Gravy's 2005 Liberty Renegade

Gravy

Ant Anstead of Dirtbikes
Supporting Member
That's where I've been mistaken. I understood all of that: I just incorrectly labeled KPI as the angle created between your the imaginary castor line drawn up continuous from upper and lower bj and up from contact patch center point to that imaginary intersection point. I can't for the life of me think what to call that angle. (I guess I would label it imaginary effective steering thrust angle or trail).
And this angle changes based upon backspacing and tire diameter...
Screenshot_20240228_121627_Photos.jpg
 

Gravy

Ant Anstead of Dirtbikes
Supporting Member
Corban you're right on the money. I didn't see your post on trail until after I did this rudimentary screen draw lol.


Anyways my observation is that every time I've changed a tire size up on my vehicles I shoot for a little less measured caster because that trail angle increases. And too much return to center is the culprit often for death wobble and (steering lift in extreme cases I think in mountain/!motor bike land too much trail feels like floppy steering like on a chopper). But it's obviously a fine balance: too little caster means too little return to center and wandering steering.
 
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Gravy

Ant Anstead of Dirtbikes
Supporting Member
Okay I feel completely silly but these tires are 245 75 17s.
They look freaking rad we'll see how much they rub 😂

20240228_132516.jpg

I went with the narrower ones up front to hopefully preserve ball joints (I'm actually not worried about the uppers since the RRO ones use an Escalade ball joint) and keep the rub to a minimum...
20240228_132632.jpg

They almost look right because the stock JL steel wheel has so much backspacing that with a little adapter spacer they don't look too bad.
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bryson

RME Resident Ninja
Supporting Member
Location
West Jordan
That's where I've been mistaken. I understood all of that: I just incorrectly labeled KPI as the angle created between your the imaginary castor line drawn up continuous from upper and lower bj and up from contact patch center point to that imaginary intersection point. I can't for the life of me think what to call that angle. (I guess I would label it imaginary effective steering thrust angle or trail).
And this angle changes based upon backspacing and tire diameter...
View attachment 170182
The angle you're talking about is still just the caster angle whether you're measuring it above or below the centerline, and doesn't change with tire size - I think what "trail" (might have a different name when talking 4-wheel vehicles?) refers to is the distance between the 2 lines (at ground level). That certainly does change with tire size, and I'd assume it does indeed have an effect on handling.
caster with trail.jpg
 

Gravy

Ant Anstead of Dirtbikes
Supporting Member
Trail distance increases yes. Also you're assuming the spindle and contact patch centerline is dead centerline to the BJs. Usually it isn't. Any change in tire size and backspacing moves that distance from contact patch centerline to ball joint centerline point in both the x and y axis.
"Trail angle" or "effective steering angle" would be a compound of all these things.
I think we're both saying mostly the same thing but I'm saying poorly.
 
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