Topic: Increasing Frame Rake on CB750

[Stoli]

I've read the theory but was hoping to find someone who has first hand experience with increasing the rake of a CB750. I found several project threads of guys who did this but the threads ended before they were able to report back on handling, etc.

I was contemplating increasing the rake by about 10 degrees which will in turn increase the trail from 3.74" (stock) to somewhere between 5" and 6" depending on the wheel size I go with. Obviously this will make the bike harder to corner, but I am willing to sacrifice that in this case. At what point does increased trail become a safety issue? I've read that as long as it is under 6", it should be OK. Anyone out there with experience in this area?

[lucky]

I've read the theory but was hoping to find someone who has first hand experience with increasing the rake of a CB750. I found several project threads of guys who did this but the threads ended before they were able to report back on handling, etc.

I was contemplating increasing the rake by about 10 degrees which will in turn increase the trail from 3.74" (stock) to somewhere between 5" and 6" depending on the wheel size I go with. Obviously this will make the bike harder to corner, but I am willing to sacrifice that in this case. At what point does increased trail become a safety issue? I've read that as long as it is under 6", it should be OK. Anyone out there with experience in this area?

Try to keep the trail down to 5 inches.
Otherwise the steering will feel very heavy and the handlebars will want to flop to one side or the other. It will affect slow speed turns. Tight turns will become harder.

Remember you can alter the  trail with different methods. That will make the steering not feel so heavy as well.


You can use  raked trees, springer forks with longer rockers,and forks with different amounts of offset.
Some springers have no offset in many cases.

40º total rake would be quite manageable.

Slightly wider handlebars give you more leverage too.

One thing I would keep in mind is that the stock CB750 frame is only 1/16 inch thickness and was not designed for the stresses of the extra rake.
Remember these frames are 40 years old in some cases.
Most aftermarket frames that are raked are twice that thickness AND the tubing is larger diameter than 1 inch too.
Also if you have a rigid frame that also puts even more stress on the frame with no rear suspension.

IF I wanted that rake I would get a Cycle One  frame that is designed that way from the very start.  OR you can get a hard tail kit and alter the rake but maybe just go with 35º like the  H-D V rod, Suzuki Intruder or the Yamaha V Star.

If you keep the rear suspension but use Cycle-X shorty shocks, that will help the frame not take so much stress.

Remember that the down tubes of the frame take most of the stress.
That is why on choppers with a lot of rake the down tubes are increased to as much as 2 inch diameter tubes or double down tubes.

Ideal trail is about 4.5 inches.

[Stoli]

Thanks Lucky. So I was wondering if you have experience riding a CB750 with 6" of trail, or have you just read the same articles as I have?

[Stoli]

Case in point. Here is a pic from a project thread where a guy is building a drag style bike. He raked the frame an extra 10 degrees and put on 21" rims. According to my calculations his trail is 5.9".  Some say that is too much, others say it is OK.



Unfortunately the thread died before there was any handling report.

[lucky]

Thanks Lucky. So I was wondering if you have experience riding a CB750 with 6" of trail, or have you just read the same articles as I have?

I have built frames (and ridden them) from scratch with 42º rake and DNA springers and it was not a problem. With slightly longer custom rockers I could have even made it easier to steer. But it was not so bothersome That I had new rockers made.
Bitter End Choppers would have made me some new rockers If I wanted them.

Six inches of trail would make the bike very hard to steer and it would be constantly wanting to flop over to either side when at lower speeds. It would make your arms tired.

5.9 inches of trail "would work" but do you want to be fighting it all the time?

Also remember that as far as calculations go that some 21 inch wheels and tire combinations can have the same OD as a 19 inch front wheel and tire.
It all depends of the tire profile.

Notice 16 inch rear wheel and tire combos compared to 17 inch wheel and tire combos.
Almost the same OD. The tire profile is a big factor.

The rake of a 1976 CB750 was 28º with 4.5 inches of trail.EDIT correction( 3.8 inche trail) Offset not given.
With stock front forks and 10º of added rake would make 38º rake. Not very extreme at all.
Trail? You would need to remeasure it. I do not think it would be over 5 inches
but it depends on the length of the front forks.

If I was doing any project I would not use math formulas with so many variables. I would just carefully measure the rake and trail so I KNOW what is actually true.

If I build a frame I want to have the wheels with tires I am going to use from the very start of the project. I would also make sure I have 5 inches ground clearance.
I have used 4.5 inches ground clearance but that is very close to the minimum.
I like frames that are flat on the bottom with nothing hanging below the frame so that anyone can easily jack up the bike.

[Xnavylfr]

You might find more INFO here, cause we CUT and HACK pretty much every part on the CB750.
http://hondachopper.com/

Look in the CHOPPER section when you open the site, you'll see some home built and modified plus some complete builds with aftermarket frames.
I think there is a post in the GARAGE section showing HOW to change the RAKE of a stock frame!!

Xnavylfr(CHUCK)

[Stoli]

If I was doing any project I would not use math formulas with so many variables. I would just carefully measure the rake and trail so I KNOW what is actually true.

I agree that it is a good idea to measure to make sure you know what you wound up with, but formulas should be used to give you an idea of where to start.

The rake of a 1976 CB750 was 28º with 4.5 inches of trail. Offset not given.
With stock front forks and 10º of added rake would make 38º rake. Not very extreme at all.
Trail? You would need to remeasure it. I do not think it would be over 5 inches
but it depends on the length of the front forks.

If my high school math is still good, then I figure the trail calculation on a CB750K to be the following:



Trail = sine(A) x B - C

where:

A = Rake Angle (my manual lists stock rake as 27 degrees)
B = 1/2 Front Tire Diameter (or distance from ground to center of axle which was 12.75" on stock bike)
C = Triple Tree Offset  (this is 2.45" on the stock triple tree)

Assumptions:

1. Tree rake = 0 (parallel to steering column)
2. No changes to Rear wheel/tire dimensions and frame angle to ground remains the same as stock
3. There is a little slop unless you lengthen the fork to ensure the frame angle stays constant but with
    less than 10 degrees of rake, that shouldn't be much of a factor
4. This formula is for "true trail" which is the perpendicular distance between the steering axis to the
    tire contact point. As opposed to the common way it is measured (parallel to the ground).


So, for a stock CB750:

trail = sin(27 degrees) x 12.75 - 2.45  =  3.3"   

* The shop manual lists trail as 3.7" but that is the distance as measured parallel to the ground.
   True trail is measured perpendicular (as noted in 4 above), so 3.3" is about right.


If we increase the rake by 10 degrees we get:

trail = sin(37 degrees) x 12.75 - 2.45 = 5.22"

Lucky - that puts the false trail at about 6.5" !!!!  So on the one hand you are saying that 10 degrees of rake is "not very extreme at all" but on the other hand you are saying that that kind of trail will make my handle bars want to flop from side to side at slow speeds. The contradiction is confusing to me.

This is why I am hoping that someone who has built a drag bike out of a CB750 (like that pictured above) will chime in and relate some experience. 

[Xnavylfr]

Here's one for an example!!

http://www.hondachopper.net/Gallery/053/index.html

This may help also!

http://www.chopperhandbook.com/rake.htm

Xnavylfr(CHUCK)

[Stoli]

Thanks Chuck. Good info.

The choppers use mods like a springer fork and/or a raked tree which will both decrease the excessive trail caused by the increased fork rake. I was hoping to not have to do either while still increasing the rake to the max possible without killing myself. I am starting to think that might be about 8 degrees or so. I'm not building a chopper so a springer is not a solution for me. The raked tree might be a solution but I'd rather not go there either. 

[Xnavylfr]

If you increase RAKE then you will need a slightly longer frontend to make the bike sit level. For us CHOPPER guys, a small change in angle requires a +6" in fork length.
Here is another site that you can plug in the rake angle and all variables BEFORE you strart buying forks and find out they are TOO short. By the way a GOOD place to get FORK tubes is Forks BY FRANK.

http://www.rbracing-rsr.com/rakeandtrail.html

Xnavylfr(CHUCK)

[Stoli]

I was thinking about a larger wheel/tire in  the front and a longer swingarm and an appropriate wheel/tire in the back to get the bike back to level.

[Xnavylfr]

You have to remember on tires sidewall height and width . Even though you may have a 16" wheel now? with say a 130/90 tire if you go to a 18" wheel with a 140/80 tire the REAL difference is ONLY about an 1/2". Because the difference between a 16" and 18" is NOT actually 2" ,it is only 1" in diameter difference!!! That is how us CHOPPERHEADS can run 16" tires on the rear instead of 18" and still stay ALMOST level!!!

Xnavylfr(CHUCK)

[Stoli]

You have to remember on tires sidewall height and width . Even though you may have a 16" wheel now? with say a 130/90 tire if you go to a 18" wheel with a 140/80 tire the REAL difference is ONLY about an 1/2". Because the difference between a 16" and 18" is NOT actually 2" ,it is only 1" in diameter difference!!! That is how us CHOPPERHEADS can run 16" tires on the rear instead of 18" and still stay ALMOST level!!!

Xnavylfr(CHUCK)

Noted.

Maybe my best option is to increase the rake by 10 degrees and try to find a tree with an offset that is around 1" longer than the stock. That would keep the trail to under 5".

[Retro Rocket]

I was thinking about a larger wheel/tire in  the front and a longer swingarm and an appropriate wheel/tire in the back to get the bike back to level.

Remember, changing rim size on the front will also change trail slightly....

[754]

You can run stock fork with 10 degrees rake..but relocate sidestand, an deal with your pipes.. Highpipes make it easy..

[Stoli]

You can run stock fork with 10 degrees rake..but relocate sidestand, an deal with your pipes.. Highpipes make it easy..

Any good leads/links on high pipes or do I need to have them custom made?

[754]

I am trying to talk my buddy into making a set..like i have..if it goes well we will try to build more...those started off as a turbo headpipe, some look better than others.. Then I put the sidestand higher up the frame behind. Footpeg/motor bolt.

[lucky]

If I was doing any project I would not use math formulas with so many variables. I would just carefully measure the rake and trail so I KNOW what is actually true.

I agree that it is a good idea to measure to make sure you know what you wound up with, but formulas should be used to give you an idea of where to start.

The rake of a 1976 CB750 was 28º with 4.5 inches of trail. Offset not given.
With stock front forks and 10º of added rake would make 38º rake. Not very extreme at all.
Trail? You would need to remeasure it. I do not think it would be over 5 inches
but it depends on the length of the front forks.

I understand your confusion.
38º rake is not extreme but trail is a separate issue.

Many choppers on CB750 Choppers website that are raked have springer forks or girder forks.
Those type forks have no offset. the fork legs are in line with the steering stem axis.

Some photos show telescopic forks but do not tell you they have raked trees.
Just be careful when reading about  each bike or looking at photos. Some info is missing.

I would keep things simple and use "regular trail measurements" not "true rake".

I just built a CB750 that had 36º rake with stock CB750 front forks and stock triple clamps and it need nothing. It was rideable and not uncomfortable at all. That bike was sold.

BTW... Some books list CB750 rake at 27º10'

I would just call it 28º with bearing slop.


Please do not use photo without permission.

[Retro Rocket]

 

The rake of a 1976 CB750 was 28º with 4.5 inches of trail. Offset not given.
With stock front forks and 10º of added rake would make 38º rake. Not very extreme at all.
Trail? You would need to remeasure it. I do not think it would be over 5 inches
but it depends on the length of the front forks.

Lucky, the stock trail on a K1 to K6 is around 3.8 inches, no where near 4.5. I agree with what you said about measuring the trail, it should be measured at the ground, not the wheel as in the earlier diagram and the best way I have found to measure it is with a plumb bob for the horizontal line through the axle to the ground and a lazer pointer for the line through the middle of the steering head, mount it with some tape.....

[lucky]

The rake of a 1976 CB750 was 28º with 4.5 inches of trail. Offset not given.
With stock front forks and 10º of added rake would make 38º rake. Not very extreme at all.
Trail? You would need to remeasure it. I do not think it would be over 5 inches
but it depends on the length of the front forks.

Lucky, the stock trail on a K1 to K6 is around 3.8 inches, no where near 4.5. I agree with what you said about measuring the trail, it should be measured at the ground, not the wheel as in the earlier diagram and the best way I have found to measure it is with a plumb bob for the horizontal line through the axle to the ground and a lazer pointer for the line through the middle of the steering head, mount it with some tape.....

Thanks for catching that mistake.

[Xnavylfr]

LUCKY<  That Cycle One frame has been GOOSE NECKED and the steering neck has been lowered and not on a straight backbone . If you modify a STOCK CB frame neck to get +10 degrees you have to add 6" to fork length.  BEEN THERE, DONE THAT!

http://s465.photobucket.com/user/Xnavylfr/media/newbikeupclose-1.jpg.html?sort=3&o=31

Stock frame with 36 degrees rake.

Xnavylfr(CHUCK)

[Red Good]

Check this out , kind of an interactive rake and trail setup . Click on the virtual bike line . Will give you an idea what it will look like . Based on an hd frame .

http://www.classic-steel.com/

[Xnavylfr]

I've heard rumors that RED GOOD  would know a whole lot about that CYCLE ONE FRAME!!!   I could be wrong , but that's what I've heard!!



Xnavylfr(CHUCK)

[crazypj]

Thanks Lucky. So I was wondering if you have experience riding a CB750 with 6" of trail, or have you just read the same articles as I have?

 I've only ridden one and didn't like it.
Low speed steering is REALLY heavy and as Lucky says it want's to flop from one side to the other.
As a styling exercise that will be trailered around it will be totally great, as something to ride, unless you manage to avoid any traffic/low speed it will work.
As with everything, just because it can be done and has been done, it doesn't make it sensible or practical (after all, you 'could' cut your head off but personally I wouldn't recommend that either)

[Xnavylfr]

READ this info it will help a LOT!!

http://www.chopperhandbook.com/rake.htm

A better understanding of RAKE and TRAIL and how they relate to each other.


Xnavylfr(CHUCK)