Cycle News - Archive Issues - 1970's

• • InI The diagrams an d numbers are th e ones we us e fo r th e GP fr ames. We have fo und th at n o two frames or swingarms are the same. After y o u h ave cut out th e p ieces be sure that you have the frame and swing arm scribed where y ou want it before y ou start welding the pieces in place. ' " Back to the forks. Th e ones on the YZ80 are buil t by Showa an d are the same forks that are used on the XR 75, so these m ods will work on the XR. After you have the forks out of the , triple clamps remove the fo rk b oots and you will see a snap ring that holds the forks together . Take a p air o f snap ring pliers and remove the snap ring. Use the proper tool here, don't try and pry it out, you m ay dam age the snap ring or the for k sli der. Plus it hurts whe n y ou slip and punch a screw driver through your hand. A snap ring of this type has a right an d wrong way of going back, in. The snap ring has two sides to it, one side is sharp edged and the other side is kind of ro unded. Look at th e snap ring close an d yo u can see these two sides: Always install this type of snap ring with the sharp edge away from the thrust side . As pressure is pushed against the snap ring, like the for ks topping out, the sharp edge of the snap ring is forced in to the groove and the chances of it co m ing out are next to impossi ble. With the tubes out of the sliders, remove the seals and fork stop sliders. The only thing th at is lef t on the tube n o w is the sn ap ring th at limits the fork tr avel. The best way to remove this snap rin g is to hold the fork tube in a vise and with a small screwdriver lift one end of th e snap ring up and slide it out of th e groove. Dents in the' fork tubes are kind of hard on fork seals , so the best th in g is to take a piece of radiator hose, and sp lit it down the middle. This will , acer By E.C. Birt Honda h as controlled Mini Cycle racing for some time;"but now it's slipping away from them. Yamaha has. released their all new YZ SOB. This little yellow zon ker has been available to the public for abou t nine mon th s now t, an d is th e two stro ke tuners dream co me true. As far as a stocker is co n cern ed it is almost unbeatable; win records across the Uni ted States are . stacking up in th is class and Yamaha is the bike to be at. I'm going to show yo u how to build y o ur engine into three different sizes, but first we're going to look at suspensions. All the horsepower in the world won 't ge t y o u that ch ec kered flag if you can't keep th e bike on the ground. Before we break out our hacksaw and welder, let's look at a few points th at we should keep in mind. Be sure you understand wh at you are doing before you start modifying your fram e. You can get too much travel; don 't put six in ches of travel in the rear when you only h ave four in ches in the fro n t forks. If you don't remember this y o u m ay w in d up with a very dangerous motorcycle. Compression' of the rear suspension without th e fron t suspen sion also being compressed most often occurs when you go in to a turn with the Diagram 1 I I ,I 1 -2.00 0 - 1 - 9.4" ----." 15.2 50-- I -1 power on. This gives the front end too m uch rake and the bike won 't turn. Next thing to keep in the b ack of your m ind is th at y ou want the swing, arm parallel with th e ground hal f-way " thro ugh its ar c. Th is is where the swing arm spe n ds 85% of its rac ing ti me. Also by h olding this point in min d we won' t be affe cting the rake of the bike that much. Another reason for m ain tainin g this parallel line is that ch ain problems will be minim ized. The shock ang le is very important and you want to be sure th at y o u hav e the bottom of your shocks at least o ne inch farther back than the ' top of the shock. This is calle d shock trail. I like two inches o f sh ock trail for goo d shock life. Neve r let this shock angle under 90 degrees be tween the swing arm pivot and the upper shock m ount. This mi ght cau se the shock to bend, plus it 's terribly hard on the seals and le akage is sure to follow . When you start cu t ting on your frame do n 't use a cu t ting torch. Neither the frame nor th e swing arm n eed more heat on them than is n ecessary . Mak e all cuts with a hacksaw or cu t off wheel. If you use a flame type torch you stand a good chance of changing the temper of the metal and h aving the frame break. Do it right the first tim e and you wo n 't have -to go back and do it again. Use a he ll-arc welder to do all th e necessary welding to the frame and swing arm . Before y o u start to do all the trick stuff to your bike, figure it o ut on p ap er . Don 't rush, know just wh at you want to do before you start. Most of today 's minicycles in th e 80cc clas s have 4.0 to 4.5 inch es of tr avel in the front forks and with this known figure - we can use this as a maximum fo r rear wh eel travel. Maico " uses a 40-60 percent ratio for their lo wer m o un t on the swing ar m . Sixty percent of the swingarm is in the front of the lower mount and 40 percent is behind the lower mount. To ac hieve th e rear wheel travel th at you want for your bike in relationship to ' th e swingarm , try and stay in the ratio of 35 to 40 percent fo r op ti m um handling. When you are figuring this ratio, be sure that you make your m easu remen ts from the center of the ax le, with the axle pulled all the way back in the adjusters. The sto ck shocks are too long for the midshock treatment. For repl acem en t shocks you have the all new Arnaco Golden Sh oc k that is designed for the midshock mods or the Boge shock. I h ave heard by the grapevine that Koni has a new minicycle . shock in the making "but it h asn 't been rel eased yet. On all our bi kes we use Arnaco sh oc ks fo r three reasons: 1. Arnaco has built the shocks to o ur specifications for mid-shock useage and with special valvin g, 2. Shocks can be built in almost any length from 10 in ches to 16 in ches and a stroke from two inc hes up. Spring weigh ts from 40 to 11 0 lbs. are also available. 3. They cost $65.90 and th ey are guaranteed again st rip, ravel, and tear. Look at Diagram ,, 1" of the swin garm ' and y o u can see that the lower shock moun ts cen te r line is 5.79 5 inches from th e center of the ax le' to the center line of the lower mount. From the shock mount to the swing arm we have 9.45 5 inches. This swing arm is 'set up on a 38 -6 2 percent ratio , by using 3.0 inch es shock travel and 10.5 inches fro m eye to eye we have a rear wheel travel of 4.839 inches. The bottom of th e shock h as 2.000 in ch es of tr ail . Th is leaves us .750- inch clearance b etween the fender and tire. Shock springs have always been a problem to figure ' ou t , In the new Webco catalog there is a spring formula listed under S&W springs th at is a go o d pl ace to start. See Diagram ,2 ;. , Between Arnaco , Boge, or S&W y o u can ge t springs from your lo cal de aler th at will range from 35 lbs. to 13 5 lbs. If you h ave . been keeping track y o u know that we can only use safely as much rear wheel travel as we h ave in the fro n t forks. Sin ce we have only 4.5 inches of fork tr avel in o ur YZ we 're in trou ble with 4 .839 in ch es o f rear wheel travel . Diagram 2 " MY R=T where R is rec om me nded spring rate M is s um of 65 % ma ch ine weig ht and 50 % ride r weigh t Y is swing ar m lengt h from pivot to a xle eye, divided by swing ar m len gt h from pivot to lower s hoc k ey e. Exampl e: Rider weighs 180 po und s. Mo to rc yc l e w ei«h s 2"5 po unds (filled w ith g as- an d ready to rac e). Distanc e from swi n g arm p ivot to ax le is 18" . D istance f rom sw in g arm p ivot to sh oc k eye is 15" . . M = 65 % 24S + 50 % 180 = 249 .25 Y =18 + 15=1.2 MY = 249.25 x 1.2 = 299.1 MY 299.1 R - 4-- 4 74. 8 lb. sp ri ng ra t e = = = In th is ca se, a 75# sin g le rat e spring is indic at ed;: o r, if t h e rider p ref ers. a 60 /90 progressi~e spri ng ( 60 We have two ways o ut; 1. If we use sh oc ks with 2.5 inches o f travel we will have 4.032 inches of travel in th e rear and just sq uea k by with 4 .5 in ch es of front travel. Here we would want to usc; 10.0 inch eye to eye shocks so the back of the bike is no higher than necessary, and the swing arm stays almost parallel to the gro und. 2. Change the front forks so that they have .500 inch more travel to handle almost 5.0 inches of rear wheel travel . " Now I' m gettin g ahead of my sel f and we will go back to the front fo rks in a minute; but let's finish up the rear of our bike first. Diagram 3 y '- i A B ~ 90 7 5). make a good cover fo r the vise to clamp the tubes in~ Now spray the tube with so me dychem an d machine a n ew groove .5 00 inch lower than the groove that is in the tube now. Be . sure th at y ou dupli cate the original as far as depth an d radius. After you have the new gro ove m ach in ed into the fork tube the nex t step is to set the tube up in an end mill and redr ill the fork tube bleed off holes. Space these three dampener holes just as they are stock, but .500 inch lower as we did the gro ove. Drill these h ole s wi th a No . 55 drill bit. Some of the older model tubes have an inner wall ; if your forks h ave this inner wall do n 't drill th ro ugh it. Yo u will find this inner tube righ t after y o u break th rough th e firs t tube. After yo u have the fo rk tubes re-m ach in ed and drilled, hel i-ar c th e three old holes shu t and hand file them until they ble nd into the fork tube. Polish the fork tube where the weld m ar ks are with 320 emery cloth. - The gro ove that' t he sn ap ring sat in before the modifications will no t n eed to be wel ded up because the sli der will cover up the old gro ove when th e fo rks are at their fullest p oint o f travel, an d will never reach the fork seals. Sounds like a lo t o f wor k, an d it is. But the ti me you save racing will m ake it all worthwhile. Next session we will 100 k at h o w to make engines to fit the class of your ch oice . • 13

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