Race chassis

[Frits Overmars]

I need all the help I can get so as me and Mr.JS were building up another FXR we have decided to go with these numbers J has cut and re-welded the steering head to steepen up the rake and we are just working out how much to chop out of the swinging arm. We measured J's bike and it is 1230 with work having been done to shorten the swinging arm so I reckon we can get close

Cutting and rewelding the steering head will place the front wheel nearer to the engine. You will be able to get away with a fair amount of steepening because the steeper fork angle will reduce the horizontal movement of the front wheel in suspension, so you will need less clearance between the wheel and the exhaust header.
The new front wheel position will improve both the wheelbase and the front/rear weight distribution. The steeper rake will also mean that braking will charge the front fork springs less than before (imagine the opposite, an almost-horizontal front fork, and you'll see why), so you may be able to fit softer springs.

Shortening the swing arm may further improve (i.e. shorten) the wheelbase, but it will also counteract the much-needed front-biased weight distribution.
My advice would be to try the steepened rake, combined with fork yokes that give the desired trail, without shortening the swing arm yet.
You can always do that later on. Afterwards, it will have been the same amount of work, but you will have learned more.

[cotswold]

Cutting and rewelding the steering head will place the front wheel nearer to the engine. You will be able to get away with a fair amount of steepening because the steeper fork angle will reduce the horizontal movement of the front wheel in suspension, so you will need less clearance between the wheel and the exhaust header.
The new front wheel position will improve both the wheelbase and the front/rear weight distribution. The steeper rake will also mean that braking will charge the front fork springs less than before (imagine the opposite, an almost-horizontal front fork, and you'll see why), so you may be able to fit softer springs.

Shortening the swing arm may further improve (i.e. shorten) the wheelbase, but it will also counteract the much-needed front-biased weight distribution.
My advice would be to try the steepened rake, combined with fork yokes that give the desired trail, without shortening the swing arm yet.
You can always do that later on. Afterwards, it will have been the same amount of work, but you will have learned more.

Thank you for that Frits

[mr bucketracer]

Thank you for that Frits

don't listen , he don't know what he's taking about ;-)

[Henk]

don't listen , he don't know what he's taking about ;-)

And you do? Mr drink this, it melts plastic cups?

[husaberg]

And you do? Mr drink this, it melts plastic cups?

Henk hes being sarcastic, if you look at the GPR frames, you will notice a much steeper head angle that the NX4 which they share running gear with.
Plus the firewater is just unbranded arthritis medicine.

[cotswold]

So if I were to raise the back up that would have the desired effect, would a slightly longer shock do it or am I barking?
Seems that dropping the forks through a bit and raising the rear plus the cutting and welding we've already done would be putting more bias onto the front?

[mr bucketracer]

Henk hes being sarcastic, if you look at the GPR frames, you will notice a much steeper head angle that the NX4 which they share running gear with.
Plus the firewater is just unbranded arthritis medicine.

lol , you worked it out

[mr bucketracer]

And you do? Mr drink this, it melts plastic cups?

ok only changes it colour lol

[mr bucketracer]

So if I were to raise the back up that would have the desired effect, would a slightly longer shock do it or am I barking?
Seems that dropping the forks through a bit and raising the rear plus the cutting and welding we've already done would be putting more bias onto the front?

your bike is already to fast in a stright line let .

[Frits Overmars]

So if I were to raise the back up that would have the desired effect, would a slightly longer shock do it or am I barking? Seems that dropping the forks through a bit and raising the rear plus the cutting and welding we've already done would be putting more bias onto the front?

Raising the back would slightly reduce both rake and trail, increase the cornering clearance and raise the center of gravity. The latter is not favourable by definition, though in most practical cases it has a positive effect on handling.
Dropping the forks will also reduce rake and trail, but it will also reduce cornering clearance. It will also move the front tire contact patch closer to the CG, so you'll put a bit more load bias onto the front. But moving masses like battery, fuel tank or radiator forward will help more. And moving the rider further forward will make a huge difference.
Take a look at the bike below; notice where the footrests, the seat and the rider are.

[Frits Overmars]

your bike is already to fast in a stright line let .

That's not racing. Even my daughter could do that. There's no fun without curves (pun intended).

[cotswold]

your bike is already to fast in a stright line let .

not this one, bog stock engine just pipe and carb
i was happy with the pace of the 50 that Terrene now owns but it had to make way for my CVT 50 that I'm keen to get going, hope that will be fast everwhere

[lodgernz]

Been thinking about what would be suitable wheels for a freetech 50 with 95/75r17s front and rear slicks. The Honda NS-1 came out with 17x2.15 and 17x2.75 wheels to take a 90 front tyre and 100 rear tyre. They're really cheap, probably fairly light given their size and readily available in Japan. They're the same design as the Honda NF4 RS125 too which I think look pretty cool. Thoughts?

The NS-1 has a steel frame, as someone has mentioned, and the rear suspension has no linkage, so is not progressive. The spring is progressive-wound, but that's not the same thing. It's also quite long with a wheelbase of 1285 - 1315 depending on chain adjustment.
And yes, they are readily available in Japan, if you're very rich. They're highly sought-after.
I have an old one, and it will be my next bucket project, with quite a few mods. I use those same wheels on my NS50F and NS90F buckets, BTW, and although they are pretty much the same design as RS125 wheels, they weigh a ton. Roughly.

[Frits Overmars]

The NS-1 has a steel frame, as someone has mentioned, and the rear suspension has no linkage, so is not progressive. The spring is progressive-wound, but that's not the same thing.

Indeed, not the same thing. There's nothing wrong with progressive springs, provided you fit them on dampers with the same built-in progressiveness, which is very difficult to achieve. So it's better to use a linear spring on a linear damper in a suspension geometry with built-in progressiveness.
A linkage system can provide any progressive behaviour, from zero to infinite, but with careful selection of the upper and lower shock mounting coordinates you can achieve up to 15% progressiveness even without a linkage system. And 15% progressiveness is sufficient for racing. In other words, you don't need a linkage system.


If the distance from the swing-arm pivot perpendicular to the shock (the length of the yellow line) increases during suspension movement, you have progressiveness.

[Michael Moore]

Linkages are useful for packaging when you can't fit a direct-mount damper where you have space for it. MX and big touring bikes can benefit from the large amounts of progression that can be obtained. But they do also offer a very large number of bad choices, and sometimes those can be had by being just a millimeter or two away from the spot that gives you the curve you are looking for.