Safe cruising speed

Decemberist · Jul 8, 2012

In trying to preserve my motor for as long as possible what do you guys think is a "safe" highway cruise speed without putting too much stress on the engine. My 74 850 sits at 4000rmp at around 65mph. However, the bike seems to be wound out at that speed. It is a great original condition commando with only 6k on the odometer. Basically, I am wondering if any long term damage i.e. stress at speeds around 75mph if any?

1up3down · Jul 8, 2012

I took a 4000 road trip on my Commando, rode 8 hours a day at 70mph, right around 4200rpm

It IS a long stroke air cooled motor, I would not feel comfortable holding it at 4500rpm all day, that's just me.

If you want a higher cruising speed, then change the gearing rather than hold higher rpms.

concours · Jul 8, 2012

4000 rpm gives me 80 mph :mrgreen: with a 22t sprocket. Cruuuuuuuiiiiiiiissssssseeeeeee

AussieCombat · Jul 8, 2012

Yep, can't see the point in revving a motor that pulls hard from 3,000.
It's a 22 for me.
Criusing at 100 - 110kph (60 - 70mph) puts me at.. 3,300 - 3,800 respectively,
and no issue at all getting off the lights two up, or sitting on our "biult up area" speed limit of
60 kph (35mph) in third.
Provided rear sprocket is standard..
This might help.....
19 tooth.... 15.83 mph. per 1000 revs.
20 tooth.... 16.66 mph. per 1000 revs.
21 tooth.... 17.49 mph. per 1000 revs.
22 tooth.... 18.33 mph. per 1000 revs.
Figures from the Norton Owners Club.
AC.

Decemberist · Jul 8, 2012

AussieCombat said:
Yep, can't see the point in revving a motor that pulls hard from 3,000.
It's a 22 for me.
Criusing at 100 - 110kph (60 - 70mph) puts me at.. 3,300 - 3,800 respectively,
and no issue at all getting off the lights two up, or sitting on our "biult up area" speed limit of
60 kph (35mph) in third.
Provided rear sprocket is standard..
This might help.....
19 tooth.... 15.83 mph. per 1000 revs.
20 tooth.... 16.66 mph. per 1000 revs.
21 tooth.... 17.49 mph. per 1000 revs.
22 tooth.... 18.33 mph. per 1000 revs.
Figures from the Norton Owners Club.
AC.

Sold! a 22 tooth sprocket it is, I too think that the engine has great pull from 3000rpm on would benefit from a larger sprocket.

Hobot · Jul 8, 2012

I've asked this same question in early 2000 on 3 forums to basically be assured by a number of famous long distance riders and builders, that a decent condition Cdo engine can do 5000 all day and night no problemo if oil level watched it stays a decent engine a long time. Best sense to me of the harmonics occurs 3500-4500 in vertical twins but 4500-5500 in my solid mount 10,500 redline Asian Vtwin Ducati knock off. I like that a Cdo pulls forward better in top gear than my 70 hp/365 lb balloon tire cycle does in 6th @ ~70 mph/5000 rpm.

Two things that put me off 22T is 1st gear is a bit too much launch commitment on clutch release onto loose stuff or tight creepy sections and 4th is too lugging-choppy to go slow enough, 30-ish, in these sections to stay out of lower gears to preserve sleeve shaft bushes or oil wedge on cam lobes.

Matt Spencer · Jul 9, 2012

CHUCK A TWENTYTHREE TOOTH GEARBOX SPROCKET ON IT .

4000 then should be adequate . Factory stated 5.900 Max CONTINUOUS .
I believe a sound motor may be safe to 7500 intermittant. 7000 definately .

Ordinarilly you go slightly less stupid in 1st & second to obiviate the risk of overreving,
due to the continuation of acceleration of the flywheel if you stuff it up .Such as when
you belt it into second with your heel throttle WFO and light up the rear tyre .
Actually , a harsh shift desychronised and to eager on the throttle can cause this.
Its liable to be ammuseing in the damp , if youre Watching rater than piloting .

Reggie · Jul 9, 2012

If sounds like you're running a 19t sprocket?
The only thing to consider is that you will lose a small amount of performance (acceleration) by changing to a larger sprocket and setting off in first will require just a bit more clutch slipping, but all If that doesn't bother you and you prefer cruising in a more relaxed manner, go ahead, your 850 will be OK, but you can get 20 and 21 t sprockets as well. You will also need some links adding to your chain.

Brooking 850 · Jul 9, 2012

Mine loves cruising at 4200 RPM all day, unfortunately our speed limits are below the 70 ish MPH that produces, next investment is a radar detector as Mr Plod has dedollared me on one ocassion already. Happy with the 21 tooth

Decemberist · Jul 9, 2012

Based on the factory specs I believe I am running the standard 19 tooth sprocket that came with the bike? I think that switching to a 22t is my best bet because I think that 23 is too much high end and a 20/21 would make too little a difference to justify the time and money spent for the conversion. I have never changed out the drive sprocket and it looks like it can be a bugger to get at.

Hobot · Jul 9, 2012

due to the continuation of acceleration of the flywheel if you stuff it up .Such as when you belt it into second with your heel throttle WFO and light up the rear tyre . Actually , a harsh shift desychronised and to eager on the throttle can cause this.

Alrighty someone else that had this happen, on my 1st Combat in '99, when dumb as shit on everything, even sane operation, i though I had a bad clutch snicking second on cam entering steep hwy, squealing sound with loss of pull - wasn't even at WOT but was well on cam upper 6000. It delayed=scared me getting ahead of approaching traffic out of blinds so had to tame my shifting to go better. That was 19T. I then when to 20T which I like with normal Combat power up to the ton.
On wilder Peel 21T ratio via belt drive gave best all around as I liked to commute well over the ton for 10 miles in easy sweeping river valley on her.

1up3down · Jul 9, 2012

yes, you do have the time consuming task of taking the entire primary off to get to the countershaft sprocket

if you have a 19 tooth now, I would recommend going to a 21 as you would be at a sedate 4000rpm at around 70 mph

this I feel would be ideal for continuous highway cruising

unless it is important for you to maintain a stead 80mph all day, I think going to a 23 is too much simple because of the more awkward clutch slipping stuff pulling away from a stop

bill · Jul 9, 2012

you will see approximately a 250 RPM drop per tooth you add.

Decemberist · Jul 9, 2012

1up3down said:
"yes, you do have the time consuming task of taking the entire primary off to get to the countershaft sprocket

if you have a 19 tooth now, I would recommend going to a 21 as you would be at a sedate 4000rpm at around 70 mph

this I feel would be ideal for continuous highway cruising"

Sounds like a good winter project because right now I'm having too much fun riding this season (already clocked 1400 mIles) I have only owned the bike for about 6 months and I can't spare a week of the short NY riding season to get primary etc. apart. I think that a 21t is ideal for initial concern about stress at highway cruise.

mikegray660 · Jul 9, 2012

Some might say that if your 40 year old bike has only 6K on it and is in excellent shape, it may very well out live you if you continue to utilize it in the same fashion as you have for the next 40 years...... :shock:

Decemberist · Jul 9, 2012

mikegray660 said:
Some might say that if your 40 year old bike has only 6K on it and is in excellent shape, it may very well out live you if you continue to utilize it in the same fashion as you have for the next 40 years...... :shock:

Unfortunately I cannot take all the credit for it's remarkable condition. However, I do plan on taking care of the bike until my 6 year old son is old enough to enjoy the old gal as much as I have during the short time I have owned it.

Safe cruising speed

Similar threads