[dereksbelanger]

Neat! I'll be up there too. Maybe we can intorduce.

 

[Veee]

Rake matters because radius affects ride height vertically but only a percentage of rake is vertical.

what percentage of rake is vertical?
I have done it 3 different way (axle to floor, circumference, diameter) and results was same.

 

[dereksbelanger]

The vertical percentage depends on the angle of rake. For example, a 45 degree angle is 50% vertical and 50% horizontal. A steeper angle has a higher percentage of vertical. A shallower angle has a lower percentage of vertical.

Here's a simple (and ridiculous) example of how this works.

If your rake is 45 degrees and you raise ride height 1 inch, you would have to raise the forks 2 inches in the triples (because only 50% of the difference is vertical) to get back to the original ride height.

 

[Veee]

so, nobody really ever moved forks correct amount to compensate for tire difference ;D

 

[Forbin]

what difference does it make how you measure (axle to floor, diameter, circumference) and what formula you use if you get to same result, radius?
and what rake and offset has to do with it?

Rake has everything to do with front end feel/stability/steering effort, and rake is affected by changing the ride height on one end of the bike, such as when you go to a taller tire. In order to compensate, you need to raise the forks. However, the forks are on an angle as a result of rake. As a result, you need to use some trigonometry to determine the exact amount to raise the forks.

Yes, you can make a change, ride it, make a small change, ride it again, ad infinitum, and get your bike set up that way. However, being able to use some engineering skills to determine the effects of a change and what else needs to be changed to keep other parameters the same can be very helpful in minimizing the trial and error period.

---

P = pitch (degrees)
W = wheelbase (millimeters)
R = tire radius (millimeters)
A = rake (degrees)
F = front ride height at steering stem (millimeters)
K = fork length (millimeters)

Given:
W = 1430
A = 25
R = 251.9
dR = 12

Assumptions:
Steering stem is distance R behind the front axle

---

tan dP = dR / W
tan dP = 12 / 1430
dP = 0.48 deg

dF = ( W - R ) * tan dP
dF = ( 1430 - 251.9 ) * tan 0.48
dF = 9.87 mm

dK = dF / cos A
dK = 9.87 / cos 25
dK = 10.89 mm

Alternatively:
dK = ( W - R ) * (dR / W) / cos A

So it seems I was wrong before. Raising the forks 12mm for a 12mm taller tire is too much. You only need to raise them about 8.5mm*, all other things being equal.

*EDIT: Oops, I used 2R instead of R for the steering stem location. Turns out my 11mm adjustment is just about perfect for a 12mm taller tire.

 

[dereksbelanger]

Nerd

 

[tgold]

Then you sit on the bike and all those calculations go to hell.

 

[Veee]

than you run to bathroom and it goes to hell again

 

[Gerard7aa]

Rake has everything to do with front end feel/stability/steering effort, and rake is affected by changing the ride height on one end of the bike, such as when you go to a taller tire. In order to compensate, you need to raise the forks. However, the forks are on an angle as a result of rake. As a result, you need to use some trigonometry to determine the exact amount to raise the forks.

Yes, you can make a change, ride it, make a small change, ride it again, ad infinitum, and get your bike set up that way. However, being able to use some engineering skills to determine the effects of a change and what else needs to be changed to keep other parameters the same can be very helpful in minimizing the trial and error period.

---

P = pitch (degrees)
W = wheelbase (millimeters)
R = tire radius (millimeters)
A = rake (degrees)
F = front ride height at steering stem (millimeters)
K = fork length (millimeters)

Given:
W = 1430
A = 25
R = 503.8
dR = 12

Assumptions:
Steering stem is distance R behind the front axle

---

tan dP = dR / W
tan dP = 12 / 1430
dP = 0.48 deg

dF = ( W - R ) * tan dP
dF = ( 1430 - 503.8 ) * tan 0.48
dF = 7.76 mm

dK = dF / cos A
dK = 7.76 / cos 25
dK = 8.56 mm

Alternatively:
dK = ( W - R ) * (dR / W) / cos A

So it seems I was wrong before. Raising the forks 12mm for a 12mm taller tire is too much. You only need to raise them about 8.5mm, all other things being equal.

I'VE GonE CroSSEyeD!

 

[Kris87]

i'm a racer. i dont read that sh!t.

 

[Veee]

Yes, you can make a change, ride it, make a small change, ride it again, ad infinitum, and get your bike set up that way. However, being able to use some engineering skills to determine the effects of a change and what else needs to be changed to keep other parameters the same can be very helpful in minimizing the trial and error period.

problem is you did not change just ride height or just rake or just trail.
tire profile may changed which changes the way bike acts. if tire turns quicker do you really need to change ride height? what if tire is taller straight up but same leaned over? or sidewall is taller and more flexing so tire folds under rim when leaned?

 

[Veee]

i'm a racer. i dont read that sh!t.

you just ride fast and look pretty, leave rest to us slow people ;D

 

[Forbin]

problem is you did not change just ride height or just rake or just trail.
tire profile may changed which changes the way bike acts. if tire turns quicker do you really need to change ride height? what if tire is taller straight up but same leaned over? or sidewall is taller and more flexing so tire folds under rim when leaned?

As long as we're talking hypothetically, I would expect that a 120/70 is the same/similar to a 120/60 in terms of distance from the rim's edge to the ground at high angles of lean. However, I would also expect the increased rake in this situation would be offset by the flatter edge profile. Likewise for a softer tire carcass.

Is it as simple as setting the forks based upon tire height difference and leaving it there? No, I think you've made that point just fine, many times now. There are lots of variables involved. The point I'm trying to make is it can be done a bit more scientifically than just trial and error.

 

[Veee]

The point I'm trying to make is it can be done a bit more scientifically than just trial and error.

yes it can, but end result may not be exactly what science say.
tires are designed using trial and error

 

[skullvulture]

snort, what!? sorry i fell asleep on post 12 only to wake up to find a very dead horse being kicked ;D

 

[pearsonm]

Yes, you can make a change, ride it, make a small change, ride it again, ad infinitum, and get your bike set up that way. However, being able to use some engineering skills to determine the effects of a change and what else needs to be changed to keep other parameters the same can be very helpful in minimizing the trial and error period.

I'm obviously part of the minority here, but I agree. I don't have the time, money or knowledge to blow thru X sets of tires before stumbling on the magic combination.

This reminds me of Damper Rod threads. I could have saved a lot of time, effort and money had I just gone with a Traxxion drop-in kit and been done with it.

I appreciate trial-and-error. I appreciate it even more when it's kept to a minimum.

 

[brady]

I can't believe no one has mentioned rear ride height adjustment as a variable to see if we can complicate this a little more! ;D

 

[svhopeful08]

Just ordered Mich PR 2s; the front in a 70. Should correct speedo if nothing else. Plus they should be better than stock Dunlops.

I think I'm gonna go with feel-tune-feel-tune approach. We'll see what happens.

Not a racer, but getting better and better everyday. Love to tinker with tuning and look forward to learning more about rake and trail!

 

[slowpokesv]

I really like the 70 series front also

 

[herbn]

+1 good thing on the 70 series.I have pilot cts' . Seems like i ride pretty much to the edge of my rear tire and not quite on the front , is that because of the 70/60 series thing?,i could probabely lean a bit more and scrub them both to the edge. But i havn't done a back shock yet ,that might help too.