FRANKENHONDA II the 1982 DOHC CB 750F2 Build

Yes, shortening the front suspension will decrease the trail, but you've decreased the stem-to-fork offset quite a bit. The old forks had a big offset from the steering stem to the fork tubes. The new forks you have, they're almost in the same plane. That increases trail by a very large amount. Lowering the bike in the front will decrease trail by a smaller amount. You *may* be able to compensate for the offset by lowering the front of the bike, and you might even get close to stock trail numbers (for the original frame) that way. But it's highly unlikely you'll be able to get anywhere near a trail number low enough to require a steering damper. You'll have to take measurements and do the calculations, but in all likelihood, the steering damper will prove to be unnecessary, and you'll save some weight by leaving it off.

Charles.
 
Here you can see how offset effects trail. Excuse the poorly edited drawings, but you get the gist.


Charles.
 

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ChopperCharles said:
Yes, shortening the front suspension will decrease the trail, but you've decreased the stem-to-fork offset quite a bit. The old forks had a big offset from the steering stem to the fork tubes. The new forks you have, they're almost in the same plane. That increases trail by a very large amount. Lowering the bike in the front will decrease trail by a smaller amount. You *may* be able to compensate for the offset by lowering the front of the bike, and you might even get close to stock trail numbers (for the original frame) that way. But it's highly unlikely you'll be able to get anywhere near a trail number low enough to require a steering damper. You'll have to take measurements and do the calculations, but in all likelihood, the steering damper will prove to be unnecessary, and you'll save some weight by leaving it off.

Charles.
Click to expand...
Wont be able to lower it anymore unfortunately she sits very low im actually worried it may be a little too low, wont know till i ride it.

I'll have to sit downnand work it out when im back home from work in 6 weeks.

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cb250nproject said:
Wont be able to lower it anymore unfortunately she sits very low im actually worried it may be a little too low, wont know till i ride it.

I'll have to sit downnand work it out when im back home from work in 6 weeks.

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Click to expand...

Did you measure? 4-4.5 inches ground clearance is the lowest you can really go. And with that, don't hit any big bumps lol.
 
FYI, I went with 93-95 GSXR front triples for this reason. They have more offset than later trees, and they work with Hayabusa forks. (I used the 94 GSXR forks though, because that allowed me to run a mechanical speedometer and OEM gauges). A little more trail is not a big deal, it makes the bike stable. I'm just saying that the steering damper is likely completely unnecessary.

Charles.
 
ChopperCharles said:
FYI, I went with 93-95 GSXR front triples for this reason. They have more offset than later trees, and they work with Hayabusa forks. (I used the 94 GSXR forks though, because that allowed me to run a mechanical speedometer and OEM gauges). A little more trail is not a big deal, it makes the bike stable. I'm just saying that the steering damper is likely completely unnecessary.

Charles.
Click to expand...
Yea these are 2008 gsxr 750 forks, they are definitely shorter but im unsure the difference in offset.
Once i get home i will have to look im not too sure what the stock rake and trail is on the DOHC is. The powers been out all night on site and is still off so i was conserving my battery so the alarm would go off.
Hopefully there is power in the office then ill be able to do a bit of R&D, i think cognito moto had a thing on it saying that tue change wasnt to drastic with this conversion but we'll see.

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Has anyone reading this done the Gsxr fork conversion on their cb 750 dohc and simply pressed a new stem in ? Im interested to see how it handles with a 17" rim on it

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cb250nproject said:
Has anyone reading this done the Gsxr fork conversion on their cb 750 dohc and simply pressed a new stem in ? Im interested to see how it handles with a 17" rim on it

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Click to expand...

Hold on, can you give me this info:

Wheel size front and rear
rear shock length
what swingarm
Length difference of front end
tire sizes being used
 
cb250nproject said:
Im going to have to ask the wife to measure it up sorry im overseas atm ... leave it with me im trying to work out how to explain clevis to clevis :/

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Click to expand...

Lol!!! Ur drop in the front is right at 3ish inches, bit i need the shcok lengtg to figure in any rear drop. If its the same, 3 inches is actually sizable and will have a big effect up front.
 
Ok with the tires and everything you're .5 lower in the rear, and 4 lower up front. That's before factoring ib the rear shock, which I think is longer that stock. Stock I think was around 13. If that's true, basically the front end of the bike is 4 inches lower, and the rear is stock. That's a big difference homie. I'm not sure how rideable that will be.
 
jag767 said:
...Stock I think was around 13.
Click to expand...

Stock length in the F is 14". Either way, you're right: that's a pretty extreme alteration
 
Cookie said:
Stock length in the F is 14". Either way, you're right: that's a pretty extreme alteration
Click to expand...

Ok. .5 rear drop, 4.0 front drop. So 3.5 front drop normalized. That still may not be rideable.
 
When you say not rideable, do you mean because the forward lean decreases rake angle and thereby trail? To some extent that would be countered by the smaller offset, right?

What is the offset in the GSXR triples? I believe stock is 45mm?


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cb250nproject said:
So how could this be countered? Getting triples to lower the forks ?

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Click to expand...

Assemble the bike and measure the rake. If it's poo poo that's one way yes. I've only seen that done for 1 inch though, you'd likely want to do at least 2 inches.
 
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