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Discussion Starter #1
So I stumbled on some gsxr600 front forks. They were stupid cheap so I couldn't pass it up. Unfortunately this is a cx650c so the front rake is stupid at 32°. With the front forks on with no sort of extension that drops the front to around 26-27°. I plan on adding some 50mm spacers to the forks to bring the bike back up to the right height. But then the rake is still at around 28-29°. So now the question is should I cut the neck off and reframe it at a lower rake? The gsxr is at 23.5° if I remember correctly.

With shortening the bike I cant see it causing it to handle poorly. It would likely improve handling? And with moving the neck and lowering the rake angle to around 24-25°. That's not going to make it hard to ride or handle poorly either?

I'm still wrapping my head around these.
201172
 

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The rake angle is virtually meaningless. It is the trail that is important. Trail is the distance between where a line drawn through the steering head (NOT the forks) meets the ground and where a vertical line through the axle meets the ground. Larger trail provides greater stability at highway speed at the expense of requiring more steering effort at low speeds. Less trail makes low speed steering easier but can result in twitchy handling at speed *= more concentration needed to keep it in a straight line).
The guys that built the original choppers increased the rake so that they could ride bikes designed for rutted dirt roads (usually pre-war and war surplus Harleys) on the then-new paved highways and fork length to keep the front end from becoming too low. By the late '70s bikes were designed with adequate trail for highway stability but "choppers" had become fashionable and people were "building" them with the idea that if some rake was good more must be better (it wasn't) so manufacturers responded by designing "factory customs" (like your CX650C) with the appearance of large rake angles but with more normal trail values, usually by moving the axle ahead of the fork legs.

Fast forward several decades. Choppers have gone out of fashion and cafe style bikes are in fashion (usually made by people that don't understand what the changes they want to make will do to the way the machine works)(people like you, who ask questions and try to understand instead of blindly copying the "style" are to be commended but alas are all too rare).

SO by lowering the front end with shorter forks you have decreased the rake and thus the trail. But by changing from leading axle forks to ones with the axle centred you have increased the trail. But how much trail is right for you?
For reference, a stock GL650 has 117mm of trail, a CX650E (the sport model) has 105mm and the CX650C has 126mm.
If you are building a bike to use on the race track (where you expect to have to concentrate intensely all the time but at least all of the traffic is going the same direction) you would probably be OK with 90-100mm but if you expect to use it for long highway trips and want something with more stability something in the 115-125mm range would be better. For general use (in town + highway), I would shoot for something like 105-115mm.

And then there are the effects of lowering the front of the frame. This will decrease ground clearance (you started with 155mm; I wouldn't go much less than 125mm). It will also alter the angle of the floor of the fuel tank (it needs to slope toward the location of the petcock in order to make all of the fuel available). It can also lower the front of the seat and increase the tendency to slide forward on it.

As for using preload spacers in the forks to raise the front end, the best way to do that is to determine the optimum length by using the calculation here How your suspension works

If it was me, I would start by calculating & installing the spacers, then measure the trail and decide whether it is suitable for your planned use. If that is OK, make sure the tank is still lower at the rear and go from there.

BTW: You didn't mention wheel & tire sizes and types. That can have an effect on handling too.
 

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BTW: Welcome to the forum. Please add your location to your profile and your bike's model and model year to your signature so that you don't have to remember to tell us every time and we don't have to keep asking when you forget (see Forum Settings link in my signature).

And welcome to the world of antique vehicle ownership (they own us, not the other way around). Your bike is about 4 decades old and the Previous Owners may or may not have done the maintenance necessary to keep it safe & reliable so it is highly recommended to download the Factory Shop Manual for your model (available through the CX Wiki - link in my signature) and go through all of the service procedures, regardless of whether your bike has reached the specified mileage.
I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. Check the date codes on your tires and replace them if they are over 5 years old no matter how good they look & feel because old rubber simply cannot flow around the irregularities in the asphalt well enough to grip, especially if it is cool or wet. If your bike still has the original rubber brake line(s) (should be replaced every 2 or 3 fluid changes = 5 or 6 years) I recommend shopping for modern stainless braided ones (they last practically forever and double the life of the fluid).

The best advice anyone can give you about customizing any vehicle is to get it safe & reliable in more or less original condition and use it for a while before you start making any changes so it can tell you what changes it needs to make it do what you want/need better. That approach almost always results in something you actually want to keep and use but making changes based on style or on what someone else (who may or may not really understand how the changes affect the way it works) has done often results in a piece of expensive yard art that you can't stand sitting on for more than a few minutes and might even be dangerous.
 

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get new fork tubes if using a newer upside down fork set or a cx frame or a gl and even a gl frame its tough to find the right length ujm forks
the cx650c forks are 3.5 inches longer than a regular 500
1.75 longer than a gl650 set so puttingt the steering right with any modern stock ujm fork is pretty much a no go
you cant drop the neck that far as you will lose connection with the upper tubes
the frame is set up as a cruiser its the wrong bike to do this with wait till you get to doing the rear of the frame
 

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Discussion Starter #5
Hey thanks guys! I'll be doing a lot more math before I ever cut anything. The bike will be getting 40 years worth of service done.

As for riding it around wasn't an option when I bought it. Was able to get it running. But old tires and not getting maintained for that long made it reckless to take it out.

As for the plans on the bike. It's mostly just for short city rides and zipping around. It will be getting custom just about everything. Metal fab is always a lot of fun for me. The seat, pegs, and handles will all be heavily considered before its ever finished. I want a bike I can be comfortable and be safe anywhere I go.

I know its not just a quick thing to slap together. I gave myself a few years to do this. I would never tear into something without understanding how it works. But that's why we do this right? To learn and grow? To make something at the end of the day we can have pride in? I take a lot of joy in learning and using my hands.
 
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