suggestions.
CYCLE WORLD Magazine 1499 Monrovia Ave. Newport Beach, CA 92663.
Number One Products 4931 Encinita Ave. Temple City, CA 91780.
