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Discussion Starter · #1 · (Edited)
So, I am a bit of a mad scientist, and can't leave anything alone... I turboed my 92 cb250 nighthawk. Took a few years learning how to turbo something with carbs, But I got it running really well.

Now to a new problem, the clutches keep slipping at full boost.

I have tried ebc's heavy duty springs(10%) and both their HD and CF disks.

I have also tried KG factory springs (20%) and their high performance disks.

I have also made some spring spacers to pre-load the KG springs a little for higher clamping force.(only a slight improvement).

I haven't tried mixing and matching disk and springs.

KG has worked the best so far. With the spacers. 1st and 2nd hold fine. But 3rd around 8krpm at 9-11psi slips.


Anyone got any ideas? KG springs with some other disks?



Thanks!
Mark
 

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Hey Mark,

No answers here unfortunately, but some ideas to noodle that may be helpful.

The CB250 looks like it makes 21hp factory. I would suspect that the clutch is designed around that power and torque output, and now that you've boosted it you have upset the balance between the friction coefficient and the torque- hence the slippage. What do you think about that?

I have to believe a bike with more power has something different about the clutch. A GSXR at 120hp can't have the same clutch as a 20hp 250. I am not sure if that is different friction materials, surface area of the plates, or number of plates. Does anyone happen to know?

Another thought is what does the Honda crowd do when they are boosting civics etc? Gotta imagine they run into this too. I would guess they buy an upgraded clutch because they are available, unlike for your 250.

I wonder if EBC or KB would have any recommendations for you, have you tried hitting their support up?
Would a different oil with different additives affect the friction?

I do have a '02 XR400 clutch laying out in my shop. Happy to take some measurements or photos if at all helpful. But that wouldn't really help ya fix your slippage.
 

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Discussion Starter · #3 ·
I think you are right! My guess is I am around 35hp now (cant wait to have a good clutch and dyno it) I havent thought of different oil with different friction modifiers yet. I had been running some yamalube I had left over from a friends bike. and most recently I put in some random motorcycle oil I got from the local autoparts store (I have been going threw a lot of oil with every clutch change)
I have talked with EBC, and they didnt really know, but thought their HD springs and CKF disks. when I tried the CKF disks I by accident put in some new thinner plates, and that soured that test. I think I need to get some 1.6mm steel plates and re-try. Are the stock CB250 plates aluminum or steel?
I have been trying to talk with the people at KG factory, but haven't been able to get ahold of anyone yet.

I do know a lot of other bikes have 6 or 8 clutch springs, when the CB250 only has 4. This means less clamping force. Also the diameter of the cb250 disks are quite small. The bigger bikes would have larger disks with larger surface area. long with more springs for higher clamping force.

I have actually done a lot I the modified car communities, specifically Nissan. built a few motors, turbo-ed about 6 vehicles. They have options of some very aggressive clutch material, 4 and 6 puck disks, with heavy duty pressure plates. and if you really want to spend some money, they do have some twin or more disk clutches for the crazy high HP people.

I have been trying to cross reference part numbers in the hope of finding another bike that "can use" the same part number, but might have better options. I have found stuff like this in the past. A lot of times it ends up being a marketing error, and the company didn't realize or think that the lowly 250 would care about having the high horsepower clutch option from an ATV or something. But no luck in this case for the cb250 clutches.
I am willing to get custom clutch disks if the company is confident they can be better then the other options.
 

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Your comment about the plate thickness is an interesting concept. As far as parts you can make yourself, those ones are doable in my opinion-you could figure out how to make them out of thicker material. Might do the trick, and it sounds like you have a set of thicker plates to try. I think if you went thicker past those plates you may find the weak point is clamping pressure of the springs, which won't be affected by reducing the ROM needed to engage the clutch as done by thickening the plates. Kind of a crazy idea but I wonder if texturizing the metal plates, like with a wire brush, would have any affect?
 

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Discussion Starter · #5 ·
I don't think I have the tools to make plates myself, the inside splines are a bit out of my wheelhouse.
There are some companies that sell thinner than stock plates as stock. I got a set by accident. and they end up lowering the clamping force by a lot. I have found one company that sells steel plates the proper size (1.6mm) I just dont know if stock is aluminum or steel. If steel it isn't much of an advantage to buy. But if steel, I have heard they have better texture, and hold up better, at the cost of possible warping, aggressive bite.
I just got back from a nice long 109mile all back roads ride with the KG disks and modified KG springs, and they would work well for someone with a little less power. Still havent been able to try the KG springs with EBC CKF disks.

maybe thin plates and an additional disk.. I might have to see if the math adds up, or it will be too thick...
 

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Discussion Starter · #7 ·
Here you go
I got a big writeup over here. I am a bit insane I think, got to modify everything!

I have been working on testing multiple clutches. In a week or 2 I should have some good information.
 

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Discussion Starter · #8 ·
an update, just incase someone finds this random thread 14 years from now.
Barnett clutches seem to be the winner, They are holding 29HP at the rear wheel, with the 1/8th inch spacers and the KG springs. I haven't tested to see how well they hold without the springs, or spacers, I have been too busy riding it!
 

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So, I am a bit of a mad scientist, and can't leave anything alone... I turboed my 92 cb250 nighthawk. Took a few years learning how to turbo something with carbs, But I got it running really well.

Now to a new problem, the clutches keep slipping at full boost.

I have tried ebc's heavy duty springs(10%) and both their HD and CF disks.

I have also tried KG factory springs (20%) and their high performance disks.

I have also made some spring spacers to pre-load the KG springs a little for higher clamping force.(only a slight improvement).

I haven't tried mixing and matching disk and springs.

KG has worked the best so far. With the spacers. 1st and 2nd hold fine. But 3rd around 8krpm at 9-11psi slips.


Anyone got any ideas? KG springs with some other disks?



Thanks!
Mark
So my main question is this how much faster is the bike and what about wheels and tires did you swap out and go with a 16 inch front with actual disk brakes do you have any pictures. I'd like to talk I got a 97 cb250 nighthawk and wanted to make it into a sport bike style and modify with full fairing to. This is my email maybe you will hit me back I put a picture of mine [email protected]
 

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Discussion Starter · #10 ·
I have 17x3.5 inch Warp9 wheels, with Shinko 705 130/70-17 tires front and rear.

I think the bike is a decent amount faster, Its doesn't have enough power to wheelie under acceleration alone, but its very quick up to 60ish. above that it slows down a lot. at 75, you gain about 1mph a second, and its still gaining at 85. My guess is 0-60 times are around 6 seconds, but I haven't really done any testing/drag racing. maybe some time this year I will get down to the track and test.
I got a few pictures, and a big writeup over here The Turbo Hawk
I went with RMZ250 supermoto caliper and front hub, but I don't know how well that will bolt up to the stock forks.
 

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So what other forks will work I can't afford those Ducati ones because I'd like to get the front tire in closer like my k9 gsxr1000 yeah I started working on this bike because I work for lkq pick your part scrap yard it came in with a title and not torn up I snagged it for 400$
 

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If the Barnett clutch is still a little weak you can try a couple of things. Shortening a spring makes it stiffer. You could slooowly grind it down and not overheat it. You need to keep release distance still and not get coil bound so a shim might be required.
The other suggestion would be to either reduce the clutch plate count by adding another plate and removing a friction, or carefully removing some friction material from the disc by widening the grooves. To keep a clutch or brake from grabbing you add surface area to give it some slip. If you reduce the surface area of the friction material you effectively increase the psi on the clutch pack without increasing the spring load on the basket. This will eliminate possibly distortion or possible failure of the basket. These bike clutches last so long and are so smooth in operation I wouldn’t think they had minimal lining area. Just a thought. We used to play with brake lining area in the old stock car days when you had to use stock parts for the year and make of the car. I would be careful with aggressive lining material since the oil in shared in both the engine and turbo. Don’t want semi metallic material or Kevlar going through the goodies:~]
 
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