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Old 08-30-2013, 04:37 AM   #676
DoktorT
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Quote:
Originally Posted by Beemerboff View Post
But that is their opinion, and their results, and if anyone has a logical explanation as to why shorter rods work better, and some documented test results I am certain we would all like to hear them.
Indeed, any controled variables testing by competent persons who publish results and conclusions for peer review are going to trump all the butt dynos in the world combined when it comes to REALITY. Just keep the salt grains available until you find such.
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Old 08-30-2013, 08:56 AM   #677
supershaft
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Originally Posted by Beemerboff View Post
There was a good article in an old Petersen mag on rod lengths.

Working with a lightly modified 302 and changing nothing but the pistons/pin location they found that long rods worked better everywhere.

Their reasoning was that with long rods the piston spent a shorter time at TDC , then moved away from TDC slower.
So it was still to catch more of the pressure build up, and at the point where it could make most use of the pressure, when the angle between rod and the crankpin radius was around 90 degrees, the piston was further up the bore and subject to more pressure.

All made sense to me, and of course the cams and possibly ignition timing could also be optimised to suit the longer rod too, putting it further ahead.

But that is their opinion, and their results, and if anyone has a logical explanation as to why shorter rods work better, and some documented test results I am certain we would all like to hear them.
Shorter rods do have advantages. For the most part greater mechanical leverage and less tendency to ping. At higher rpms increased piston dwell trumps those advantages. Most say around 4000rpm in a lot of engines including ours.

Reality? I have worked with long rodded airheads. Butt dynos trump dynos all the time in reality but I have worked with long rodded airheads on the dyno too. From a stock 1.91:1 to around 2.15:1 works! If I had a real hot rod airhead, I would run long rods in a 94mm bore.
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Old 08-31-2013, 04:23 AM   #678
Beemerboff
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A longer rod always has a straighter push at the crankshaft, and that will translate into better mechanical advantage, not less.

And the piston on the shorter rod spends more time at TDC where it isn't moving, and if it isn't moving it isn't making any power.

It is hard to see any point where a short rod has any mechanical advantage that would enable to overcome these to disadvantages and get in front.

Just why things change at 4000 revs also needs a bit more explanation.

FWIW the optimum dimensions for a 500cc air cooled two valve motor running to around 7500 revs were determined around 60 years ago, all square at 86 mm , with a rod stroke ratio around 2 to 1 - BMW just lost the plot trying to achieve a narrow motor.

A 2.15 rod ratio is certainly a step in the right direction, and would appear to partly alleviate the damage caused by the ultra short stroke.

I will leave it to someone whoactually has had a long rod motor an a
dyno to confirm if it is optimum.
But they would probably running at higher revs where the reduced stress of the longer rod due to the straighter push would be a big advantage too - BMW crankcased are reputed to be quite flexible when you get the revs/ power up!

Richie Moore would have some of the answers, but like most sensible, professional tuners he will probably keep his hard earned information for those who use his services.
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Old 09-03-2013, 10:44 AM   #679
supershaft
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Quote:
Originally Posted by Beemerboff View Post
A longer rod always has a straighter push at the crankshaft, and that will translate into better mechanical advantage, not less.

And the piston on the shorter rod spends more time at TDC where it isn't moving, and if it isn't moving it isn't making any power.

It is hard to see any point where a short rod has any mechanical advantage that would enable to overcome these to disadvantages and get in front.

Just why things change at 4000 revs also needs a bit more explanation.

FWIW the optimum dimensions for a 500cc air cooled two valve motor running to around 7500 revs were determined around 60 years ago, all square at 86 mm , with a rod stroke ratio around 2 to 1 - BMW just lost the plot trying to achieve a narrow motor.

A 2.15 rod ratio is certainly a step in the right direction, and would appear to partly alleviate the damage caused by the ultra short stroke.

I will leave it to someone whoactually has had a long rod motor an a
dyno to confirm if it is optimum.
But they would probably running at higher revs where the reduced stress of the longer rod due to the straighter push would be a big advantage too - BMW crankcased are reputed to be quite flexible when you get the revs/ power up!

Richie Moore would have some of the answers, but like most sensible, professional tuners he will probably keep his hard earned information for those who use his services.
Like I have said, I have worked with long rodded airheads in the dyno room and out. It sounds like you should read up on it. Some tuners prefer shorter rods for more mechanical leverage. You have figured that backwards. Shorter rods have more angularity and that translates into mechanical advantage.

Right at TDC? That I have never payed attention to. Right around TDC? Longer rods have the piston staying around TDC much longer. Increased TDC piston dwell.

What changes above 4000rpm? The rpm. Flame front travel time has its limitations. RPM doesn't. When the revs get high, it's nice to have a piston that waits for the flame front to push on it. Therein is the greater higher rpm torque. Some tuners even think that when the piston finally does get moving at a greater speed than a shorter rod in order to make up time for hanging around TDC, it really helps enhance the pressure differential on the intake stroke thusly getting an advantage on not only the power stroke but the intake stroke as well. For the most part, it slows things down AND speeds things up all at the right times. That has its advantages for cam timing as well.

Your 60 year old optimum dimensions for a 500cc have long since been proved erroneous. That statement isn't even worth discussing IMO. Look into Honda's last four stroke engine that was designed to compete with a two stroke of equal displacement. Even getting not to close to two strokes had Honda using dimensions far from that nonsense.

Sensible Professional tuners? More than a couple have written books that cover everything they learned on this subject.
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Old 09-08-2013, 11:32 PM   #680
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Anyone knows the dimensions of R100CS -82 fork seals and steering bearing? Or maybe even SKF part numbers?
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Old 09-09-2013, 01:03 PM   #681
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Try snowbum. I just got all the carb o-ring sizes and part numbers. Guy likes his data...for better or worse.
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Old 09-09-2013, 03:12 PM   #682
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This might be 205'd, but a link for 8" round headlights.

http://headlightbmwmotorcycle.com/Pr...scription.html

I am not sure if they suit LHD countries, look like they do - I could be wrong tho'.

Cheers

~OZ
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Old 09-09-2013, 07:42 PM   #683
Kai Ju
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Originally Posted by oz_airhead View Post
This might be 205'd, but a link for 8" round headlights.

http://headlightbmwmotorcycle.com/Pr...scription.html

I am not sure if they suit LHD countries, look like they do - I could be wrong tho'.

Cheers

~OZ

The lens pattern is for what looks to be RHD countries.
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Old 09-09-2013, 11:54 PM   #684
disston
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Quote:
Originally Posted by oz_airhead View Post
This might be 205'd, but a link for 8" round headlights.

http://headlightbmwmotorcycle.com/Pr...scription.html

I am not sure if they suit LHD countries, look like they do - I could be wrong tho'.

Cheers

~OZ
These were available last year but the guy sold out. Not sure if this is the same guy. Terrific deal. I think they weer even cheaper last year.
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Old 09-10-2013, 07:39 AM   #685
boxerboy81
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These were available last year but the guy sold out. Not sure if this is the same guy. Terrific deal. I think they weer even cheaper last year.
They were less than $50 last year! Sold like hotcakes!
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Old 10-16-2013, 07:29 PM   #686
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Piston rings non-oem r100

I tried to get the Hastings rings as suggested by Lornce, my neighbour but part number changed or not available. I did check the website also but no love.

My VW Porsche mechanic is very familiar with these engines and offered these rings which do fit and seem to be right.

However I am used to rings being tighter in the bore and these measure .025" in the bore. But no rings are offered as oversize unlike for cast iron barrels which are sold .025" over. I believe BMW OEM rings for /7 have oversize too.

Sooo going to use them. Tell me I should --- please !!

Here is a pix of the box. Dont know the maker but maybe your supplier can read the barcode and get some for you too ! ooops not allowed to upload attachments

Fraser in Toronto
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Old 10-16-2013, 08:37 PM   #687
Wirespokes
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I tried to get the Hastings rings as suggested...
Call them! They're very helpful and probably have something for you - even if they have to make them up.
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Old 11-29-2013, 03:42 PM   #688
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i have used them with no luck, the ring has not the same shape as the original and the oil ring is different too.
they work but with lower compression, if you have the money the original ring is much better.
also from 81 the barrels are cover with nikasil, very unlike you have any need to oversize rings

i put nikasil barrels from ebay, my r 100 is from 78, you can swap them, used and with std rings
now is working well and does not burn oil
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Old 12-02-2013, 07:15 AM   #689
Donny1973
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Rear Shock Covers

Anyone know if the aluminum /5 shock covers are being reproduced?
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Old 12-02-2013, 09:34 AM   #690
Caddy82rats
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Anyone know if the aluminum /5 shock covers are being reproduced?
http://www.motobins.co.uk/bmw-parts....20valve%20Twin

Motobins UK PN: Part No.93391
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