Topic: how do i figure how much difference in trail with longer rear shocks?

[Xnavylfr]

Changing front tire will not change your RAKE!!  RAKE is the angle at which your neck is welded!! in relation to a line at 90degrees  straight to the floor. In your pictures it looks like you have RAKED triple trees because the neck is not at the same angle as the forks. If you are not careful , you can get yourself in to a NEGATIVE trail situation and that means trouble!!!
Even my avatar bike with the LONG springer has 4.5"+ trail.

http://s465.photobucket.com/user/Xnavylfr/media/StretchArmstrong_zpscc15a241.jpg.html?sort=3&o=1

Raising the rearend will change the rake SLIGHTLY , but you have to worry about TRAIL.  I've seen choppers with over 45 degrees of rake and still have close to 6" + trail!!

Xnavylfr(CHUCK)

[Ewan 500K1]

Changing front tire will not change your RAKE!!  RAKE is the angle at which your neck is welded!! in relation to a line at 90degrees  straight to the floor. In your pictures it looks like you have RAKED triple trees because the neck is not at the same angle as the forks. If you are not careful , you can get yourself in to a NEGATIVE trail situation and that means trouble!!!
Even my avatar bike with the LONG springer has 4.5"+ trail.

http://s465.photobucket.com/user/Xnavylfr/media/StretchArmstrong_zpscc15a241.jpg.html?sort=3&o=1

Raising the rearend will change the rake SLIGHTLY , but you have to worry about TRAIL.  I've seen choppers with over 45 degrees of rake and still have close to 6" + trail!!

Xnavylfr(CHUCK)

You're wrong ya know Chuck.  Bigger diameter tyre lifts the front end , therefore the angle that the headstock sits  relative to a line perpindicular to the horizontal increases, ie the headstock rake and the fork rake both increase. So, bigger diameter tyre = increased fork rake.

See below in exagerated picture form , showing that a larger diameter tyre increases both rake  & trail

Top picture = smaller diameter wheel & tyre . Rake smaller and  = angle A, Trail shorter

Bottom Picture = larger diameter wheel & trye . Rake larger and = (angle A + angle B) , Trail longer

For trail,  Its FORK rake that matters and while this is generally the same as headstock rake, some bikes do have triple Ts that have their own internal rake such that headstock rake is not the same as fork rake.

Fork rake can also change with a whole host of other factors ... longer rear shocks, shorter rear shocks, smaller diameter rear wheel / tyre , shorter front forks, longer front forks, Triple T with own internal rake , bent front tubes on frame, bent triple T etc etc.
 
In general:
1. Doing anything that raises the rear end while keeping the front the same decreases both rake and trail
2. Doing anything that lowers the rear end while keeping the front the same increases both rake and trail
3. Doing anything that raises the front end while keeping the rear the same increases both rake and trail
4.Doing anything that lowers the front end while keeping the rear the same decreases both rake and trail

Of course if you change the angle that the headstock is welded to the frame and keep everything else the same then the fork rake (and therefore the trail) would also change. It is of  course trail that matters in terms of bike handling, but trail is directly related to fork rake by trigonometry (pythagoras's  theory if you like). Less fork rake = less trail all other things being equal. I know all this, hope it makes sense.

To get silly ... if you kept everything else stock and put on a scooter front wheel you would have a very small fork rake angle and potentially negative trail ... but all we are talking about here is minor changes to the rear shocks and slight increase in the diameter of the front tyre ... no chance of a negative trail.

Really smart chop by the way.[/list]

[Ewan 500K1]

Bent steering stem as the result of a front-ender can also reduce fork rake and therefore trail

[Xnavylfr]

You would have to go EXTREMELY larger to make any differences. Even if you were to go from a 19" tire to a 21" tire on the front it is only a 1" height difference from axle to ground (depending on sidewall size) this difference would make such a small change you'd never notice. If you were to go from a 16" front to a 21" front the difference is 2.5 " in height and that would possibly change your trail by an inch to the + side.


Xnavylfr(CHUCK)

[Ewan 500K1]

I suppose what we were trying to do was answer MM's original question . Hopefully we've given enough info to be going on with. I think we are all agreed that raising the rear shocks by up to 30mm isn't going to change the trail to point of madness, especially if the bike's stock trail is 4.5 inches. I'm keen to know what the results are anyway if the measuring stick and the protractor come out.

[lucky]

Do not forget that the motorcycle MUST be on a LEVEL floor,
 or you can just throw out all those calculations.

[Ewan 500K1]

Do not forget that the motorcycle MUST be on a LEVEL floor,
 or you can just throw out all those calculations.

Yep, wahtever surface you are using to make sure it doesnt have any surface irregularities by checking it with with a known straight edge. What I did was put a length of nice and straight 6" by 2" wood on the workshop floor (the floor is a bit rough !) then wheeled  the bike onto the 6" by 2" and checked that the wood stayed true with the bike's weight.  Most digital protractors have a "zeroing" function, so as long as the surface doesn't have any significant variations within the length of the bike it doesn't atually need to be perfectly level (ie doent need to be at zero degrees). So .. put the bike on your "level" surface. Zero the digital protractor by sitting it on your "level" surface and carry on from there. . You'll also want someone sitting on the bike when you measure the rake angle with the protractor.         ... OR ....     just borrow a set of longer shocks and go for ride an see how it feels

[lucky]

Nic you have too small a rear tyre, it should be a 4.25/85x18 and the front ought to be the 4.10x19

Thanks Bryan and Ewan.

Incorrect information.
The 1978 CB750 did have the 17 inch rear wheel.

[Nic]

Nic you have too small a rear tyre, it should be a 4.25/85x18 and the front ought to be the 4.10x19

Thanks Bryan and Ewan.

Incorrect information.
The 1978 CB750 did have the 17 inch rear wheel.

Mine's a 73 mate. Not wanting to hijack this thread any more but just to close on my original post::
 I have made some great progress regarding my head shake debacle. When I was putting the bike together some months ago  and at the putting of the forks on stage I discovered that the D washers for the top tree on either side were nowhere to be found as you would expect so I made some up. Yesterday I discovered that one of them was too fat and inhibiting the clamping of that fork top, so I ground it down so it now clamps. Then by chance I discovered both right hand side front fender bolts were loose So I tightened them up and went for a test ride, so much better after all the shakes that I couldn't find a remedy for. I put a 425/85/18 K81 on the back, will be getting the 410/19 on the front on Thursday. So yippi eye aye, looks like the bike can now be ridden past the 100kph barrier so I have some defense now for those Ahole car drivers out on the h/way. I don't see anything written here about cars and the #$%* you have to put up with riding a bike but after 30 odd years I'm just aghast at the car drivers attitudes out there, it's a bloody war zone. End of rant.

[Ewan 500K1]

it's a bloody war zone. End of rant.

get a pair of side cannons 

[Nic]

it's a bloody war zone. End of rant.

get a pair of side cannons 

Where do you get em, actually a rear cannon would be better.

[bryanj]

Lucky, i was answering Nic, not the original post mate

[mAd mOrdigan]

my next day off I'm gona buy the protractor and see what I get.

[lucky]

Lucky, i was answering Nic, not the original post mate

OK....I will go back and check that out- sorry.

[bryanj]

Lucky, no need for sorry---I ought to have put that in the post but my excuse is i am usually doing this at 06:00 ish after being at work 12 hours