Rake adjustment

[Waffa_imp]

Hey all, I've been looking around for after market triple trees for our cb. I want to decrease the rake(27 degrees) by a couple of degrees. I haven't had any luck with my search and decided to ask the forum to see if anyone has had any luck or a good place to search.

Thanks, Waff

 

[FUZZY_imp]

Why? Wouldn't think you would like handling afterwards.

 

[Waffa_imp]

Why? Wouldn't think you would like handling afterwards.

I graduated college with a concentration in 4 wheel and 2 wheel vehicle dynamics. I created a computer model where I insert the geometry of the bike to analyze and predict the handling characteristics.

I would be most concerned about finding enough information about the fork seals to see if they can handle the loads when I change the offset. It would most likely end up being trial and error on the fork seals.

 

[Deanohh_imp]

How far can you reduce the trail before it oscillates at high speed like a bent shopping cart wheel.

 

[ChipBeck_imp]

Why? Wouldn't think you would like handling afterwards.

I graduated college with a concentration in 4 wheel and 2 wheel vehicle dynamics. I created a computer model where I insert the geometry of the bike to analyze and predict the handling characteristics.

I would be most concerned about finding enough information about the fork seals to see if they can handle the loads when I change the offset. It would most likely end up being trial and error on the fork seals.

Waffa,

Wouldn't decreasing rake also decrease the loads on the fork seals? And although it would lower ground clearance couldn't you test out slightly less rake by sliding the fork tubes up in the triple clamps?

Chip

 

[Waffa_imp]

Why? Wouldn't think you would like handling afterwards.

I graduated college with a concentration in 4 wheel and 2 wheel vehicle dynamics. I created a computer model where I insert the geometry of the bike to analyze and predict the handling characteristics.

I would be most concerned about finding enough information about the fork seals to see if they can handle the loads when I change the offset. It would most likely end up being trial and error on the fork seals.

Waffa,

Wouldn't decreasing rake also decrease the loads on the fork seals? And although it would lower ground clearance couldn't you test out slightly less rake by sliding the fork tubes up in the triple clamps?

Chip

Waffa,

Wouldn't decreasing rake also decrease the loads on the fork seals? And although it would lower ground clearance you could you not test out slightly less rake by sliding the fork tubes up in the triple clamps?

Chip
That is correct, but decreasing the rake will increase the trail which I would have to fix by changing the offset. And the moment generated from a bump impact on that offset is what I'm worried about.

Since the triple clamps are parallel with the head tube angle, sliding the forks up wont change the trail (one of the main handling properties). The main goal I'm going for is the look of steeper forks. While making a few other changes to maintain/tailor the current handling properties.

 

[Deanohh_imp]

You're saying decreasing rake angle (steering stem angle) increases trail. ......sure about that?

But perhaps you could show a scale diagram of what you're trying to accomplish. I've never seen a set of triple clamps where the yokes are set to angle the fork tubes more vertical than the stem, plus have a bigger offset to maintain the same trail. Is that what you're asking?

I don't get the purpose but I hope you find what you're looking for.

 

[FUZZY_imp]

I dunno?

 

[Ack-CB11_imp]

Triples are out there somewhere, this bike has a set. Those forks are awesome with real brakes up front.

http://www.samurider.com/?p=13643/#itemImage

 

[oldgoat01_imp]

You're saying decreasing rake angle (steering stem angle) increases trail. ......sure about that?

But perhaps you could show a scale diagram of what you're trying to accomplish. I've never seen a set of triple clamps where the yokes are set to angle the fork tubes more vertical than the stem, plus have a bigger offset to maintain the same trail. Is that what you're asking?

I don't get the purpose but I hope you find what you're looking for.

OK, I admit to knowing next to nothing about this topic; however, I am in the same place. Doesn't a reduction in the rake have to reduce the trail all other things being equal? Or is that the original question, i.e., find some offset triples that maintain the trail while decreasing the rake?

 

[Deanohh_imp]

"sliding the forks up wont change the trail"

another false statement. Trail decreases even under braking as the forks compress.

 

[Waffa_imp]

You're saying decreasing rake angle (steering stem angle) increases trail. ......sure about that?

But perhaps you could show a scale diagram of what you're trying to accomplish. I've never seen a set of triple clamps where the yokes are set to angle the fork tubes more vertical than the stem, plus have a bigger offset to maintain the same trail. Is that what you're asking?

I don't get the purpose but I hope you find what you're looking for.

Sorry, I'm used to the coordinate system for the Patterson Control Model where the origin is located at the head tube intersection, and not the contact patch.

I want to decrease the rake as much as possible without the fender hitting the headers.

 

[Deanohh_imp]

You're saying decreasing rake angle (steering stem angle) increases trail. ......sure about that?

But perhaps you could show a scale diagram of what you're trying to accomplish. I've never seen a set of triple clamps where the yokes are set to angle the fork tubes more vertical than the stem, plus have a bigger offset to maintain the same trail. Is that what you're asking?

I don't get the purpose but I hope you find what you're looking for.

Sorry, I'm used to the coordinate system for the Patterson Control Model where the origin is located at the head tube intersection, and not the contact patch.

I want to decrease the rake as much as possible without the fender hitting the headers.

Sorry, I'm used to the coordinate system for the Patterson Control Model where the origin is located at the head tube intersection, and not the contact patch.

I want to decrease the rake as much as possible without the fender hitting the headers.
I guess by rake you mean the angle of the fork tubes and not the stem angle. Most would include cutting and rewelding the head tube. Or use a scale model that you could easily manipulate without destroying a nice motorcycle. Do you own a cb1100 that you will use for this purpose?

keep your coordinate system and your wheels on the ground and you might get there if the laws of physics let you.

 

[Jim21680_imp]

Wow, this got my brain gears grinding away pretty good. Nonetheless, short of having a triple tree custom-made, you're probably not going to find much...there's just no market for such a thing. Choppers, yes...but not many people are interested in going the other way. Lastly, even if you were to have a new tree milled out, I don't see how you could ever trust it. There's just too many forces that stem needs to cope with.

 

[Waffa_imp]

I am going the other way which is why it might be hard to find. And it is only a slight change, so I wont need to chop the frame. I just want to change the fork angle, not stem.

Can you help me understand how moving the forks up in the clamp, or compression of the shocks change the trail when the fork angle is parallel to the head tube angle?

 

[Jim21680_imp]

This seems to depend on whether or not the rear suspension is compressing or remaining static. (Is the frame rotating or not)

 

[Deanohh_imp]

Ahhh, Socratic dialog......brilliant.

Nevermind, it disappeared.

what are the other components that determine trail, and which one(s) of them changes when the front of the bike is now lower than the rear.

 

[Jim21680_imp]

Yes, after thinking about this...it's completely dependent on that variable; under real-world riding, the rear suspension will typically decompress under front braking, rotating the frame forward, and therefore decrease trail.

Now on a side note...2 big reasons you won't want to do this:
1. You void the warranty.
2. You'll likely invalidate your insurance policy. I'd bet my next paycheck they'd consider this a "frame modification", even though you didn't touch the frame.

 

[Waffa_imp]

I think Jim hit it right on the nose.

Trail is only affected by head tube angle and tire contact point. The only way trail would change from sliding the forks is from the change in center of gravity rotating the frame. I'm not sure how much this will change the trail, anyone tried it yet, notice any difference?

 

[Deanohh_imp]

Given the dimensions involved, a capable frigonometry student could calculate exactly what the change in trail is.

This post is not directed at any particular person nor is it intended to offend any person or thing on the planet, living or dead.