Suzuki GN400 Thumper

[ILoveThumpers]

Enough lurking, time to start a build thread.

So here's me. And here's what I do.

And here's the bike the day I got it from my father-in-law. It was given to him by his father-in-law but only had one owner. Basically it hadn't been ridden in about 5 years. I drained the carb, exchanged the oil and it started on the second kick. 20k ish miles and bone stock.

My goals:
[list type=decimal]
[*]Remove anything unnecessary.
[*]Improve the handling for in-town driving and maneuverability.
[*]Convert the 6v system to 12v
[*]Make it more comfortable to sit on/controls position/handlebar position.
[*]Not injure myself in the process!
[/list]

Oh, also I would like it to look something like this when I'm done:

What I really don't care to do:
[list type=decimal]
[*]"The Ton" I have no desire to go 100+ MPH on this bike. It's for transportation to/from work primarily and maybe a motorcycle show or two if it comes out pretty enough.
[*]Compression work. It's a big-ish single and already at 9.5:1 compression. No electric start so I have no desire to raise compression.
[/list]

This site really is awesome; in my limited time lurking and researching here I've already found that another bike used the same style wheels (mags, which I love) and there's an 18" rear available to match my 18" front. that should solve some of the weird "squish" I currently experience on this thing.

I'm right at 6' tall with long legs so my knees are just under my chin when I sit in the driver's position... the seat just has to go! The chopper handlebars are a nuisance as well so those are the two changes I'll be making this season.

The rest of the season will be riding and planning. The "real" build will start in the late fall when it's just too cold to ride... or if I get bored between then and now. (Yeah, right)

-Deek

 

[ILoveThumpers]

Suzuki GN400 Thumper: Geometry Planning

The bike stock has kind of a "chopper" look; I can't decide if I like it or not but it really is kinda ugly at first glance. Despite how it looks, it handles okay... at speed. 45mph or more and it's fairly solid other than a little "squish" in the rear end. Shocks are plenty stiff so I think it must have to do with that nasty tire back there.

The main issue I have with this bike is at low cruising speeds and stopping/pulling away from stops. It seems to take a lot of effort to get the thing to move as quickly as I want it to. Let's just say it's "stable"... too stable.

So, on to geometry. Here's where it stands:

Most of those figures come from the Motorcycle World test drive; the ones that didn't were either measured directly or calculated. All calculations are unladen. I'll probably re-do the CoG with me on the bike so I have a good reference for the seating changes I'm making. (Center of gravity was measured using the method described at this resource.)

Changes planned:
[list type=decimal]
[*]Larger rear wheel (18") but shorter tire
[*]Maybe slightly longer rear shocks (0.5" longer)
[*]Lower profile tires 100/90 so slightly shorter diameter
[*]Shorten forks around 3"
[*]Move seating position back
[/list]

Plugging those values into this calculator give me the following measurements: (I haven't diagrammed it yet... soon)
26.24 degree Rake
3.23" Trail
53.56" Wheelbase

So that's going to move the center of gravity forward and up (unladen) but the seating position change will definitely move it back. I'll actually have to make the changes to measure it properly.

-Deek

 

[rockcitycafe]

Re: Suzuki GN400 "Thumper"

3.23 on a short wheelbase like that sounds a little short... my 750 is 3.7 on a 58" wheelbase and it's stable and still turns quickly, that sounds like it might wind up a little twitchy, also, be sure to measure trail at full front compression, it's especially important when the front wheel is loaded under heavy braking

 

[ILoveThumpers]

Good point. The wheelbase is short indeed; I've considered a longer swingarm to extend it a bit. I've tried comparing the new dimensions to other bikes but can't find one that's precisely the same. Still looking, though. Motorcycle World typically made those measurements with a 160lb rider. I'm right at 200 so I really should measure it myself.

I'm reading in Foale about ground trail and mechanical trail now. I'll probably add those to the diagram eventually along with CoG with rider on.

I'm not opposed to a steering dampener but would rather not require it for normal riding!

-Deek

 

[ILoveThumpers]

Suzuki GN400 Thumper: Geometry Examples

In searching for examples of the GN400 made into something "other" I've found a few examples. There are 2 that are of interest with regards to my plans for the geometry of the bike:

This minimalist take on the bike, by Holiday Customs presented at Pipeburn.

The builder chose to slide the forks up in the tree but as best I can tell, kept the overall length the same. It's difficult to tell how far down the tube he went; I would guess between 2 and 2.5 inches. (There is very little detail available about this particular build.)

Another is this little brat by Chris Errecart and Adam Bendig. Chris shortened the shocks 3.5" AND lowered the tree so the clip-ons would clip above the top tree... That's something like 5.5" shortened!

Doing the math on that little devil:
24.29 degree Rake
2.75" Trail
52.71" Wheelbase

From the article linked above: "The finished GN400 is ridden almost daily by Adam and he has some road trips planned that will test the mettle of that yoga-mat saddle. He says that the bike is as reliable as gravity and he’s out on the B-roads whenever his understanding wife and three kids will allow."

Chris Errecart owns the bike now; in the comments of that article Chris says "If you take a look at the suspension on these bikes in stock form, you can see there was more than enough to be removed and still maintain some function. There are still several inches of travel, springs are still in place, and an appropriate amount of fluid in there as well. I've got over 500 miles on this setup and I don't have any complaints."

There's a daily driver with a more aggressive take on the GN400 than I have for sure.

-Deek

 

[ILoveThumpers]

Fit 18" spoked wheels front and rear, conventional forks (not leading axle) 1-1/2 " longer rear shocks or, better option, relocate top mount couple of inches further back
It's a light bike without a massive amount of power, doesn't need long wheelbase.
I would fit a longer swing arm if it's 'twitchy (probably twinshock RM or DR)

Don't like spokes, personally... they'll be 18" anyway, is there a reason you suggested spokes other than aesthetics?

Centerline forks would help with the trail... what about flipping the forks around so the axle is trailing? (I want to keep build cost down if I can, but not at the expense of a shitty handling bike!)

Are you suggesting raising the rear instead of lowering the front, or both?

Moving the top mount is not a problem; those shocks lean a lot and I'll be doing some welding back there anyway.

-Deek

 

[Big R]

Re: Suzuki GN400 "Thumper"

Signed up.....

Spoke wheels have the advantage of fitting whatever size rim you'd like. Pretty much any GT / GS front spoke hub will bolt right in, and the rear shouldn't be too difficult either.

 

[ILoveThumpers]

Spoke wheels have the advantage of fitting whatever size rim you'd like. Pretty much any GT / GS front spoke hub will bolt right in, and the rear shouldn't be too difficult either.

Gotcha. I don't dislike spokes, but I do prefer cast. I think it's an 80's kid thing...

-Deek

 

[rockcitycafe]

Re: Suzuki GN400 "Thumper"

a daily ridden bike with bad geometry sometimes never shows itself until it's put into a bad situation, then all hell breaks loose, like coming around a bend and getting the front wheel off the ground, and when it comes down again, loading the front suspension to the point that the trail becomes dangerously short and causes a tank slapper... best to engineer it from slow turning to twitchy, rather than going the other way

 

[johnu]

Re: Suzuki GN400 "Thumper"

I like this project already, the frame and suspension mods on bikes are what really interest me. You might want to add your counter shaft center lines too if you are playing around with different swing arms and longer shocks. Good luck with the build.

 

[ILoveThumpers]

Pretty sure Buell Lightening was only 55" wheelbase?
With 'race kit' it's 105bhp (at least the one I rode was 8) )
Handling was great
Reversing forks will be a real bad idea, you'll probably have 0~1" trail?
Lower front and raise rear
You can probably find a GS550 wheel to match front

Reversing forks increases trail, not decreases. Remember the steering head center line is in front of the wheel axle center line already. I did the math on that configuration and the trail jumps to about 7".

Moving the top shock position will raise the rear a bit. Working on that (and about a thousand other scenarios!) at the moment.

There are a few GS450 wheels on eBay at the moment. One even has the brake assembly but I need to see if the current brake for the 16" will fit.

What's with people shipping tires on the wheel? Talk about a waste of shipping!

-Deek

 

[ILoveThumpers]

a daily ridden bike with bad geometry sometimes never shows itself until it's put into a bad situation, then all hell breaks loose, like coming around a bend and getting the front wheel off the ground, and when it comes down again, loading the front suspension to the point that the trail becomes dangerously short and causes a tank slapper... best to engineer it from slow turning to twitchy, rather than going the other way

I've measured fork travel (and rear suspension travel) and I'm in the beginning stages of working out the extreme case scenarios. Foale has a nice section about steering head drop and the effect on lean/steering angle. It's a bit over my head at the moment but I'm learning. In any case, I've not ran a scenario yet that takes the geometry out of the realm of "normal" except for reversing the axle position.

-Deek

 

[canyoncarver]

Re: Suzuki GN400 "Thumper"

Following. I have a fair bit to learn in this department.

 

[ILoveThumpers]

Re: Suzuki GN400 Thumper: Geometry Examples

I wouldn't ride that bike on a dare. Under three inches of trail=deathtrap.

As reliable as gravity? That bike would make me fucking terrified of gravity.

LOL!!! I took the "reliable" comment to mean the motor. It's a well-established platform used on dirtbikes for lots of years.

But yeah, that suspension scares the shit out of me.

I measured 6.5" of total fork travel. If he took 3" out leaving him with 3.5" of travel. After pre-load (if there is any) I'm betting he has MAYBE 2" of travel? No thanks.

-Deek

 

[ILoveThumpers]

Following. I have a fair bit to learn in this department.

Me to. We studied this stuff in trigonometry in high-school because our teacher was a gearhead. But I've forgotten, well, all of it.

Not a single resource I've found will go anywhere near any kind of "rules of thumb" for any of these measurements. The Foale book mentions a "magic" rake established by racers of 27 degrees but then takes much pain to describe the ridiculous variations in leading/trailing/centerline axle positions.

There are some accepted ranges mentioned... rake from 23 to 30 degrees, trail between 5.5 and 3.5 inches (all assuming a given wheelbase and tire diameter, of course)

The lesson I've learned thus far is that you can make almost any "normal" suspension work at some speed, but at the sacrifice of something else... it's all a trade off and you have to tailor it to your goals.

-Deek

 

[ILoveThumpers]

Would you consider raking the frame a bit? A little more rake in the steering stem would add trail and achieve some of the lowering that you are looking for. In my rarely modest opinion, if a bike needs a steering damper to keep it from going into tank slappers, it's much like putting lipstick on a pig.

I'm not opposed to it; a modest change would be a quick cut'n weld job on the top. Based on what I've read thus far, I don't think I'll need to, but then again I'm not done with calculations yet.

There are factory bikes with steering dampers; these are obviously crazy powerful and meant to be ridden way faster than I ever care to go on two wheels but it's a valid design element in certain situations. Not mine, though!

 

[ILoveThumpers]

I love watching those skills videos. It's simultaneously inspiring and humbling. As in "DAMN! I want to ride like that!" then "Shit... I'll never ride that well."

I just don't have the time to devote to improving my skills to that point. If it were my job (like it's theirs) maybe I could get there but sadly, I sit behind a desk all day.

-Deek

 

[ILoveThumpers]

Yeah, was a bit tired and not thinking straight...

No worries. I've been soaking in it for a couple of days; that's the only reason I caught it as readily as I did. It was a bad idea regardless... 7+" of trail would be nice on a drag bike!

-Deek

 

[Big R]

Re: Suzuki GN400 "Thumper"

I remember we were discussing something else recently......what was it? Oh yeah - a Suzuki GN400.

 

[scrambledpegs]

Re: Suzuki GN400 "Thumper"

This is an amusing topic. Ha lipstick on a big line killed me!

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