My motohack always needs something

I dunno about DB but I’m up for a geek out anytime.
I did do some quick trigonometry on my longer swing arm and with a few assumptions figure a 1 3/8 longer swing arm would decrease the effect of a 4” stroke (big bump) on my steering axis by a .100” or so. But then it all is held up by springs. But I convinced myself that a longer swing arm would be more stable.

although I’m still wondering if I should move my swing arm shock mounts back 1 3/8. Any thoughts?

Us off road racing guy geek out on suspension and especially long travel suspension. 32 to 36 inches of travel is the normal with 5 inches of bump stop travel. We like to set up so your sag yields a little more than 55% of the travel in the upwards direction and 45% down. We want the wheels to fall into the bottom of the "whoops" and dampen all the way to the tops of the whoops. When set up right you notice that the wheels are going crazy and the cab is not moving up and down at all. Here's BJ Baldwin's suspension at work:



Like a lot of other things when it comes to "Adventure" style equipment, you end up with a bastard that falls somewhere between the full "off road" set up and a full "on road" set up. Full on road suspension is really forgiving of being fairly far off on your set up as you really don't need much travel. Running your suspension as stiff as you can comfortably tolerate is not a bad thing. Being super "plush" for off road doesn't always handle very well on road and getting the rig to take a set when you throw it into a turn can be a little difficult from the weight transfer as the suspension compresses towards the outside of the turn. Of course a anti sway bar helps considerably.

OK, from the best I can tell by scaling the pictures your original swingarm was 10.625 inches from pivot to axle and 7.475 inches from pivot to shock mount for a ratio of 1.42:1
After lengthening they go to 12.375 and 9.225 respectively for a ratio of 1.34. So the change in effective spring rate was +12%
I have used 15 inches as the length of the shock and an initial angle of 30* from vertical, after lengthening the angle increases to 38*. This reduces the effective spring rate to 91% of the original value.
So the net change is an increase in effective spring rate is an increase of 2%. I wouldn't be in a hurry to change anything, particularly if you are happy with the way it rides.
All of which is a long winded way of saying that @brstar was right.

Your instinct is right in that the longer swingarm will give less variation in trail over the total suspension travel. The trade off (and there always is) is that it has increased the moment of inertia about the steering axis, this combined with the reduced righting torque from the lower trail means that any headshake is likely to be more violent.

OK, this is fun thread, but I feel a lot like Cole Trickle in "Days of Thunder" discussing the setup of his race car with Harry Hogge --



You builders are amazing. Me, I just drive 'em. I blame it all on my dad. When I started high school every kid was required to take one "shop" class. Choices were Sewing, Cooking, Metal Shop, and Wood Shop. My dad pushed me into Wood Shop.
Shoulda picked Metal.

When it came time for me to select a class, the only thing that was available was cooking class. Instead, I quit school and got a job at a motorcycle shop. Best choice I ever made!

I think we all know that in the end resistance is futile.




yes burned powder coating stinks

I’m not so concerned with spring rate, thanks Steam powered, but the effect on trail and ride height has to be tested!

The shock relocation did raise the ride height about 1/2” and made the steering just a bit heavier. I countered this by dropping the front preload and all steers just great. According to my measurement it has about 1 1/4” of trail unloaded and about 1/2” loaded. I can now use the preload as an easy adjustment if I feel the need to increase the trail a bit in the future.
I can also put the shock back in the forward position to decrease trail a bit more if I go to a taller tire someday.
And I can move the swing arm pivot to the max trail position if I go to a shorter tire.
So I’m pleased with the increased trail adjustments and I’m pleased with the trail and steering as it sits now.
The temperature was upper 20’s F and the wind was 20-30 mph but I got 1 1/2 hrs of test riding. It steers nice and easy but still holds a line just fine. You need to pay attention but it’s not twitchy and much easier to steer in the wind than the big fat car tire.
It’s all looking good.
Tomorrow SWMBO rides.

That’s a pretty cool video

Thanks for taking the time to do the calculations. The one thing that still puzzles me is I don’t have any head shake. None. Not even a low speed wiggle. Of course that’s a good thing but with the small amount of trail I have I expected to have some. When I had the big car tire up from I had just a bit that I balanced out with tire pressure. But that was quite a few changes ago.

You do know that I’m just trying to get my rig to steer like yours!

Cat Skinning, Ver 3.04A

The cat has been skinned. And my knuckles too.
Now to move on to the next thing.

Still trying to get my head around how just changing the shock mount changes trail. Ride hight? Preload? Total movement? Would it not just change how much your trail changes in Bump and Droop? What am I not seeing?

It changes the height and the steering axis angle so the point where the steering axis meets the ground. Imagine if you put a 3 foot long shock in and where the steering axis would point.

Since the swing arm pivots around the fixed down tubes it also pulls the axel back while the ride height increases.

I don’t know how much effect this has on steering where the trail is longer but I see a significant effect on this bike where I’m running very little trail.

I suspect there to be a point where head shake is a concern as I approach negative trail when the shock compresses under normal use.

But this is just what I have deduced and may all be hog wash. Others here know much more.

I think I'll have to draw it out on paper. Once I see it the light bulb will go on, maybe ! LOL

Probably ride height, it should be noted his testing is conducted in miserable cold and windy weather, I suspect he really has no idea how it's going to steer, but he is giving himself a hell of an education whether it's valid or not remains to be seen. Thank dog the temp has no effect on how those shocks work

In certain circles Octanes changes and mine to could be referred to as mental masturbation or examining the lint on ones belly button but WTF winters are long up here.
I'd go for a seance to see what Luke things but I'm pretty sure he'd laugh and say whatever !

Could be the skinny front tire isn't grabbing enough to give you any,your tire pressures must be close to correct, I suspect your welding projects are straight and more rigid than most.

I think starting with a ridged set up is the basis for minimal head shake. I’d like to think mine is about as ridged as a bolted on affair can be. But someday it’ll probably break and change my thoughts entirely.
It’s cold today but no wind. I think I’ll go for a ride (SWMBO is backing out!). Then back to the shop to pull the tub and re-torque the heads. Something I’ve been ignoring for far too long. And make a change to the external framework so I won’t have to pull the tub in the future.

On building my Racecar I fussed over a lot of details. Not really knowing what I was doing I basically copied the Old car for dimensions and used a commercially available front suspension. Not sure if any of you know of Ted Lathorp. Built stuff for British Cars. https://www.fastcarsinc.com/product-category/front-suspension/ We did modify it slightly so I could run more negative camber. Where I was going with this is I have a very adaptive driver, who is quite fast but gives me almost no feedback. He just drives. I think I am similar with my hack, I just adapt to it's idiosyncrasies and probably wouldn't really notice subtle changes unless it was drastic. So I marvel at how HighO can fathom out what he needs to change, fab it up perfectly then figure out what the next step is.

I got a whole pile of scrap parts.