Why is 4K the magic RPM number? - BMW Luxury Touring Community
 
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post #1 of 25 Old Aug 18th, 2008, 11:40 pm Thread Starter
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Why is 4K the magic RPM number?

I've seen a number of threads with comments about keeping the rev's up on an LT.. like around 4K. When I'm lounging my way home along a stretch of road with gentle curves and mild grade transitions ... (speed limit 30mph) I let her go around 2K.. so long as there is no lugging.. downshifting as needed to keep the rpms around 2-2.5... if there is any need.. I bring the revs up to around 3. Why is 4K seemingly the magic number?

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post #2 of 25 Old Aug 19th, 2008, 12:01 am
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Re: Why is 4K the magic RPM number?

I think your definition about lugging may be different from mine. Try this one out the next time you are at 2K on a higher gear and see what happens ...

Peg the throttle (i.e accelerate quickly)

If your engine feels like it is stalling rather than accelerating quickly and smoothly then you are lugging the engine. It is working below it's best performance/tourque curve. It's not a good place for an engine to be.

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post #3 of 25 Old Aug 19th, 2008, 2:31 am
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by asleeplessknight
Why is 4K seemingly the magic number?
Power band.
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post #4 of 25 Old Aug 19th, 2008, 2:49 am
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Re: Why is 4K the magic RPM number?

I think it depends on what you're trying to do at any given time. Like asleeplessknight, I normally allow Katie to burble gently along at 2-3k, which is roughly 40-60mph in fifth. Yes, it means I have to downshift to accelerate quickly but the box is plenty good enough to allow quick changes so it's no hindrance. On my bike, 4k and above seems to move the engine to a different map and fuel consumption drops sharply so I tend to avoid that region unless I'm accelerating.

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post #5 of 25 Old Aug 19th, 2008, 6:42 am
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Re: Why is 4K the magic RPM number?

I love riding this bike on country roads around 55+ in 3rd gear at about 4500 rpm. It is soo sweet in that range

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post #6 of 25 Old Aug 19th, 2008, 6:47 am
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Re: Why is 4K the magic RPM number?

I used to keep the revs low. Then learnt about controlling the slowing down by using revs instead of brakes. Saves pads, effort and is safer. I ride from 4k to 5k mostly now and although she makes a right noise, I think she likes it rough.

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post #7 of 25 Old Aug 19th, 2008, 7:22 am
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by mpillis
I love riding this bike on country roads around 55+ in 3rd gear at about 4500 rpm. It is soo sweet in that range
When you're on the twisties it makes sense to keep the engine spinning so that you can drive through the bends. I find that Katie needs to be under power to track well. Take a bend on a slack or balanced throttle and she'll often twitch a bit but slowing down into the bend and driving through is guaranteed to have her running on rails.

Quote:
Originally Posted by c00k1e
I used to keep the revs low. Then learnt about controlling the slowing down by using revs instead of brakes. Saves pads, effort and is safer. I ride from 4k to 5k mostly now and although she makes a right noise, I think she likes it rough.
I don't see that there a contradiction between slowing down via the gearbox and keeping the revs low under normal driving conditions. You just change down to a gear where the retardation is as high as you need it to be. I use the technique all the time, stemming from learning my trade on a series of British bikes. Not only were the brakes often marginal anyway, with cable operation you were more than likely to pull a nipple off if you heaved on the lever. Using the gearbox for slowing down was the most practical way to ride, the brakes only being used to bring the bike to a halt most of the time.

A friend once had a 1947 AJS Model 18 sidecar outfit, which had the 16 bhp pre-War 500cc engine in the lightweight frame from the 5bhp Matchless 350cc WD G3L mated with an unbraked chair. Unfortunately, the bike had the brakes off the G3L, too, and they were about the size of a shoe-polish tin! She always said that she couldn't do an emergency stop. The best she could manage was an emeregency slowing-down manouvre!

I suppose what riding those old bikes taught their riders was to look a l-o-n-g way in front so that you weren't taken by surprise very often.

Keith


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post #8 of 25 Old Aug 19th, 2008, 11:26 am
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Re: Why is 4K the magic RPM number?

Motorcycles are like women. Get on them to hard when they are not ready and they are going to shut down. Downshift too quickly when they are revved up and they’ll skid out of control. Each one is different and you have to find and work that sweet spot just right.
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post #9 of 25 Old Aug 19th, 2008, 4:01 pm
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Re: Why is 4K the magic RPM number?

I get more miles per tank at lower rpms.

At one time, it was cheaper to use the engine as a brake than it was to use the brakes. Throw out bearings didn't last past 60K.

Right now, most of my riding is around town. So, I'm having fun trying to figure out how to get more miles per tank, save a few bucks when I refill the tank, reduce my portion of US dollars going to foreign countries (two dollars for a pound! and $1.50 for a Euro!), and reduce demand for oil from foreign countries.

Most of the time when I'm on the open road I don't have deadline to be where ever I'm going, so I ride around the speed limit in the right hand lane.

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post #10 of 25 Old Aug 19th, 2008, 4:01 pm Thread Starter
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Re: Why is 4K the magic RPM number?

It all makes sense.. and is how I've grown to feel "her moods"... and that is probably the sweetest part about her ride... it is WHAT you want it to be, WHEN you want it to be. No surprises.... ... except at crawling speeds...of course.....WHERE did I leave the retractable training wheels again?.... on the schwinn you say? NOT.

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post #11 of 25 Old Aug 19th, 2008, 5:35 pm
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by asleeplessknight
I let her go around 2K...
As I do.

This is the kind of engine you can find in a car. You sure can drive @ >4k in a car. But I don't.

It sucks so much fuel, become unstoppable and is much more noisy. IMHO

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post #12 of 25 Old Aug 19th, 2008, 9:50 pm
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Why is 4K the magic RPM number?

Quote:
Originally Posted by asleeplessknight
I've seen a number of threads with comments about keeping the rev's up on an LT.. like around 4K. When I'm lounging my way home along a stretch of road with gentle curves and mild grade transitions ... (speed limit 30mph) I let her go around 2K.. so long as there is no lugging.. downshifting as needed to keep the rpms around 2-2.5... if there is any need.. I bring the revs up to around 3. Why is 4K seemingly the magic number
I have custom ear plugs and you virtually don't hear the engine at any revs. So 4000 rpm is not a problem.

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Bill
05 K1200LT
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post #13 of 25 Old Aug 20th, 2008, 7:09 pm
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by Frenchy
As I do.

This is the kind of engine you can find in a car. You sure can drive @ >4k in a car. But I don't.

It sucks so much fuel, become unstoppable and is much more noisy. IMHO
I agree. I find a dramatic change in fuel economy above about 3,000 RPM in high gear. This is about 60 MPH indicated or probably 54 MPH in reality. I find this works great for commuting to work and back and I get 50+ MPG consistently. Every not and then I kick it up a notch to have some fun in the twisties, but that drops the MPG to low to mid-40s very quickly as will 70+ on the interstate.

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post #14 of 25 Old Aug 21st, 2008, 12:19 am
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by Voyager
I agree. I find a dramatic change in fuel economy above about 3,000 RPM in high gear. This is about 60 MPH indicated or probably 54 MPH in reality. I find this works great for commuting to work and back and I get 50+ MPG consistently. Every not and then I kick it up a notch to have some fun in the twisties, but that drops the MPG to low to mid-40s very quickly as will 70+ on the interstate.
I tried keeping the revs up for a while to see how it worked and I couldn't stand it. I just feel like I'm racing the engine and that constant scream gets on my nerves. I find myself cruising, like several others, at around 2500-3000 on the highway in my daily commute and I find that comfortable and economical. In all my years of riding speed has just never done anything for me. I like to enjoy my ride without have to have the concentration of an Indy car driver.

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post #15 of 25 Old Aug 21st, 2008, 1:17 am
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by Voyager
This is about 60 MPH indicated or probably 54 MPH in reality.
It's more like 57 mph. The Mk II speedo is out by about 3 mph throughout.

Keith


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post #16 of 25 Old Aug 21st, 2008, 2:21 am
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Re: Why is 4K the magic RPM number?

FWIW, on my '06 LT the speedo is definitely 5 mph high on almost all speed ranges. I used my GPS to obtain the actual speed.

I do know I would never compare my LT's engine to a car engine. Not one that operates at low RPMs at least. Any 4 cyclinder engin of comparable size (2.2 L or less) will be reving pretty high just to keep the vehicle up to speed. Anything below 3K is definitely just asking for trouble.

Maybe not trouble for the engine, but potential trouble for you. The main reason I keep the engine in the powerband is safety. You never know when you may need to rapidly accelerate to avoid an accident. When I was first learning to drive, this particular issue was stressed heavily.

Using the powerband and engine for braking is also a much safer way to control your speed (for the most part) than your brakes. It's the same thing when stopping. Those that stop by just pulling in the clutch and then shifting to neutral/first when stopped are unprepared for potential road hazards. It's what I would call comfort/lazy driving rather than prepared driving.

I am curious about those that feel the RPMs are too high, the noise to much, or what not, as that is just something I don't understand at all. If anyone would care to explain it, I'd like to understand it better. Maybe it's because I've riden for 30+ years and been on many different motorcycles. Most of them having a redline of 10K or higher. Or maybe I just follow that older philosophy of ride it like you stole it. I always get a rush when riding a motorcyle, but if it didn't feel like a motorcycle (fast, powerful, responsive, etc.) I'd never have bought an LT in the first place. I'd have bought a Mazda Miata or similar convertable.

And while I admit it would be nice to get better gas milage, it's never going to happen for me out here in CA. For a couple of reasons...

1) Highway speeds are 65 ~ 70 mph. Most people drive 10 ~ 20 mph over the speed limit, but just doing the legal speed limit gives me 40 ~ 45 mpg. I can live with that.

2) A static motorcycle (i.e. one that stays the same speed as the surrounding traffic) is often invisible. When I started to ride they called it defensive driving, but in my early years of motorccyling I found defensive driving to be potentially life threatening. Meaning I had several incidents where I almost died because of jerks in cars not seeing me and running me off the road, almost in to a guard rail, or flat out hitting me (only luck saved me the time I was hit as they turned my forks in to a S shape and I flew 20 feet straight up).

I no longer recommend defensive driving to anyone. I drive aggresively now. Not like a squid! What I mean is I always go a few mph faster than the surrounding traffic to the right of me. I do not let the traffic on the left go the same speed as me either. I keep myself as a moving target (so to speak) as movement attracts the eyes (we are a predator type species you know). In my last 20 years of driving I have had zero traffic incidents. So, it's worked for me. YMMV.

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post #17 of 25 Old Aug 21st, 2008, 7:00 am
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by Rockdoc
It's more like 57 mph. The Mk II speedo is out by about 3 mph throughout.

Keith
No, I've checked it against several radar units and my other vehicles and it is off about 10% across the range. 3 MPH at 30, 6 at 60, 8 and 80. It is not a constant error value, but a percentage error. There is a slope problem in the speedometer calibration not an offset.

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post #18 of 25 Old Aug 21st, 2008, 7:33 am
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Re: Why is 4K the magic RPM number?

In that case it looks like BMW have changed the speedo mechanism back to the same type they used on the Mk I bikes. Mine's just over 3 mph out all the way, again checked against my GPS. I remember the discussions after the introduction of the Mk II bike, which is when I joined the board, and most other speedos were also about 3 mph out.

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post #19 of 25 Old Aug 21st, 2008, 5:09 pm
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Re: Why is 4K the magic RPM number?

I think as others have mentioned previously that how you ride your current bike, whatever it may be, depends a lot on what you have ridden in the past. Folks who have ridden sportbikes will ride their LT that way also. People who have ridden or continue to ride dirt bikes will tend to keep then engine speed up because that is what they are used to. I have always ridden cruisers and that's how I still ride and I am content with how the LT handles. That coupled with me being a generally laid back person means that I do not ride with our local BMW club because I will just not be willing to go fast enough to keep up, making the ride fun for no one. I find myself more at home with a cruising club and so that's where I ride.
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post #20 of 25 Old Aug 22nd, 2008, 9:16 pm
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Re: Why is 4K the magic RPM number?

I shift at around 2 to 3 k. Unless I'm heavy traffic or entering a highway. My bike is happy with the lower revs. However, I weigh 160lb and I'm easy on the throttle. I dont lug the motor, I just like to ride easy.
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post #21 of 25 Old Aug 22nd, 2008, 9:48 pm
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Re: Why is 4K the magic RPM number?

The argument is always there about replacing brakes or clutches when coming to a stop, I use brakes with downshifting with as little clutch slippage as possible. I'll still be in the right gear if I have to scoot but it's a hell of alot cheaper replacing brake pads than clutches etc.
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post #22 of 25 Old Aug 22nd, 2008, 10:21 pm
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Re: Why is 4K the magic RPM number?

It is very evident from this thread and past ones that there is NO "magic" number. Like people ride different style bikes, people also ride differently within one type of bike.

Neither mode of riding the LT really hurts it very much. In my opinion though, riding nearly all the time below 3500 RPM probably shortens the life of the engine FAR more than riding much of the time well above 4,000. Still, a large number of LT riders will not likely see much over 100K miles during the time they have it, so will pass on any possible engine life shortening damage to the next owner. The old "brick" engine will likely see 150-200 K even if abused somewhat.

It all boils down to how each individual likes to ride. I was one for the higher RPM ranges anytime except for just droning down the freeways, where I just rode along in 5th gear. Any other time though, I usually kept the RPM above 3500, shifting around 5000 unless in the twisties or otherwise just having fun. Then it was shift at 6-7000. I nearly always shifted at an RPM that would not let the engine drop below 3500 after the shift.

That was the way I most enjoyed the performance potential of the bike, and liked the far more snappy response to throttle. Also, as others have pointed out, compression braking going into curves is FAR better if the revs are up, keeping one off the brakes so that the curve response does not suffer from braking, which will try to stand the bike back up, especially if any front braking is used.

Highway riding though just begs for loafing along at low RPM. Just down shift if you see a possible need for accelleration coming up. It just sucks if you grab a lot of throttle when a need arises, and almost nothing happens. The LT is pretty much a whale at low RPM, but can be surprisingly sprightly if the revs are above 5000.

The LT is a pretty tough bike, so however you ride you are not going to create all that much of a problem with engine longevity.

Now if the new model will just have a reliable FINAL DRIVE, and no failure prone clutch slave cylinder! It will be the perfect bike for me. The LT nearly was.

I don't want to achieve immortality through my work...I want to achieve it through not dying.

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post #23 of 25 Old Aug 22nd, 2008, 10:27 pm
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by dshealey
It is very evident from this thread and past ones that there is NO "magic" number. Like people ride different style bikes, people also ride differently within one type of bike.

Neither mode of riding the LT really hurts it very much. In my opinion though, riding nearly all the time below 3500 RPM probably shortens the life of the engine FAR more than riding much of the time well above 4,000. Still, a large number of LT riders will not likely see much over 100K miles during the time they have it, so will pass on any possible engine life shortening damage to the next owner. The old "brick" engine will likely see 150-200 K even if abused somewhat.

It all boils down to how each individual likes to ride. I was one for the higher RPM ranges anytime except for just droning down the freeways, where I just rode along in 5th gear. Any other time though, I usually kept the RPM above 3500, shifting around 5000 unless in the twisties or otherwise just having fun. Then it was shift at 6-7000. I nearly always shifted at an RPM that would not let the engine drop below 3500 after the shift.

That was the way I most enjoyed the performance potential of the bike, and liked the far more snappy response to throttle. Also, as others have pointed out, compression braking going into curves is FAR better if the revs are up, keeping one off the brakes so that the curve response does not suffer from braking, which will try to stand the bike back up, especially if any front braking is used.

Highway riding though just begs for loafing along at low RPM. Just down shift if you see a possible need for accelleration coming up. It just sucks if you grab a lot of throttle when a need arises, and almost nothing happens. The LT is pretty much a whale at low RPM, but can be surprisingly sprightly if the revs are above 5000.

The LT is a pretty tough bike, so however you ride you are not going to create all that much of a problem with engine longevity.

Now if the new model will just have a reliable FINAL DRIVE, and no failure prone clutch slave cylinder! It will be the perfect bike for me. The LT nearly was.
Why do you think operating below 3,500 RPM will shorten engine life compared to operating above 3,500?

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post #24 of 25 Old Aug 23rd, 2008, 8:51 am
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by Voyager
Why do you think operating below 3,500 RPM will shorten engine life compared to operating above 3,500?
Well, you did ask.

High throttle openings at low RPM put high loads on the crank and rod bearings, which are moving at lower speed than the engine is really designed to sustain for longer time periods. Rod and crank bearings depend on oil shear properties to keep the bearing surfaces from touching. Oil shear pressure depends on speed differential between the two surfaces. At lower speed the oil can more easily be squeezed out allowing the surfaces to get closer, or worse, contact momentarily.

The LT, or any engine designed to run at relatively high RPM, has much smaller crank journals than one designed to run at low RPM, like a big V-twin.

Designing The sizes of the crank journals in any engine s a compromise between bearing surface speed and bearing surface area. If the surface speed gets too high, the oil film can be sheared apart, which damages the oil, so in high RPM engines the journals have to be considerably smaller, which unfortunately also reduces the surface area of the bearings. Small surface area works just fine as long as the surface speed is high enough to develop a sufficient "hydrodynamic wedge" in the oil film based on oil shear properties. Put high power on them at too low speed, and combination of small surface area and surface speed too low to develop enough oil film pressure combines to allow the bearing surfaces to be forced too close together. If you look at rod and crank bearing inserts you see that the ends have a little low angle chamfer designed in. That is to cause the oil to be squeezed into the bearing surfaces as the bearing rotates. that starts the "hydrodynamic wedge" that continues around the bearing surface. The pressure developed by that wedge is what keeps the bearing surfaces from touching.

The same thing can happen between the piston skirt and cylinder wall, but the surface areas are much higher, and the combustion pressure puts much higher load on the bearings than it does on the piston sides when the connecting rod is at an angle, since the angle never reaches anywhere near 45 degrees to the cylinder.

The study of bearings and lubrication is called "Tribology". There are books available on the subject. I took that course, many, many years ago. Still have the book somewhere.

So based on things I have learned as an Engineer over decades, and involvement and study in drag racing in the '60s, it is my desire to protect my engine bearings by keeping the RPM up, and never "lug" the engine. In my opinion, more than 1/3 throttle opening below 2500 RPM for any time is lugging the LT engine. I never cranked the throttle hard open when starting out until the RPM was up a little, and always shifted so that the RPM would be above 3000 after the shift.

I think the LT engine thrives between 3000 and 6500, probably not liking either side of that very much.

I don't want to achieve immortality through my work...I want to achieve it through not dying.

David Shealey
Dandridge, TN
EX: '01 Black LT, BAT BYKE (Totaled at 110,000 miles)
IBA SS, BB, BBG, 10/10ths.
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post #25 of 25 Old Aug 23rd, 2008, 10:49 pm
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Re: Why is 4K the magic RPM number?

Quote:
Originally Posted by dshealey
Well, you did ask.

High throttle openings at low RPM put high loads on the crank and rod bearings, which are moving at lower speed than the engine is really designed to sustain for longer time periods. Rod and crank bearings depend on oil shear properties to keep the bearing surfaces from touching. Oil shear pressure depends on speed differential between the two surfaces. At lower speed the oil can more easily be squeezed out allowing the surfaces to get closer, or worse, contact momentarily.

The LT, or any engine designed to run at relatively high RPM, has much smaller crank journals than one designed to run at low RPM, like a big V-twin.

Designing The sizes of the crank journals in any engine s a compromise between bearing surface speed and bearing surface area. If the surface speed gets too high, the oil film can be sheared apart, which damages the oil, so in high RPM engines the journals have to be considerably smaller, which unfortunately also reduces the surface area of the bearings. Small surface area works just fine as long as the surface speed is high enough to develop a sufficient "hydrodynamic wedge" in the oil film based on oil shear properties. Put high power on them at too low speed, and combination of small surface area and surface speed too low to develop enough oil film pressure combines to allow the bearing surfaces to be forced too close together. If you look at rod and crank bearing inserts you see that the ends have a little low angle chamfer designed in. That is to cause the oil to be squeezed into the bearing surfaces as the bearing rotates. that starts the "hydrodynamic wedge" that continues around the bearing surface. The pressure developed by that wedge is what keeps the bearing surfaces from touching.

The same thing can happen between the piston skirt and cylinder wall, but the surface areas are much higher, and the combustion pressure puts much higher load on the bearings than it does on the piston sides when the connecting rod is at an angle, since the angle never reaches anywhere near 45 degrees to the cylinder.

The study of bearings and lubrication is called "Tribology". There are books available on the subject. I took that course, many, many years ago. Still have the book somewhere.

So based on things I have learned as an Engineer over decades, and involvement and study in drag racing in the '60s, it is my desire to protect my engine bearings by keeping the RPM up, and never "lug" the engine. In my opinion, more than 1/3 throttle opening below 2500 RPM for any time is lugging the LT engine. I never cranked the throttle hard open when starting out until the RPM was up a little, and always shifted so that the RPM would be above 3000 after the shift.

I think the LT engine thrives between 3000 and 6500, probably not liking either side of that very much.
Ah, you didn't mention engine load before, you just said operating RPM. I agree that large throttle openings at low RPM aren't desirable, but cruising at small throttle openings at low RPM is perfectly fine for an engine like that in the LT.

And there is a trade-off between the bearing loads from combustion pressure sourced forces and the dynamic forces from the reciprocating components. As RPM increases, the bearings can withstand greater loads, but they also need to withstand greater loads as the force from the pistons reversing direction become very high.

My tribology experience is pretty dated also, but I do remember enough to know that tribilogy relates to surfaces in relative motion and isn't just an issue related to bearings and lubrication! It is much broader than that.

I don't think a broad statement such as "operating below X RPM will cause greater engine wear" is accurate. That was the point of my question. And, yes, I did ask.

2017 KLR650 "Mule"
2007 K1200LT "Starship Enterprise", VOICE II, Navigator V, Motorrad Communicator
1987 Kawasaki Voyager XII
1976 Kawasaki KH400
1973 Kawasaki 100 G5
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