Rod Length Ratio of the new GS

Dr. Curve · 04-08-2004, 06:59 PM

Whats the opinion out there on the rods now being used in the new GS with its longer (2.5mm) stroke...........are they longer or the same length as before. The current oilheads with their RLR of 1.77 is one of the lowest in the current crop of motorcycles out there, with only Harley producing a engine with a lower RLR. Current engine design has been going for the higher RLRs for a long time with some Ducatis having a RLR of more than 2.1. BMW also increased their RLR in the fours over time in a effort to smooth the engine out even more.......... and many of us wondered if their decision to reduce the RLR to 1.77 from the previous airheads 1.91 was a good decision. The long rod BMW engines that have been built, both air head and oil head, have proven strong in HP at higher RPMs and enjoy more peak torque at a higher RPM than a stock rod length (125 mm) bike. On this new engine BMW has the choice of keeping the rods short which helps keep the engine narrow and increases torque at low RPMs....... or........ trying to raise the ratio by extending the piston pins on up toward the crown.

By the looks of the engine photos that have been released it appears as if the rods are going to be as short as ever.......which offers wonderful torque down low but limits top RPMs. If the new rods are 130mm long, 5mm longer than the the current units........then the new RLR will be 1.78 as contrasted to the current 1.773 which is very neary the same. For BMW to have really raised the RLR up to current designs the rods would have needed to be lengthened to at least 140 mm (1.91 same as airhead) which clearly did not happen.

This means that we can be assured of a new engine with great torque right off idle and into mid range............but with limited red line and a tapering off of torque up top, just where many of us would like to see it start to pick up.

Any chance that BMW will offer the S units with a higher RLR than the other twins in the new line? I wish they would but until we can get the specs there is no way to know for sure. I predict that the new rods are no more than 5mm longer than the current units and that it is possible that they are still the same in length as the current units which would yield a RLR of 1.71......which would make this one bad puppy on bottom end.

Any thoughts on the new rods in the new engine and what BMW direction BMW has chosen to go? Long rods, same rods, or shorter rods?

*Note of course: The new rods need to be at least 4.25 mm longer (129.25 mm) than current units if the RLR is to stay exactly the same (1.77) as it is now.

captainasty · 04-08-2004, 08:11 PM

Hey Doc, How's it going? If they are using the same crank and piston as in the R1200C then the rods are shorter.

CYoung · 04-09-2004, 05:40 AM

You need to hope for a new R1100S with the bigger bore but shorter stroke and longer rods

Personally, I can't wait for an updated S based on the new boxer motor regardless of the final RLR. Its gonna be great!

CYoung

roger albert · 04-09-2004, 07:25 AM

Captain Nasty,

The crank is not even vaguely the same crank. Can't be what with the different offset, smaller counterweights, a journal moved etc.

I doubt we'll see the longer rods here. It was a trend in motor design, but seems to be going back the other way now, not just BMW. I just ran across an article touching on that this week. I'll try and see if I can find which/where and share it. At any rate, though certainly existing, the long rods vs shorts rods are a very small effect in the overall motor design, and have as many cons as pros. Anyway, those are my thoughts. I don't have any real opinion o what BMW has done. There's a FACT there somewhere (what is the new length and rlr) but I don't happen to know it. Like Jim, I am curious though.

Dr. Curve · 04-09-2004, 07:37 AM

Well CY, I had thought ,as well, that BMW would offer us a new S engine with a 68mm stroke, shorter rods, high pin pistons, and a very flat combustion chamber.........but they went the other way somewhat. I believe that BMW will always heed the need for engine design that offers high torque at low RPMs and the lower the RLR the more advantage of rod angularity in the first 90 degrees of crank travel. Harley has always been the leader in this department ..............with the noteable exception being the XR750 which had a RLR of aprox. 2.12.

I am curious about the new rods. If they are the same length as before, which BMW could have accomplished by moving the pins in the new pistons up toward the crown just 2.5mm, then the RLR will be reduced to about where the current Harleys are. This will assure that the engine has solid touring and heavy load credentials, thus making camp ground turn arounds and other low speed maneuvers a piece of cake, with no bogging down or stalls in the forecast.

Still..........the RPM red line will remain inherently lower over time if the RLR is not increased.

Had BMW offered us a engine with a 68mm stroke, 100mm pistons, and 125mm rods, then we would be looking at a boxer twin with a red line of 9500 instead of the 8400 or so we now live with. Peak HP would be increased and max torque would be in the higher RPM ranges rather than down so low like now. Great for a S but not so hot for a touring bike.

I will buy whatever they send of course, but can't help but wonder at the new direction.

roger albert · 04-09-2004, 09:37 AM

Hi Jim and all,

> I believe that BMW will always heed the need for engine design that offers high torque at low RPMs and the lower the RLR the more advantage of rod angularity in the first 90 degrees of crank travel.

Agreed, that's why this doesn't strike me as being particularly a "new direction"

I'll buy what they send if I like it. I'm back to kind of enjoying the old airhead proiject. It's proven I still have a bunch to learn on tuning Mikuni flatslides. I have a bunch more power now (coupled with a big bore kit and big valve heads which along with the carbs has moved the good torque quite a ways up the rpm range) but don't feel as comfortable using it. Quite a bit less controllable. Always good to have an opportunity to learn. Just a bit frustrating in the meantime.