Untame
Been Around the Block
LOL... thanks! After I got the bike I tried to move the convo over to the Project forum.they are in your intro post. not here.
LOL... thanks! After I got the bike I tried to move the convo over to the Project forum.they are in your intro post. not here.
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?
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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
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?
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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.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.
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.Just thinking outloud... what about reversing the forks so that the wheel is offset in the other direction ("trailing axle forks")? This should basically zero out the offset. Not at my computer, so I can't check the geometry.
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.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.
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.Who makes the best quality clip-ons?
you'd be a fool for blindingly following strangers advice on the internet - this forum included. So good on ya for looking deeper.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?
Awesome information. I really appreciate you taking the time to spell it out for me.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.
I cannot stress how much you will nerf the bikes handling by shortening the forks beyond 20-25% and unless you swap for a stiff forks spring it can be dangerous. have you ever ridden in a car that has had 75% of travel removed? Having been a mini trucker I can tell you this is not the way. lol.
https://woodcraft-cfm.com/Who makes the best quality clip-ons?