The Chopper Compendium

Springer Rules of Thumb:
I have written down what I believe to be the rule for setting up a springer rocker mounting system. We have been working on and refining the rule and I will put out the initial writing while redefining the rules. I recognize the needed trail number to be 3.375” of ground trail with the bike in the unweighted position. I also adhere to a rear tire width that is 200% larger than the front tire width to set my trail at 3.375. Attached is a rule I use to account for the difference of rear tire width if smaller or larger than the 200%.

The 90* angle rule: this rule was set up to dictate the proper measuring and assembly of the springer rockers. First and foremost do not have any weight on the frontend. I block the front of bike frame with 2x4 and shimming stock. I set the front of the frame.75 to 1" above the final position of the weighted frame (containing motor transmission and rider). For lighter riders (200# or less) I set closer to .75", for riders 200# or more I go a full 1". This rule dictates the position of the rear (rigid leg) rocker attachment hole. That in turn will tell you where to place your spring leg attachment hole.

The horizontal zero rule: set a horizontal zero line that intersects centerline of axle, centerline of spring leg, and centerline of rigid leg. Proceed the same way you would set the 90* rule with bike setup.

The tire width rule: trail should be set to 3.375” of ground trail when the rear tire is 200% larger than the front tire (i.e.: front tire is 90/90/21 and rear tire is 180/55/18 the rear tire width is 200% larger). If the rear tire is not 200% larger the following equation will help adjust the trail.

Trail Adjustment= [{(rear tire width/front tire width)x100}-200]0.01
-if the number is positive add the answer to the 3.375” of initial trail
-if the number is negative subtract the answer from the 3.375” of initial trail

Eample: 90/90/21 front tire, 250/40/18 rear tire
[{(250/90)100}-200]0.01=
[{(2.78)100}-200]0.01=
[{277.78}-200]0.01=
[77.78]0.01=.78~
.78+original trail of 3.375= 4.155” of total adjusted ground trail

- About .78” (disregarding of infinites and using only three decimal places) of trail adjustment would be needed to account for the use for the example tires. Had the answer been a negative number, you would have subtracted from the original ground trail of 3.375.

Eample: 90/90/21 front tire, 140/90/16 rear tire
[{(140/90)100}-200]0.01=
[{(1.556)100}-200]0.01=
[{155.56}-200]0.01=
[44.44]0.01=-.44~
-.44+original trail of 3.375= 2.935” of total adjusted ground trail

Rule 1: place the blade (the long end) of a carpenter's square along the centerline of the steering axis. If not using raked trees, raked cups, or offset trees you can simplify this and use the centerline of the rigid leg of the springer. Align the tongue (the shorter leg) of the carpenter's square with the centerline of the axle. If using the centerline of the rigid leg, the point of the square will be where you should place your rigid leg rocker attachment hole. With this hole made you can in turn make the spring leg rocker attachment hole by setting a level at horizontal zero along the centerline of the rigid leg rocker attachment hole. Follow the horizontal zero forward to the intersection of the centerline of the spring leg. The point of intersection is where the spring leg rocker attachment hole should be placed. Once the frame is taken off the cribbing and the weight is applied to the frame, the rockers will preload the spring leg and naturally set the rocker attachment holes 10-20* of horizontal zero.

Rule 2: expanding upon rule one, if the 90* rule cannot be utilized in a flowing rocker design you may have to go to the horizontal zero rule, and that is: if a 90* cannot be achieved then set a horizontal zero line that intersects centerline of axle, centerline of spring leg, and centerline of rigid leg. This will place the unweighted rocker attachment hole in horizontal alignment with the axle. There by getting the rocker holes to be at a 10-20* off of horizontal zero along the centerline of the rigid leg attachment hole.

A good diagram from Gary Weishaupt shows the 90* angle rule and how it will affect trail and design. An example of using the horizontal zero rule would be Gary’s rude crude radiused rocker. By setting the rocker holes to zero unweighted, when the weight is applied then the springs automatically preload and set the proper angular relationship. Gary’s diagram and the point marked PR is what we are trying to achieve, realize this chart is of the frontend in the weighted position

here is garys radiused rocker in the weighted position


Some things to take into account is
1. I am using ground trail
2. I am using trail in the unweighted position
Building in the unweighted means that the cyclical action will never go below the listed ground trail. I feel good about building this way. You don’t have to be afraid to disagree or add on also. Also you will see the 90* rule is based on steering axis centerline. This coincides with true trail measuring. I believe this is why it sense.

i have made the list into a pdf so that you can print it out and laminate or just add it to the springer build that Gary posted up.

Thanks, this really helps.

ok im getting a huge headache reading and rereading this while i dont think im incapable of learning this im thinking its going to take a lil while . would you be willing to show this process step by step through pics or video sort of an idiots guide ?

Curt i have been wanting to do a few videos so i will try to get something for you. As for the rules, the tire is just to figure out how much to add or subtract from 3.375 the 90* rule just sets the hole for the rocker to attach to the rigid leg. the horizontal rule is only if you cannot get the 90*

Very intersesting read. I particularly like your "tire width rule". Thanx.

i appreciate it krymis and i like the tire rule too never thought about tires changing it that much

Very interesting krymis. I've been studying all you guys have written on springers. I will use these rules to check the bike I'm building and the springer I'm using. It's an old Randy Enterprises. It's 34.5 from the top of the lower tree to the CL of rear leg pivot. Anyway...I am using a 200 rear and 100 front tire so...
Can you explain what the tire width's and their relationship to each other has to do with what the trail should be is please? Also when you say "ground trail", how is that different than just trail? Or is it the same? Thanks.

irish,
from my riding style i have found that a 200% larger rear tire is the best size to use compared to the front. So if you have a 100mm front tire a 200mm rear tire goes right with the stated idea. This is especially helpful when you go with a real skinny or real fat rear tire in adjusting the trail #. which in my case i use 3.375" of ground trail.

Real trail is measured from a perpendicular line that intersects the centerline of axle and the imaginary line of the steering stem centerline. This is usually about 90% of the ground trail.

let me mind you that the answer from the equation you get is in inches so if you want mm measurements you would need to divide by 0.039370 for a mm answer.

also a new amendment to the tire width equation
I will from here on out base my equation on a 2.5" ground trail. That give approximately 2.25" of real trail.
The equation will also change from a 200% wider rear wheel to a rear tire of the same size.
Trail Adjustment= [{(rear tire width/front tire width)x100}-100]0.01 instead of the original
Trail Adjustment= [{(rear tire width/front tire width)x100}-200]0.01

so now the answer you get will be added to the answer because i will not build a bike with a skinnier rear tire than front tire. That answer will now be added to the 2.5"

Eample: 90/90/21 front tire, 250/40/18 rear tire
[{(250/90)100}-100]0.01=
[{(2.78)100}-100]0.01=
[{277.778}-100]0.01=
[177.778]0.01=1.778~
1.778+original trail of 2.5= 4.278” of total adjusted ground trail


the reason i did this is after a weekend of long conversations with prominent chopper builders from the 70's, i realized in the 70's they really did not have the huge tires. They were mainly a max of a 150. So i thought about it, ran a few equations and came up with answers that were not that far different from the original equation. I also started all this because i wanted to make my 250 rear bikes handle a little better. In doing so this equation helps me to get a target trail # that would help the bike not be so cumbersome to lean into a turn. I will also round my trail to 4.25" for simplicity sake. I try to round to the nearest 1/8" .

just remember this is what works for me. Trail is really not as important as one would think. You will learn to adapt to a bike that handles poorly example point is a stock harley. My 2008 rocker C has 6.2" of trail with a rake of 37* and a 1* raked and offset trees. Its a LSD 250. The bike is an ok handling stock bike.

krymis wrote:irish,
from my riding style i have found that a 200% larger rear tire is the best size to use compared to the front. So if you have a 100mm front tire a 200mm rear tire goes right with the stated idea. This is especially helpful when you go with a real skinny or real fat rear tire in adjusting the trail #. which in my case i use 3.375" of ground trail.

Real trail is measured from a perpendicular line that intersects the centerline of axle and the imaginary line of the steering stem centerline. This is usually about 90% of the ground trail.

Thanks krymis for the explanation. Of the two choppers I've built, both ended up with close to 4 inches of positive trail. The first bike had a springer and the second a hydro fork. Both were rigids with around 40 degrees of rake and about 6" over in length and I thought both handled good for what they were. They both handled moderately tight twisty roads decently. I've owned numerous stock geometry bikes, standards, cruisers and sport bikes for comparison. I measured the trail on the old springer I have now and it's right at 4.5" just sitting there at 45* with the axle hole 13 inches off the ground. Should be close to good anyway.

well here are some more tid bits for you...
here is a cad drawing of sugar bear rocker #1 on a 90/90/19 wheel and tire setup on the horizontal rule


here is the same rocker and tire with the 90* rule


here is one of the #4 rockers set up on the 90/90/21 wheel and tire setup on the 90* rule

so i posted these to add some more info on what i am working on and found.
1. sugar bear does not use trail as much as he uses flop factor ({F=((t*sinR*cosR)/90)*D} where t=trail R=rake and D=wanted degree of turn to one side, i use 40) as realized as a drop in the steering neck.

2. he tries to set his trail to .125 or less to 0"

3. As his springers and rockers get larger the trail matters less and less. You have a longer torque arm (axle to solid leg fulcrum) and the perceived smaller diameter wheel. The angle on the front axle becomes so large it is difficult to have a force over come the assembly with out lifting the wheel off the ground.

4. the neck barely moves due to the low flop number

5. using and limiting to 4 rocker sets typically will do nothing but add more static ground trail as rake increases. as seen in the 35* rake cad drawings.

6. if the assembly goes negative trail, the # is so small that the disturbance is unrealized be for the correction occurs

7. the dropped rockers are a big factor in the whole assembly

you can apply these few ideas to any rocker system frontend like leafers springers and harmans