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Long 5th Gear

N

nevadaslim

New member
I was wondering if anyone has tried installing the log 5th gear that Siebenrock sells,

http://www.boxer2valve.com/motorcycle/2322889-993.html

I'm new to Airheads, just got my 1980 R100T restored and on the road. What I noticed is I'm turning around 4000 RPMs @ 65mph on the freeway. Seems like it would be a little easier on my old motor, and less vibrations on my old body if I could cut the RPMs down a little.

This is supposed cut about 5% off the RPMs. Which isn't a lot, but probably enough to notice.

So I'm wondering what the pros and cons are of installing a taller 5th gear.
 
A lot depends upon how and where you ride the bike.

Pro: lower rpm *when cruising at high speeds (e.g.80mph interstates)

Cons: (1) the gear is not too spendy but there is no such thing as opening an airhead gearbox, replacing one gear, and buttoning it back together. With that vintage bike you’re almost guaranteed to need some additional work in the tranny. $$ can add up quickly.

(2) At lower cruising speeds (60-65) you may find the bike starting to lug a bit, and that’s hard on the engine.

(3) Top gear roll-on and passing response will be reduced.

The long 5th is a common mod on lower-geared GS bikes; on the taller gearing of an RS or RT it may be best suited for bikes that see a lot of high-speed use. And if it’s vibration you want to diminish you might find better results for the money in tuning and a long hard look at overall engine condition (valve train, carburetor, etc.

I have run an R80ST for long hours at 5300-5500 rpm with no issues, and the bike actually feels smoother in that range than at 4000-4200 rpm.

All IMHO, of course.

Best,
DeVern
 
Mr. Nevada
Have you cruised your bike at the wished for lower RPM for a time?
If it is in fact smoother at the lower Rev point, maybe this would work for you.
But as DeVern stated its never as straight forward or as cheap as you may think.
And you don't want to lug it, especially when its warm or hot, got to keep the oil pressure up and out of the ping, or predentanation [ sp?] zone.
But what the heck its only time and money eh?
Happy Holidays!
 
Your bike just gets happy at 4000 rpm! These engines like higher rpm. I think the R100T will do well over 100mph the way it is. The speedo goes to 140 mph. I've had a number of BMWs including R80 and R100RTs, and the R100T is the finest runner.


nevadaslim said:
I was wondering if anyone has tried installing the log 5th gear that Siebenrock sells,

http://www.boxer2valve.com/motorcycle/2322889-993.html

I'm new to Airheads, just got my 1980 R100T restored and on the road. What I noticed is I'm turning around 4000 RPMs @ 65mph on the freeway. Seems like it would be a little easier on my old motor, and less vibrations on my old body if I could cut the RPMs down a little.

This is supposed cut about 5% off the RPMs. Which isn't a lot, but probably enough to notice.

So I'm wondering what the pros and cons are of installing a taller 5th gear.
Click to expand...
 
Ride it a year before you do anything (rash). Get to know the bike.

Get used to those engines running at higher RPM’s than you may be used to. The sweet spot up there is where there is a equal balance of pulling as well as pushing.

Charlie
 
In top gear the sweet spot on my r75/6 is around 5K, give or take a bit. It's just happy there, these motors don't take to lugging well, that and a to slow of an idle. Let it spin.
 
If you want longer gearing, the less expensive solution would be to swap out your final drive, rather than opening the transmission. If you’re presently running a 3.00 final drive, a 2.91 should be available.
 
khittner said:
If you want longer gearing, the less expensive solution would be to swap out your final drive, rather than opening the transmission. If you’re presently running a 3.00 final drive, a 2.91 should be available.
Click to expand...

Yeah, but then you are stuck with taller gears all around including first gear and you don't need a taller first gear if you are using the bike off-road as it was designed.

Interesting thread. This tall top gear would drop high speed rev by 400-500 rpm. Don't know if that would be bad or good. I guess it depends on how much the bike is ridden in top gear on big roads etc.

.
 
leafman60 said:
Yeah, but then you are stuck with taller gears all around including first gear and you don't need a taller first gear if you are using the bike off-road as it was designed.
.
Click to expand...

OP’s motorcycle is a 1980 R100T—that’s a naked or handlebar-faired touring street bike, as designed. It’s not a dual-purpose G/S that would be used for off-roading.
 
I see Harley guys doing some mod to decrease RPM on the interstate. My opinion is it isn't worth the money spent. How much time do you actually spend riding over 60 MPH? A few hours here or there is not going to decrease engine life significantly. 4,000 RPM on a bike that red lines at 7,500 is nothing.
 
The taller 5th gear and cir clip was done last year on a R100GSPD, and it’s the best drivetrain modification I’ve ever done on any of my bikes!*
I have owned it for over a decade and now wish it was done earlier.
It's much easier to ride at highway speeds. (5,000 rpm = about 80 mph)

Years ago I used to run AirHeads on the road at 4000 / 4500 rpm.
After having problems with carbon buildup on pistons and heads, was advised to keep at or above 5,000 rpm.

Bill

* it’s tied for best mod with the Siebenrock 1000cc kit on the R90.
 
Last edited:
tanker4me said:
I had the taller 5th and circlip done last year on a R100GSPD, it’s the best drivetrain modification I’ve ever done on any of my bikes!*
I have owned it for over a decade and now wish it was done earlier.

On AirHeads I used to run at 4000 / 4500 rpm, on the road it’s now 5000+ rpm.

Bill

* it’s tied for best mod with the Siebenrock 1000cc kit on the R90.
Click to expand...


Say that again, please. You installed a taller 5th gear. Whereas you used to run 4000/4500 RPM, you are now running 5000+ RPM.

Wouldn't that be the other way around????

.
 
Last edited:
Oops! I see how it reads that way, I will try to edit it for clarity.

I was commenting on running at a higher rpm than seemed normal to me.
My experience before Airhead ownership was with 10,000rpm 2-strokers & 4,000 rpm Brit-bikes.

Most of the Airheads torque is between 3,500 & 5,000 rpm so I was keeping it on the lower side of 5,000.
At 4,000 - 4,500 rpm my Airheads were accumulating carbon buildup on the pistons and heads.

I was advised to stay at or above 5,000 rpm on the highway.

Bill
 
Last edited:
tanker4me said:
Oops! I see how it reads that way, I will try to edit it for clarity.

I was commenting on running at a higher rpm than seemed normal to me.
My experience before Airhead ownership was with 10,000rpm 2-strokers & 4,000 rpm Brit-bikes.

Most of the Airheads torque is between 3,500 & 5,000 rpm so I was keeping it on the lower side of 5,000.
At 4,000 - 4,500 rpm my Airheads were accumulating carbon buildup on the pistons and heads.

I was advised to stay at or above 5,000 rpm on the highway.

Bill
Click to expand...

I’m still not following you. How did installing a taller 5th gear help you achieve higher rpm on the highway?
 
? Last year the taller 5th gear was installed at the same time the circlip was, and stated it is the best drivetrain mod I’ve ever done.

The comment on running at 5 grand has nothing to do with the 5th gear mod.
I was commenting (like others have) on the fact that an airhead in stock tune should be run at 5,000+ rpm on the highway.
I’m getting ready for a ride to La Canada for an airhead coffee and conversation meetup this morning.
It’s 95% freeway to get there, with light traffic it’s going to be top gear at 5,000rpm.

Standard gearing indicated speed at 5,000 rpm was 80mph, taller 5th gear installed it’s 85 mph.
I’m gonna guess that real speed is 5 mph or so lower.
It doesn’t sound like much, but it makes a big difference on the highway if doing long distance rides.
As always mpg will vary, on a trip to Northern California and back the low was 32 mpg, high was 44 mpg.
The carbs were rebuilt afterwards, and the bike hasn’t been out yet to check for any effect on mpg.
 
Last edited:
tanker4me said:
? Last year the taller 5th gear was installed at the same time the circlip was, and stated it is the best drivetrain mod I’ve ever done.

The comment on running at 5 grand has nothing to do with the 5th gear mod.
I was commenting (like others have) on the fact that an airhead in stock tune should be run at 5,000+ rpm on the highway.
I’m getting ready for a ride to La Canada for an airhead coffee and conversation meetup this morning.
It’s 95% freeway to get there, with light traffic it’s going to be top gear at 5,000rpm.
Click to expand...

So, before you did the 5th gear change-out, you would have been doing 5300 or so at the same speed..., right???

.
 
Last edited:
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