Topic: 400f sprockets and the highway

[JZEROE]

Hey all,

I actually already posted this (yesterday) as a response to a 2-year-old conversation between GregK and TTR400/Kevin, but it's so buried that I'm not sure if anyone can see it. Please pardon me if you've seen it twice; got some parts orderin to do, and could use some advice.

So I just got a 400f, and it definitely wants a little goosing on the bottom end. But I'm hesitant.

I had a 550f, and I dropped a tooth on the front sprocket. Definitely helped me take off from the light, but it also *killed* my highway driving: at 75MPH, it was around 7500RPM, which was a little nerve wracking on a 35-year old 550. Between what I had read on the forums, and bike's behavior at that speed, 8K seemed like a forbidden zone.

I know the 400F has a much higher redline, but that doesn't mean I necessarily want to get there—even though my machine is in pretty good shape and only has 12K on the clock. So what do you guys cruise at on the highway (I live in CA... freeways are unavoidable)? And what RPMS will a 16-tooth front sprocket bring you to at around 75?

Thanks!
J

[TomC]

Hi Jzeroe
     The CB400 Four is geared well for freeway use. Sixth is good when all is well. Up hill, in to a strong head wind I would recommend dropping down to fifth gear.
     A CB400 Four does not need to run all the way out to the 10,000 rpm redline. But if you have a problem with running to 9,000 rpm on a regular basis I do not think that you will enjoy riding a CB400 Four.
     TomC in Ohio

[Gordon]

At first I thought you were thinking about lowering the rpm's on the highway because it sounded like you don't like the high rpm's, but you're asking about going for a smaller sprocket on the front? 

I'm not sure you're going to like the results of that since you do so much highway riding.  The 400 already revs so high to begin with.  If you're not getting enough torque at low rpm's there may be something not quite right with the engine/carbs.  I've only had one 400F, but it was plenty quick off the line with stock gearing.  Nothing at all like my 550 that I did switch to a smaller front sprocket for quicker acceleration. 

[socalenduro]

won't dropping 1 tooth in the front be a bigger change than adding 1 tooth in the rear?
% change and all that...

maybe changing the rear sprocket would be a good point in between

[JZEROE]

I actually have VERY little experience with this 400f: I rode it a bit when I bought it on Monday, but it's been monsooning all week here in SF. I've barely gone 5 miles on the thing, and haven't even cleaned/synced the carbs or replaced the 30-year-old air filter yet. Glad to hear that it's got plenty of get-up-and-go; I'll hold off on replacing the worn out chain and sprockets until I've sorted out the breathing.

[crazypj]

The redline was advisory when new, don't worry about it.
 If you want a low rpm cruiser, that's what you'll have to buy.
 BTW, CB550F will happily spin to 13,500rpm, 7,500 and it just starts making power.

PJ

[JZEROE]

My 550 would have spun up to 13,500, sure—if you wanted to use a conrod as a kickstand.

[Bodi]

You'll get around 6500 at 75mph with the smaller front sprocket. I have done this and run approximately 6k at indicated 70mph . I have somewhat larger than stock tires and the speedo is probably erroneous.
This does improve acceleration and 6.5k isn't a strain on the 400 from my experience. 75mph is pretty tiring on this bike though, the vibration and wind blast plus the narrow seat - it is not a highway cruiser by any reckoning. Fine for a short commute.
The gearing calculation is pretty simple, the engine rpm for any speed will change by the percentage change in sprocket size. Dropping 17t to 16t front sprocket will raise engine rpm 6.25% (17/16=1.0625). Going from 38t rear to 39t raises engine rpm by 2.6%. 38t to 40t is just over 5%.
You can do both... 1 16t front an 40t rear will raise rpm about 12%.
Rear wheel torque in any gear/rpm combination increases the same amount - as does acceleration. The 17-16 change means more shifting in town, the lower gears get "shorter" as their speed range with reasonable rpm gets to be less. It isn't too bad - but I think it's about as low as I would recommend on the gearing.
Changing the front sprocket is better in some ways. It's much easier to change. One tooth doesn't change your chain length enough to make it unadjustable as is.

[crazypj]

My 550 would have spun up to 13,500, sure—if you wanted to use a conrod as a kickstand.

 must be something wrong with it then, mines done 106,000miles

PJ