So, I need a new chain ...

[robindean]

So, I need a new chain.

My bike is still operating but the chain is old and stretched out. I imagine the gear next to the engine might need replacing as well (what is that called?). This puts me at a bit of a crossroads about something.

I live in Chicago. The average freeway speed here when traffic is at a minimum tends to be around 70-75 mph. Now, my bike easily keeps up but that puts my RPMs in upwards of 6000. I realize that our bikes are ultra strong when it comes to high RPMs but I thought I'd at least inquire as to whether or not changing my gearing will bring a more meditative zen to my bike.

Thoughts?

I'd like to know the following:

1) What is the basic analogy for loss/gain in terms of an altered gearing.
2) If I were to alter this, what limits should I observe?
3) If I stay stock ... good GRIEF what is the part number for the gear next to the motor? The diagrams at bike bandit have me a little confused because I can't see the difference between the trans gearing or the drive train (they have them on separate pages and they both look similar to my newb eyes).

I'd like to really lock into this topic as I have a couple of weeks to my next paycheck. Hence, I can make sense of it and get what I'm after with some help from you cats!

P.S. When replacing the chain + that gear ... should I also be considering a replacement for the gear that is actually on the wheel? Do they wear out as well?

 

[HoughMade]

Some people say to always replace the front and rear sprockets when replacing a chain and other than the $$$ there's not a downside to doing that, but that advice seems to have become common in the age of aluminum sprockets. Our sprockets are steel (at least mine are). Personally, I would not replace the sprockets unless they show wear which can be a hooking of the teeth.

As for changing the gearing, I did. I have a SOHC XS400, so it is not directly applicable to you, but i went from a 39 tooth rear to 37 tooth and I am very happy with it at higher speeds.

 

[drewpy]

I run a 17 front and 39 rear which = 1 tooth up on gearing. some say that you should have an odd and even number for each sprocket, but I can't see any difference.

BTW i'm running 6k @ 70mph the power comes in at 7k and max torque at 8k, to far down and you would have to change down a gear due to the engine bogging down.

 

[robindean]

Very interesting!

My bike feels as if it has the most power when I hit the throttle at around 4000 RPMs. I'm not sure why mine is different. Strange. Maybe I need to go through the tuning process for all three of the mentioned power points that I've read about.

To my newb eyes, I see the mention of a couple teeth here and a couple teeth there ... what's the difference in RPMs if I add or remove a couple of teeth? Obviously, assuming we're talking about the rear sprocket, more teeth will mean higher RPMs and less means the latter.

How far is too far? Specifically at HoughMade, where are your RPMs at 55, 60, 65, 70 and up?

I'm thinking that I'd actually like to replace some other stock elements with after market whoop ass on a simple level, such as air filter and exaust ... maybe jets? Assuming I do, it might be nice to go down a few teeth on my rear sprocket ... or should I add teeth to my front? What's the difference?

LOL ... lovin' the topic. Thanks for the input. Keep it comin'!

 

[HoughMade]

My RPM dropped about 500 in 6th gear when I changed sprockets. At 55 mph, I now turn around 5000 rpm, at 60 mph, 5500 rpm, at 70 mph, 6500 rpm.

 

[bcware]

I just want to clarify why you want to change things. You said "meditative zen": to me, this doesn't mean anything. Does the noise the bike makes displease you? Does cruising at "high" rpms worry you for some reason?

You're not going to hurt anything by cruising at 6 grand going 70+ mph. The xs400, like most small bikes, is going to run at higher rpms if you want any power.

Changing teeth on the sprockets has its benefits and costs. If you change the teeth as described above you will trade a higher top speed for slower acceleration. Most people do the opposite; trade top speed for quicker acceleration.

 

[lhaolpa]

Adding one tooth to the primary[front] sprocket will lower the rpms at a given speed as this raises the ratio of the basic gearing.Replacing the secondary[rear] sprocket with one with one or two fewer teeth will do the same thing,but with less dramatic results.With one tooth less or more,you'd have to be very aware of engine speed and road speed to notice a difference.With the primary,it's much more noticeable,as the primary is smaller and has a much more pronounced effect on the secondary.If you just want a little relief from the high rpms at cruising speed,change the secondary one or two teeth smaller,and you'll get a more relaxed speed at cruising. lha

 

[drewpy]

Very interesting!

My bike feels as if it has the most power when I hit the throttle at around 4000 RPMs. I'm not sure why mine is different. Strange. Maybe I need to go through the tuning process for all three of the mentioned power points that I've read about.

you have the dohc model, with the 2 inlets your getting faster low rpm air flow so its possible you will be getting a bit more torque lower down the rpm range, but 4k is low for max torque!

mine takes off at 7 k to 9.5k

check your ignition timing

 

[robindean]

Excellent feedback.

@HoughMade - your RPMs are identical to mine. I hit 5000 at 55. Perhaps the previous owner already did some work on 'er.

@BC - I meant humming/chanting vs. screaming. You've got the right analogy. I'm not too concerned about the high RPMs. I'm simply testing the waters to get them lower for more of a hum/growl. Assuming I lose power this way, I'm hoping to regain it with after market parts that are easy to install.

Anyone have a chain/sprocket change-out kit that they prefer? One that has a series of different sizes to dabble with?

I'm probably going to need a new speedometer ... this Reagan administration noise isn't going to work if I boost the top end.

 

[Daddio]

I was trying to figure out my gearing and a member here "Foul" gave me a link to an awesome website to help get a definitive answer. I was changing the rear wheel size and the chain length quite a bit so this website was very helpful.
http://www.gearingcommander.com/

 

[robindean]

That is a very cool link. Bookmarked.

In toying with it, I threw 18/33 into the final drive ratio for "custom". That seems to put me where I want to be but seems quite drastic compared to what others are saying.

I intend on purchasing after-market parts and would like to know if you folks think I can regain power with an air filter / exaust combo?

If my ratio seems crazy, do tell.

 

[jayel]

That is a very cool link. Bookmarked.

In toying with it, I threw 18/33 into the final drive ratio for "custom". That seems to put me where I want to be but seems quite drastic compared to what others are saying.

I intend on purchasing after-market parts and would like to know if you folks think I can regain power with an air filter / exaust combo?

If my ratio seems crazy, do tell.

since the stock drive sprocket was only a 16T a 18T may not fit due to clearance between chain and engine cases, usually up or down one tooth on the front is all you can do, down chain turn radius get to tight, up clearance issues result

 

[robindean]

Ya know, for an '82 XS400J ... they aren't presenting any gear ratio for 6th gear.

Anyone willing to load up that page and validate that the presented gears are correct?

Moreso, off hand, anyone know what the gear ratio for 6th is? I posted 1/1 just for kicks and it came up with a reasonable prediction.

With the crank case size issue, I'm debating perhaps a 16/31 combo. Thoughts?

 

[jayel]

here's a calculator that has sixth gear but you need to know primary gear, clutch ring gear and all gear set ratios ect. http://www.rbracing-rsr.com/gearspeed.html

 

[bcware]

@BC - I meant humming/chanting vs. screaming. You've got the right analogy. I'm not too concerned about the high RPMs. I'm simply testing the waters to get them lower for more of a hum/growl. Assuming I lose power this way, I'm hoping to regain it with after market parts that are easy to install.

I am not sure how much of a difference you would "feel" going up or down one tooth, but on bigger racing bikes people seem to think it makes a noticeable difference in acceleration/top speed.

What does everyone else think that has actually done it? HoughMade? How does it feel? Is the noise level reduced significantly?

If you combine the tooth change with exhaust and intake mods you're probably going to lose a lot of low end torque. You'll make more peak horsepower and probably torque, but it will be at high rpms, which, as I have gathered, is not where you like to be. For daily, relaxed driving a stock setup is probably superior.

 

[Daddio]

One tooth in front is typically worth around two teeth in back.

 

[HoughMade]

I seldom ride on interstates. I spend a significant amount of time around 55-60 mph. The roughly 500 rpm drop at that speed takes me out of an rpm range where there was a vibration. Now, instead of cruising at the vibration point, I transition quickly through it on my way to 55 mph. On the occasions I do ride at 70 mph for any length of time, the bike sounds less frenetic, less stressed at that speed, though that's more of a "feeling' thing and I doubt the 500 rpm drop makes any longevity difference.

In my mind, 1st gear was too low with the 39, so it feels fine with the 37. I do not notice any loss of acceleration, but I have not measured. I also gained 2, possibly 3 mpg (from 51 mpg to 53+ mpg) measured over several tanks.

But again, keep in mind that my bike is a '79, SOHC so the power characteristics are different from the DOHC at issue here and the gear ratios may be different as well.

 

[robindean]

Well, I'm thinking I'll just stay stock after all.

However, I'm not sure how many teeth are up front / in back for my specific model. I'll go count witmefingas.

 

[Daddio]

The front sprocket is easy to count with your fingas, on the left side of your bike there is a plate with your shift pattern and a cable on it. If memory serves me correctly there are only three hex heads holding it on. There is no gasket to worry about so just take it off leave the cable attached, take a quick count and you are good.

 

[robindean]

The screws on that plate are completely seized. I actually stripped two of them in an attempt to get them off. Lame

Any notion off hand what the count on the front sprocket is for stock? The rear is a confirmed 38 teeth.