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Discussion Starter #1
I have a sloooowly progressing s90 project that I would like to make into a fun 5 mile commuter.

I definitely want to upgrade the forks as the current are quite shot and generally crappy - doubt they were very good as new either. Some time ago I got decent pair of sl 100 forks (31mm vs. 27mm stock pogo sticks)

so my current plane is to use those forks shortened with sl100 or sl125 triples, yz85/kx85/cr85 17" front wheel and disc brake.

Dirt bike wheels are quite cheap and plenty available - generally the issue is that the hubs are very wide and need very wide fork spread - I haven't been able to find how far apart the forks are on 80cc bikes but would hope they are way narrower than on bigger ones.

Another issue is that I would need bracket for the disc break welded on the fork down tube. I have access to good quality TIG machine and am practicing that stuff now. Is welding a bracket a dumb idea all together? Remember this is < 10 hp bike.

If plan described above has some major holes please let me know.
 

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welded on stops has been done many times.
You probably want to pay professional weldor though as distortion of slider is quite common. (weldor is person, welder is machine you weld with)
Easier to get complete front end of later CB125, if you don't mind messing with cable disc or Suzuki GP100/125 as they came with hydraulic brake. I think Kawasaki or Yamaha may have something but don't remember trying to fit from those?
S90 only has 7~8bhp, I spent a lot of time 'playing' with them.
A CB175 piston fits and makes it (if I remember right) 112cc
With some porting, larger carb and 'decent' exhaust it will hit 90mph (70~75mph in third) but crank doesn't last very long because of the rpm and clutch has to be slipped to 10~15mph because of high gearing./

PJ
 

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Discussion Starter #3
Cool thanks,
I have actually a sl70 engine in it right now (lower 4th gear - not sure if other ratios are tighter too). And I have a ported head that I need to get new guides in waiting for future work. The little riding I did with the bike the terrible suspension was the biggest issue - I am relatively big guy - 6'4" 200lbs and live in Los Angeles where we have 3rd world country style pavement on 40mph streets. I would be still bouncing up and down 50 yards after the bump.

I like the idea of the sl100 forks as they are not that much too long and feel like well damping "real" forks. And they are way easier to come by than cb125 forks.

I don't mind the "custom" work of spacers an brackets. I take a machine shop class (more like do your own projects style - ongoing) so I have access to good lathes and milling machines etc. There are couple of really experienced weldors amongst the regulars and I think one of them will have the skillset to do a good job on the stop.

Also I don't need 90mph - just decent acceleration to 55 or so.
 

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It probably has the original fork oil? :D
Rear dampers are (Hmm, don't have a word for total shite/dangerous/crap, etc)
just about anything will b an improvement over stock.
Main problem is finding something long enough without too much travel and still have clearance on chain side.
You could probably pick up a Chinese knock off motor reasonably cheap and have a 145cc motor?

PJ
 

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yeah im pretty sure you can get a cheap 4 speed 145cc. I got a 110 under my workbench right now waiting for a suitable frame...S90s are unobtainium here. have you considered one of the xr70/xr50 clone hubs setup for a disc brake laced to a 17" rim?

This makes me wonder if a 17" rim would fit under a set of the cheap extended pitbike forks on something like an S90. my gut feeling is no, the tire will be too close to the neck and bottom triple clamp.
 

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from an email I get from the kat who built it:
The front and rear
rims are from dirt bikes. The front forks, rear shock, front and rear
brakes, and motor are all pitbike stuff off ebay. The motor is a GPX
from pitsterpro.com.


I've got a couple S90s and a lifan 150cc, I think there's a yz80 dead out back for hubs and brakes, the pitbike parts are easy to get. The forks I've been looking at locally are 40mm USD, externally adjustable comp/rebound damping etc etc and come complete with really clean milled aluminum triples that will take the S90 stem easily. They're $200.
 

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I wonder if you could adapt a single downdraft from like a FZR400 (didnt those have downdrafts???) to that engine. That would look really cool. That bike is pretty much exactly what i want to do with my chinese engine1
 

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I don't see why you couldn't, kinda been thinking about a downdraft carb for mine too....
 

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I might go to the "local" (100 miles one way) bikeyard tomorrow and see if I can myself an s90 to play with :)
 

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So wait....are those pitbike forks on that bike????
 

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Yup. the travel on them can be kinda long, especially with a 17" wheel between the uprights but spacers and springs can be reworked.
 

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Is the frame completely hand built? It looks like the basic xr/crf70 design but is definately taller. I have a couple of bare xr80 frames but they are the older twin shock models.
 

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Discussion Starter #13
Nice Bike on the picture.

However on my s90 I am going to try to stay all honda in terms of the engine. The pitbike engines don't bolt straight in - pitbikes have the xr50/crf50/z70/ct70 style two bolt mounts. S90 is with 3 bolts. I have 2 90 engines and just took the s90 engine apart - sl90 engine on the bike now.

The plan is to go with 53 or 54mm cb175 oversize piston. Webcams grinds hotcams though they are only listed under ct90 and atc90 (3-wheeler) but they are the same fit. When I start getting some machining done within decent tolerances I am hoping to get to stroking a crank so the total cc would be around 120. Also dreaming of cutting valve seats 1mm oversize and use shortened cb175/cb200 valves (same stem diam 5.5mm and apparently a little longer so new rtainer grooves can be cut and stem can be shortened.

Back to the front wheel/forks topic. I got the yz rim and it looks very cool between the sl forks - the brake disc ends up running very very close to the fork but it looks like I can make it work with the CL triples - if its too narrow cm200/cm185 triples should work. I would rather use the narrow triples to keep the narrow style of the bike.

I will be playing with the front end today - maybe can even take a picture.
 

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For what it's worth, I've got a set of forks from a 1989 CR80, and the legs are spaced about 160mm center-center. Let me know if you want more numbers, I'm in the middle of swapping them onto a different bike so I've been doing a lot of measuring.
 

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The guy said he build the chassis so I can only go with what he said.
Nifty little bike, got me interested in messing with the S90...again.....
 

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Discussion Starter #16
The bummer about small bikes is that its near impossible to put rear sets without having to resort to push only start.
On s90 it "might" be possible to have the kicker more horizontal and resting against the rear set when not engaged and have a more scooter like step kick. Not sure if the shortened kick range would still work. Hydraulic brake would be help a little as you wouldn't have the bar on the way.
 

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i wonder if i could build something to bend a big piece of square tubing like that....
 
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