Topic: Will the CBR F2 speedometer drive operate a CB750K speedometer correctly?

[xrrevin]

I'm considering using a CBR F2 front wheel on an F2 or F3 front end conversion for a '73 CB750K.  I would like to know if the F2 speedo drive will properly operate the OEM '73 speedometer.

Thanks in advance to anyone offering information gained through experience.   

Xrrevin
 

[Alan F.]

CBRs had a 17" front wheel, the gearing is calibrated for that. (Until they went electronic)
91-03 CB750 Nighthawk had an 18" front wheel, same speedometer drive but different gearing.

I'm guessing you'd need the right drive for your front wheel diameter but otherwise I think you're good, speedometer ratios were pretty standard at Honda, but drive ratios at the hub were tied to front wheel diameter.

[newday777]

The 69-78 K model speedometer are set up for 19" wheel to have the correct revolution gearing.

[Alan F.]

The 69-78 K model speedometer are set up for 19" wheel to have the correct revolution gearing.

Stu I thought the speedometer ratio was 2240:60mph. Drive gearing at the wheel makes it possible to use this same speedometer with different front wheel sizes.

I haven't bolted it up and tried it though, is my logic flawed?

[newday777]

The 69-78 K model speedometer are set up for 19" wheel to have the correct revolution gearing.

Stu I thought the speedometer ratio was 2240:60mph. Drive gearing at the wheel makes it possible to use this same speedometer with different front wheel sizes.

I haven't bolted it up and tried it though, is my logic flawed?

I'm not sure how to get the correct drive gearing to change the wheel size to use the stock speedometer. Never played with that.

[Alan F.]

With CBRF1-3 forks & 17" wheel or Nighthawk 18" wheel just use the Speedo drive gearbox from those bikes, plug & play, mix & match fitment is possible. CBR had twin disc, Nighthawk has left side only, wheels, axles & Speedo drives are all interchangeable in fit, but the speedometer drive ring gear ratio is different at the hub for different wheel sizes.

[xrrevin]

Thanks to you both for your responses and clarification of this situation.  Also, for introducing me to that compiled list of resources that are loaded with useful information.

 So, what I'm now attempting, after you suggested '91-03 Nighthawk interchangeability, is:
1. 41 mm forks ('97-98 CBR F3 owned)
2. triple trees OEM with proper offset for 18" front wheel ('91-03 Nighthawk? or other)
3. dual disk capable front wheel 18" (preferably with spokes like K3 OEM)
-
I have measured the K3 fork stem length (from the stem bearing seat under the lower tree bearing race to the top of the threads to be approx 9.25" (CBR stem is too short)
Do you happen to know if the Nighthawk trees will just bolt in to the K3 frame with at most a conversion bearing change??
Also, FYI, the fork tube center to center distance on K3 is 7.25" , F3 is 7.75".

I'm also aware that custom trees and front wheel hub, etc can be built @ roughly $1000 each, beautiful work, though.

Hopefully, this question isn't too convoluted, causing everyone to fill their glass again

Thanks,
xrrevin

[Alan F.]

Do you happen to know if the Nighthawk trees will just bolt in to the K3 frame with at most a conversion bearing change??
Also, FYI, the fork tube center to center distance on K3 is 7.25" , F3 is 7.75"

Check my K3 build thread in my signature line, I'd messed around with this idea a while back but never finished the K3. Those wheels & forks are on my 250 Nighthawk basket case, thread in the Other Bikes section...also still unfinished.....

The Nighthawk triples will bolt right up with the same bearings as the SOHC 750 uses. But the steering stem is longer. You just need to make a spacer to raise the top bearing or lower the bottom bearing to take up that difference. Or gring the weld and press the stems out, and swap your K3 stem into the Nighthawk tree and have it rewelded.

You should be able to adapt your K3 hub to CBR forks by using the CBR axle, bearings to match it up, and some custom spacers & center collar.

You'll probably need some custom spacers for the brake discs as well. It's not all that hard to do.

[scottly]

Also, FYI, the fork tube center to center distance on K3 is 7.25" , F3 is 7.75".

No, the fork tube C-to-C distance is the same on all the 750s. The later 750s had 50mm offset in the triples, compared to 60mm offset in the earlier triples.

[Alan F.]

You are correct Sir, however the F3 as mentioned would be CBR600F3. I think it's very important to note that carefully and clearly, someone could find this thread in a search and misconstrue which bike is being described.

[Nomad]

From what I’ve seen, the F2 speedo drive generally works with the '73 CB750K speedometer. You might need to check if the drive gear ratios match up, though, as there can be some variations. It’s always a good idea to compare part numbers or even test it out if you can.