Untame
Been Around the Block
LOL... thanks! After I got the bike I tried to move the convo over to the Project forum.
1980 Suzuki GN400 - cafe conversion
- Thread starter Untame
- Start date
Untame
Been Around the Block
Thanks for this calculator. Very interesting. It shows there isn't much room for adjustment. I could go with an 18in rear wheel and take 1/2 out of the shocks (internally) and end up 29 degrees with 100mm trail.4.5" overall change is pretty significant. That may still be in the realm of "safe" since this bike is kind of a cruiser thing which typically have more relaxed/stable geometry. However, without knowing how high is up you really don't know what you're getting into. People do all kinds of things to their bikes to make them look cool that render them nearly unrideable. but hey, they look good in photos!
Its fairly straightforward to check the geometry and see if you are falling into a good range. Below is a calculator you can plug in your values. Most sport-bikes fall into the range of 24-26 degrees of rake and around 100mm of trail. The most hardcore sport bikes rarely go below 23.5 degrees of rake, and 95mm of trail. things can get twitchy and unstable below 95mm trail. The OEMs with their legions of engineers have decided this small range is the best balance of stability and responsiveness. Getting close to these numbers is a good target for a sporty bike. Cruisers often have much more rake, up to 35-40 degrees more trail at 130mm+. You don't want this. it will make the bike turn sluggishly and feel heavy. Dont forget the leading axle should be factored into the forkoffset measurement.
I would take some measurements at the stock ride height, then play around with with sliding the forks up in the triple tree to see where you end up.
Another thing to consider - lowering forks by 3.5" is huge. I'm not sure the technique you are planning on doing to accomplish this but considering the GN400 probably has around 4-4.5" for suspension travel I think you are going to create a fork that has a high potential for bottoming out which can be dangerous. Most of the time forks are only lowered an inch or so and when done properly a stiffer fork spring is fitted to accomodate. Have you considered doing a fork/front end swap?
What The Heck Is Rake and Trail? A Better Understanding.
The way your motorcycle handles has a large part to do with rake and trail but what do these terms mean? Here, we break down each term and what they mean.www.motorcycle.com
On the bike I posted earlier he pushed the forks up through the triple and attached his clamp-ons there. It is a nice look. Would this also (geometrically) have the same impact on rake/trail as shortening the forks?
Maritime
Over 10,000 Posts
yes sliding up = shortening forks. If you go clipons it's the easier way. If you use bars on the clamps internal is cleaner looking but you can still just slide the forks up equal on each side/Thanks for this calculator. Very interesting. It shows there isn't much room for adjustment. I could go with an 18in rear wheel and take 1/2 out of the shocks (internally) and end up 29 degrees with 100mm trail.
On the bike I posted earlier he pushed the forks up through the triple and attached his clamp-ons there. It is a nice look. Would this also (geometrically) have the same impact on rake/trail as shortening the forks?
More likely the forks
Something to look at. As an example, I have several CB650s. They run a fork angle that is steeper than the stem angle. Very confusing, especially when you try their front end on another bike. Makes for a really long trail.
your bike has leading axle forks (putting the axle in front of the fork center). This distance needs to be added in the fork offset so its the offset in the triple clamp. Total fork offset, will be the center of the steering pivot to the center of a line from fork center to fork center, then add the offset of the center of the axle from the center of the fork. I would guess this number is around 2.5"
When you are making these measurements its important to be very accurate as even a couple mm change to the trail number will have a noticeable impact. I would not round any figure to anything less than 1/16" or preferably 1mm.
The leading axle forks are what is going to be your biggest challenge in changing the geometry. I say just bite the bullet and do a front end swap to conventional forks with the axle inline with the fork center. you'll end up with a look more in line with what you want and it will likely have better geometry numbers.
a front end off a CB500 or KZ650/KZ750 or SR400/500 would probably be reasonably easy to swap.
When you are making these measurements its important to be very accurate as even a couple mm change to the trail number will have a noticeable impact. I would not round any figure to anything less than 1/16" or preferably 1mm.
The leading axle forks are what is going to be your biggest challenge in changing the geometry. I say just bite the bullet and do a front end swap to conventional forks with the axle inline with the fork center. you'll end up with a look more in line with what you want and it will likely have better geometry numbers.
a front end off a CB500 or KZ650/KZ750 or SR400/500 would probably be reasonably easy to swap.
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If you flip the forks you will increase your trail, both from moving the patch so far back, but also because you're increasing the rake by raising the steering head. It's actually going to make things worse. Cruisers ran leading axle forks to compensate for the rake. As Doc said, you really need to go with conventional forks with the axle centered.
Untame
Been Around the Block
For giggles I mathed it out in software. If I reverse the forks, and shorten them 3.5in, and if I use an 18in rear wheel and add 1in of suspension lift, then I end up at 25 degrees of rake. But I am only able to pull the trail down to 153.
Here is a table of bikes' trail/rake and the ratio. The ratio averages out to about 4:1, but the high performance bikes are closer to 4.25:1.
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Untame
Been Around the Block
FYI Doc/Irk/Maritime (and others)... Don't want you to think I don't trust your advice. I'll be the first to admit I'm completely green when it comes to motorcycles. But I learn by digging through the facts behind opinions and proving (or disproving) assumptions. I really appreciate all of your advice!
My first preference for this build will be to work with what I've got. But if a fork swap / wheel swap / etc. makes sense and is reasonably simple and inexpensive, then I will go that route.
Where is a good source for parts like a set of forks or rear wheel? How can I verify interchangeability?
My first preference for this build will be to work with what I've got. But if a fork swap / wheel swap / etc. makes sense and is reasonably simple and inexpensive, then I will go that route.
Where is a good source for parts like a set of forks or rear wheel? How can I verify interchangeability?
Maritime
Over 10,000 Posts
EBay for parts. Swap-ability of full front end you need bearing and stem sizes mostly. There is a reference list on here for a lot of those. You could swap just the tubes easily buy getting a set of the right diameter in the length close to what you want. Whole front end is doable but may need new bearings and or a stem swap etc.
Maritime
Over 10,000 Posts
IDK there are well made no-name brand and poorly made name-brands and then expensive well made name brands. I look at reviews with a grain of salt but you can find decent ones out there. If you can get a set of OEM(Honda/Suzuki/Kawasaki/Yamaha/etc.) ones in the size for the forks you know those will be good.
you'd be a fool for blindingly following strangers advice on the internet - this forum included. So good on ya for looking deeper.
Rake and trail are simple indicators of a bikes handling. however, the entire system needs to be considered if you start making changes like you have suggested.
By shortening/flipping the forks you are affecting these other important variables:
Wheelbase - by shortening the forks you shorten the wheelbase, this will effect resistance to steering in turns, and "drive"
center of gravity - its counterintuitive but by lowering the CG you are making it harder to "tip in" to a turn - look at how high moto GP bikes are
ground clearance - reducing not only effects clearance over obstacles but effects how far you can lean over, usually the first things to touch are foot pegs.
mass centralization - if you plan to flip the forks you are moving the mass of the brake caliper forward from the pivot, this will make it harder to turn quickly and make the calipers more likely to be hit by road debris. there are reasons every manufacturer since the early 70s puts the caliper behind the fork legs.
Chassis flex - all frames flex some. To achieve steep rake angles the frame needs to be fairly rigid to handle the stress of braking. the single loop frame on these bikes is not the most rigid, its possible to get fork judder with steep rake on flexible frames. The only bike on your list with a single loop frame is the SR400 at 27 degrees.
These are just a handful of variables that are present in chassis design - one change precipitates another. If you want to go deeper the book Motochassis is a good starting point. Its a good blend of technical knowledge combined with layperson descriptions. i highly recommend it for anyone that wants to mess with chassis geometry.
That being said I can fully get on board with wanting to build something on the cheap, but I would ask why do you want to lower it so drastically? if it is simply for "stance" this can be achieved with cosmetic styling changes like swapping gas tanks, or modifying the one you have. Stance is an informal concept that can be achieved through a myriad of ways.
FYI it looks like there was a trim level on the GN400 - the "GN400E" that came with conventional forks and an 18" rear wheel. this is probably the easiest way to get the 18" rear wheel. other than that looks at parts fiches to cross reference bearing sizes, and parts overlap from manufactures is your best bet. If you have a local moto junkyard wandering around with a set of calipers works too.
If you want some low stakes empirical evidence. Get the bike running and drop the forks 3.5" through the handle bars and see how it feels.
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Untame
Been Around the Block
Awesome information. I really appreciate you taking the time to spell it out for me.
https://woodcraft-cfm.com/
If you can find a well shaped tank that has a really deep tunnel in the front to compensate for tall neck, that may work really well to place clip ons under the top triple and give you a nicely lined profile.
Untame
Been Around the Block
To keep this project simple, I like the following approach regarding the stance:
1. Put an 18" rim on the rear with Avon Roadrider 130/70-18 (25.6in OD) in place of stock 4.60-16 tires (23.5in OD, 110mm width - height might affect clearance with stock shocks)
2. Put Avon Roadrider 100/90-18 (25.3in OD) on the front in place of stock 3.60-18 (24.4in OD, 90mm width)
3. Move forks up 1in in clamp and attach clip-ons above top clamp (an effective 1in fork drop)
4. Leave fork internals alone for identical suspension preload and travel
This gets me:
1. 28.3 degrees rake (minus 1.2 degrees)
2. 102.5mm trail (minus 1/2mm)
3. 55in wheel base (minus 1/2in)
4. Less visual gap between front wheel and frame (subtle aesthetic shift from cruiser to sport)
5. Traditional visual impact of same wheel diameters and tire diameters (while maintaining widths appropriate for performance)
So, a couple minor changes (new rear rim or wheel, and move the forks up 1in) plus the new tires, and I think these subtle changes help achieve the café racer aesthetic without hurting handling, and maybe even improving the handling.
What do you guys think? A reasonable approach?
1. Put an 18" rim on the rear with Avon Roadrider 130/70-18 (25.6in OD) in place of stock 4.60-16 tires (23.5in OD, 110mm width - height might affect clearance with stock shocks)
2. Put Avon Roadrider 100/90-18 (25.3in OD) on the front in place of stock 3.60-18 (24.4in OD, 90mm width)
3. Move forks up 1in in clamp and attach clip-ons above top clamp (an effective 1in fork drop)
4. Leave fork internals alone for identical suspension preload and travel
This gets me:
1. 28.3 degrees rake (minus 1.2 degrees)
2. 102.5mm trail (minus 1/2mm)
3. 55in wheel base (minus 1/2in)
4. Less visual gap between front wheel and frame (subtle aesthetic shift from cruiser to sport)
5. Traditional visual impact of same wheel diameters and tire diameters (while maintaining widths appropriate for performance)
So, a couple minor changes (new rear rim or wheel, and move the forks up 1in) plus the new tires, and I think these subtle changes help achieve the café racer aesthetic without hurting handling, and maybe even improving the handling.
What do you guys think? A reasonable approach?
