front end geometry

#1
I was reading a thread concerning front end geometry and the presence of " offset" as opposed to 0 degree "offset", Is this referring to the placement of the axle to the center of the fork tube?:confused:
 
#3
I was reading a thread concerning front end geometry and the presence of " offset" as opposed to 0 degree "offset", Is this referring to the placement of the axle to the center of the fork tube?:confused:
Yes, FOMOGO has it right. In the mini bike world, that allowance of offset is (generally) moot. Two reasons for this: One, is that it was far more common, and simpler to produce trees with zero offset to the tubes. (Perpendicular) The reason it is done on full size motorcycles is to compensate for fork length, wheel diameter, and maintain specific axle to axle overall length of the bike for other handling considerations.

The second reason is that fork tube angle (rake) is easily adjusted to achieve negative trail: The amount of distance between contact patch of the tire as measured straight down the fork tube to the ground, to a line drawn exactly vertical from axle center line to the ground.

FOMOGO recently did a thread on this, and other members did some Google searches and posted photos. But it is not nearly as complicated to measure, as it is to talk about the measurements. Go with zero offset trees, and simply adjust your rake via neck angle to any negative measurable result.

Or in other words, so that the forks are pulling the axle, not the other way around, like the swivel wheels on a shopping cart. (Simplest way to look at it)
 
#4
Addendum- The primary reason anyone talks about offset, is that aftermarket front ends will incorporate offset trees to allow a longer fork tube- such as building a chopper/bobber, without the owner/builder having to cut and re-weld the fork tube. Generally, the motorcycle frame is built around the tube, and cutting it out and re-welding it is an involved process requiring frame jigs, and professional grade welding ability.
 
#8
Finally an informed thread on this subject, I have a homespun chopper that suffers from too much rake and not enough trail. When I turn the bars it scrubs the tire way too much, this tells me I didn't put enough thought into it. Too late now but I'd do it differently were I to do it again. A lot of good advice here, especially on the part of Havasu Dave but everyone gave some.
 
#9
Finally an informed thread on this subject, I have a homespun chopper that suffers from too much rake and not enough trail. When I turn the bars it scrubs the tire way too much, this tells me I didn't put enough thought into it. Too late now but I'd do it differently were I to do it again. A lot of good advice here, especially on the part of Havasu Dave but everyone gave some.
Ace I know this is an old thread and I have read your post here on 4 or 5 different occasions. It always read a little vague to me. This time it registered and I think I understand what you are saying when you refer to it "scrubs the tire way to much".

Are you saying that the tire drags as you turn the bars right to left when it is sitting still? Its more of a curiosity question now trying to visualize what it is doing. I am interpreting this as it is pushing or pulling the tire as you turn the bars instead of pivoting on the tire.

Have you measured any the numbers out of curiosity?

Thanks,

Doug
 
#10
Never measured but at rest it pulls which tells me I have too much rake, running it down the road it feels labored which to me is preferable to a push feel which is a more out of control feeling and is more prevalent in some more poorly made choppers I've ridden. I know what trail is and how it's measured but my issue seems more to do with the rake. The OCC chopper forks I used had built in rake through the triple trees so I might have angled it less at the head tube and gotten a better tire patch feel but then it would not have been as chopper like. I did not consult anything or anyone I just grafted the frame section onto an old Arco frame and raised the rear axle mounts to give it a better feel.

Ace I know this is an old thread and I have read your post here on 4 or 5 different occasions. It always read a little vague to me. This time it registered and I think I understand what you are saying when you refer to it "scrubs the tire way to much".

Are you saying that the tire drags as you turn the bars right to left when it is sitting still? Its more of a curiosity question now trying to visualize what it is doing. I am interpreting this as it is pushing or pulling the tire as you turn the bars instead of pivoting on the tire.

Have you measured any the numbers out of curiosity?

Thanks,

Doug
 
Last edited:
#11
At some point, doesn't the rake end up affecting the caster of the front wheel? I have a chopper bicycle with super long forks and man it's hard to ride. Like the contact patch is way further back. Head tube angled from the factory, forks extended like 3ft I think.

I never rode a bike with offset built into the forks, it sounds interesting but might take a little getting used to, at least for me.
 
#12
At some point, doesn't the rake end up affecting the caster of the front wheel? I have a chopper bicycle with super long forks and man it's hard to ride. Like the contact patch is way further back. Head tube angled from the factory, forks extended like 3ft I think. I never rode a bike with offset built into the forks, it sounds interesting but might take a little getting used to, at least for me.
Hi Massacre! The caster "is" the rake angle. Same measurement, called two different things. The caster or rake is the measurement between the vertical plain, and the angle of the forks themselves.

Angling the head tube allows a frame relationship that allows you to (compensate) to arrive at a trail setting you can live with, but still have long, angled forks.

Unfortunately and like I said before, the explanation is actually more complicated than the issue itself. Even looking at Ace's bike, we can tell that his trail is very close to zero. This will feel like he's pushing his front wheel with the frame, instead of pulling it along behind the focus of effort/inertia. Once again, I refer to the Shopping Cart analogy.

Sorry to sound like a "know it all," teacher guy. Even if you build a machine with long forks and end up with some decent trail, say four inches or so, you still have an entire steering fulcrum and angle to mess with your handling, as well as inherent limitations to steel supporting structures like vibration and bounce.

Most importantly, there is an optimum percentage of weight applied to the front axle of the entire machine and rider weight that comes in to play, and that has a huge effect on handling as well. (Refer to "weight bias") Again, looking at Ace's bike, we can see how his weight bias is going to be well to the rear of centered on the bike, making a "pushing" front end too light in weight- which compounds the push, or feeling of lack of contact.
 
#14
Is there a "Rule of Thumb" regarding rake and trail when setting up a mini bike? A range of what works and when things begin to go south?
Nothing "official," and honestly this would change based on frame and wheel sizes anyway. A rule of thumb would perhaps be based on percentages in inches based on wheel circumference. But even the speed of the machine has a bearing here, since the slower you go, the less wobble you'd have at decreasing inches of trail.

IMO, and this is just my opinion, a smaller wheeled bike should be something over an inch to three inches, while a mini cycle starting at 8" wheels would be starting at two inches going to four. But that and a dollar gets you a bottle of water. Probably plenty of information via google on motorcycle construction.
 
#16
Nothing "official," and honestly this would change based on frame and wheel sizes anyway. A rule of thumb would perhaps be based on percentages in inches based on wheel circumference. But even the speed of the machine has a bearing here, since the slower you go, the less wobble you'd have at decreasing inches of trail.

IMO, and this is just my opinion, a smaller wheeled bike should be something over an inch to three inches, while a mini cycle starting at 8" wheels would be starting at two inches going to four. But that and a dollar gets you a bottle of water. Probably plenty of information via google on motorcycle construction.
I know a trials bike isn't the same as a GoldWing. Most mini bikes are slow speed, so I would imagine a lot more leeway. What I am building will be run from 3 mph to maybe 35 if the hounds of hell were chasing me. Thanks for the opinion! jim
 
#17
I know a trials bike isn't the same as a GoldWing. Most mini bikes are slow speed, so I would imagine a lot more leeway. What I am building will be run from 3 mph to maybe 35 if the hounds of hell were chasing me. Thanks for the opinion! jim
LOL, I tend to go on and on sometimes Jim. I was out in the garage just now having a smoke and I recalled your comment on wanting a leading link, or Earles front end.

I had a vintage production "Trail Tamer" front end, which was a after market deal offered in the 70's, to make your Taco mini bike more offroad friendly. I had one on my Taco 99. Decent little engine on it, five inch wheels, and at about 40 MPH, it would start to shake at the front end.

Using the standard measuring procedures of getting the fork geometry, I can only "assume" it would be the centered effect of the front shock and actual fork. Using that, with the vertical plane, it's easy to see that the trail would have been very close to zero, if not slightly positive. But as you said, at low speed, the suspension did just fine, and worked well over terrain.

Done5.JPG
 
#18
LOL, I tend to go on and on sometimes Jim. I was out in the garage just now having a smoke and I recalled your comment on wanting a leading link, or Earles front end.

I had a vintage production "Trail Tamer" front end, which was a after market deal offered in the 70's, to make your Taco mini bike more offroad friendly. I had one on my Taco 99. Decent little engine on it, five inch wheels, and at about 40 MPH, it would start to shake at the front end.

Using the standard measuring procedures of getting the fork geometry, I can only "assume" it would be the centered effect of the front shock and actual fork. Using that, with the vertical plane, it's easy to see that the trail would have been very close to zero, if not slightly positive. But as you said, at low speed, the suspension did just fine, and worked well over terrain.

View attachment 97627
And the luck of our youth bought us a lot of leeway too. At least in my case.
 
Top