1980 Suzuki GN400 - cafe conversion

[Untame]

Picked it up for $50. Want it to be something I build with my youngest daughter (15) according to her specifications. So far she has decided she wants it in British Racing Green with light brown / tan accents (frame?) to match the quilted seat cover (duck tail seat). Maybe light brown striping also on the tank. She also wants a half fairing same color with stripes that tie into the tank stripes.

I am going to work on lowering the forks to get the bottom of the tank level with the ground (maybe raise the rear). Might go black on the rims with bare steel spokes to give the tires a larger visual presence. Want the metal bits to be bead blasted or brushed finish - no chrome or high polish. Thinking matching green paint for the headlight bucket. For exhaust, leaning towards a supertrapp design.

ALL of this subject to change as the project moves forward! No rush... she has 2.5 years of high-school left, and will have to be financed with "side money" because the household budget is tight.

Most of all I want to spend time with my daughter. And I want her to learn about wrenching. I think we will both grow through this.

 

[ridesolo]

Nice find! It looks like a good starting point and I bet you'll have a good time.

 

[doc_rot]

Looks like the perfect first project.

Take care modifying the ride height. lowering the front and raising the rear will affect the handling characteristics of the bike, primarily by decreasing the trail. Decreasing trail typically leads to a bike that become more responsive up to a point where it becomes unstable and can be dangerous. UJMs are usually pretty good about toeing the line between responsiveness and stability so its always good practice to crunch some numbers to make sure you aren't going too far.
If you are modifying the geometry purely for cosmetic purposes its typically better practice to modify the tank/seat for the stance you want.

 

[Untame]

Looks like the perfect first project.

Take care modifying the ride height. lowering the front and raising the rear will affect the handling characteristics of the bike, primarily by decreasing the trail. Decreasing trail typically leads to a bike that become more responsive up to a point where it becomes unstable and can be dangerous. UJMs are usually pretty good about toeing the line between responsiveness and stability so its always good practice to crunch some numbers to make sure you aren't going too far.
If you are modifying the geometry purely for cosmetic purposes its typically better practice to modify the tank/seat for the stance you want.

Have no idea how to "crunch the numbers" on this. Definitely want it safe. I have read about other builds on this model that dropped the forks internally 3.5in and raised the back 1in.

 

[doc_rot]

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?

RB Racing Rake and Trail Calculator

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

 

[doc_rot]

Just found the rake/trail numbers in an old Cycle world article the "GN has an unusual combination of lots of rake (29.5°) and not much trail (4.06 in.)" working backwards and making some assumptions, if you dropped the front 3.5" you probably would end up around 82mm trail which would be deep into sketchy territory.

Why are you trying to lower the bike?

Suzuki Gn400 | Cycle World | OCTOBER 1980

magazine.cycleworld.com

 

[Untame]

Very cool info. I need some time to research this thoroughly.

From what I've read, the GN400 was a transitional configuration that has more of a cruiser stance. I really want something evocative of the 70s Honda cafe style.

 

[doc_rot]

You're going to be fighting an uphill battle starting with a cruiser. Not to say it can't be done, but it will take more work than starting with a 70's Honda. if you look at real cafe racers (or the racers they were emulating) you'll notice all of them have a good amount of ride height and are not lowered in any meaningful way. Good ride height is actually very beneficial for many reasons and I would not remove it unless you value "show before go" which is OK too but its important to understand the sacrifices being made. Ultimately its your bike, your money, and your skin.

The tricky thing with modification of geometry is one change precipitates many more. Instead of lowering the forks and removing travel I would say you're better off changing forks to achieve the "stance" you're after. here are some things that will bring the GN more in line with 70s honda cafe racer:

(1) Swap the entire front end for something else. The leading axle forks will never say "cafe racer" and the stock GN forks are budget bin at best. this will allow you to reduce fork offset so you can slightly lower the front and raise the rear while maintaining good geometry numbers. Lucky for you, Suzuki used a common head-stock bearing size and there are hundreds of options for fork swaps that can be done with moderate work. All balls lists bearing sizes so you can cross reference. Picking a bike with an 18" or 19" front wheel and swapping the entire front end over will probably make things easier in the long run.

| All Balls Racing

All Balls Racing Bearing and Seals packages all the needed parts to help with rebuilding your ATV, Dirt Bike, Street Bike, V-Twin,Snowmobile, All Balls manufactures 133 swing arm kits to cover more models than anyone, from 1973 to current,

www.allballsracing.com

(2) Over-length shocks - sized to suit fork swap, you need to figure out where you land with the geometry after the fork swap and size the shock length to put you in a good range.

(3) swap rear wheel to 18". 16" rear will limit tire availability and says more chopper than cafe. this will raise the rear as well.

(4) If you want the tank level, add a spacer in the rear to raise it up slightly.

This is the basic recipe that this guy followed. This is styled as a dirt tracker, but I think these 4 changes would be a great starting point for a cafe conversion. I think this bike has a nice"stance" and looks fun to ride!

If you want to go more extreme a swing-arm swap and fabricating a new sub-frame would sweeten the cafe racer pie.

 

[Untame]

Here is the one I saw that had the 3.5in internal fork drop.

Suzuki GN400 custom: A Craigslist special

This 1981 Suzuki GN400 is one of those increasingly rare custom motorcycles that was built for under $1,000—without actually looking like it was built for under $1,000. The donor bike was an old GN400 thumper that old friends Chris Errecart and Adam Bendig found on Craigslist; it wasn't running...

www.bikeexif.com

 

[doc_rot]

looks cool but looks like there is maybe an inch or two of fork travel in the front. Most street bikes have 3.5-5.5". if you go this route you really need to get stiffer springs to prevent bottoming out. If you bottom out a fork in a turn you can crash as the spring rate turns to infinity. and based off the numbers I ran this bike has extremely low trail numbers. I would guess this bikes handling is twitchy at best and dangerous at worst.

Its really not too much harder or expensive to build something that looks good AND works well, but it will take more research and time. There is a ton of knowledge and support on this forum.

 

[Untame]

looks cool but looks like there is maybe an inch or two of fork travel in the front. Most street bikes have 3.5-5.5". if you go this route you really need to get stiffer springs to prevent bottoming out. If you bottom out a fork in a turn you can crash as the spring rate turns to infinity. and based off the numbers I ran this bike has extremely low trail numbers. I would guess this bikes handling is twitchy at best and dangerous at worst.

Its really not too much harder or expensive to build something that looks good AND works well, but it will take more research and time. There is a ton of knowledge and support on this forum.

Thanks. Safety is first. Performance second. Style third. I will do my homework for sure to make sure the chassis is set up correctly for safe riding.

 

[irk miller]

One thing with the geometry on a GN 400 is that there isn't a ton of room to raise the rear end before the chain starts sliding on the swingarm. You're going to invest in chain sliders and before you buy shocks, I'd make test blocks to add length to the stock shocks and test for sag. The green bike with the RD tank looks like the chain is resting on the swingarm without sag, but doesn't appear to have an angle, so looks like it will be off the swingarm while riding.

 

[Luugo86]

Nice starter project bike.. Happy for you man; Looking forward to seeing your journey!

 

[Untame]

Modern bikes with a similar riding purpose / style have these rake/trail specs:

Honda CB1100 RS:

• Rake: Approx. 26°
• Trail: Approx. 99 mm (3.9 inches)

Suzuki SV650X:

• Rake: 25°
• Trail: 104 mm (4.09 inches)

Yamaha XSR700:

• Rake: 25°
• Trail: 90 mm (3.54 inches)

Yamaha XSR900:

• Rake: 25°
• Trail: 103 mm (4.05 inches)

Yamaha SR400:

• Rake: 27°
• Trail: 111 mm (4.37 inches)

 

[Untame]

(What happened to the Avon Roadrider posts??? I wasn't notified that my post was deleted, nor am I aware that any forum rule was violated.)

 

[Untame]

Just found the rake/trail numbers in an old Cycle world article the "GN has an unusual combination of lots of rake (29.5°) and not much trail (4.06 in.)" working backwards and making some assumptions, if you dropped the front 3.5" you probably would end up around 82mm trail which would be deep into sketchy territory.

Why are you trying to lower the bike?

Suzuki Gn400 | Cycle World | OCTOBER 1980

magazine.cycleworld.com

Gemo

The rake/trail specified in this article are suspicious. The geometry is not working out. In this sketch I have a front wheel with a situational radius of 12in (tire measures 24.375 OD), and when I specify a 29.5 degree rake with a 4.06in trail I end up with about 2 3/8" offset from the fork centerline to the axle center (measured perpendicular to the forks). But when I actually measure this is it 1 3/8". If, indeed, we have 29.5 in rake, we are closer to a 5in trail (127mm).

I do like the idea of finding an 18in rear wheel. Would you recommend the same tires front and rear, or would the rear tire need a different profile / sidewall for optimal handling?

 

[Untame]

I have a digital angle gauge, so I need to find a level floor I can actually use for measuring the rake. This would be the easiest to confirm. Trail is going to be tricky.

 

[irk miller]

The rake/trail specified in this article are suspicious. The geometry is not working out. In this sketch I have a front wheel with a situational radius of 12in (tire measures 24.375 OD), and when I specify a 29.5 degree rake with a 4.06in trail I end up with about 2 3/8" offset from the fork centerline to the axle center (measured perpendicular to the forks). But when I actually measure this is it 1 3/8". If, indeed, we have 29.5 in rake, we are closer to a 5in trail (127mm).

View attachment 238214

I do like the idea of finding an 18in rear wheel. Would you recommend the same tires front and rear, or would the rear tire need a different profile / sidewall for optimal handling?

You can’t ignore the offset and angle of your triple tree as you measure your rake and trail. It’s particularly common in the 80s to have fork angles that differ from the stem angle.

 

[Untame]

You can’t ignore the offset and angle of your triple tree as you measure your rake and trail. It’s particularly common in the 80s to have fork angles that differ from the stem angle.

Aaaaaaaaahhhh.... I should be looking at the offset from the "stem" (pivoting axis of the forks, I presume?). If the numbers in the article are correct, I would find the forks to be positioned 1in forward of the stem. But....

So the forks might extend forward at a greater angle than the stem? I suppose that was the style then.

 

[Maritime]

(What happened to the Avon Roadrider posts??? I wasn't notified that my post was deleted, nor am I aware that any forum rule was violated.)

they are in your intro post. not here.