R90/6 vs. R100s

OK, this may seem like a stupid scenario, but here it is. I found 2 bikes, one, a 74(I think) R90/6 with decent paint, 90K miles, is on non-op. Other is a 78 R100s, 130K miles, etc... Both are about teh same price. I'm wondering what the differences between the 2 bikes is. I'd like a classic, I dig the cafe style bikes, & can do upkeep work. I just want to know the basic differences, like power, handling, brakes, etc, any quirks, what not...

May be a moot point if mama finds out about my plans before I hatch them!

Is the '78 dual disks...from the specs it looks like it should be? Even if it isn't, it has the drilled rotor(s) whereas the '74 should have the solid rotor. The '78 should have better brakes, although maybe only marginally.

Other than that, they're both points bikes, heavy flywheels, pretty much the same chassis. Obviously the 1000cc will have more power, but it's heavier. They're both high mileage and are in the range of needing the top end done or at least looked at.

To me, the brakes would be the biggest thing to think about.

its no contest, the R100 is the better bike, imo.
Frame is slightly upgraed on the R100- extra gussetting was added to the steering neck in '76 or '77. Slightly stiffer chassis, which is good. swingarm and fork tubes are consistent, as are tire sizs, so no other handling differences of note. S bars may feel better than the more upright USA bars of the /6.
switch gear is a bit different- kind of a toss up otherwise. no handlebar kill switch on the /6, iirc.
Brakes are dual disc vs single solid disc. Slight upgrade.
'74 is first year of the 5 speed trans, and has an inherent weakness or 2- notably the detent spring. however, that has likely already been repaired/replaced, as they typically snap in the 30K mile range. '74 has a kicker that the S does not. not that useful for starting , but good for setting valves and timing/pionts.
upgrade on alternator in the '78, iirc.
'74 is the first of the more "modern" airheads for styling (upgraded headlight & instruments from the /5, so both have the same basic visual aspect), so nothing gained by going older in that dept.
more power, more torque on the 100.
a running bike is generaly superior to a non-operative one.
stock paint schemes on the R100 (red smoke?) are nicer than any stock paint on the R90.
i owned both a 1974 R90/6 and a '78 R100S. R100, no contest.

Oh come on....

Having a bike or car registered as a non-op simply means that when you go down to the DMV to transfer the title and register the bike, you won't have any back fees to pay. Now, on to the bikes!

Having had both and R90 and a couple of R100's, I think that there are very significant differences between the 1974 /6 and the 78 R100. Firstly, the R90 engine is probably the best type 247 that BMW ever built. It's strong, reasonably smooth and it'll last well over 1/4 million miles with regular service and reasonable care. In my opinion the /6 represents one of the high points in Airhead design and they're becoming both a bit scarce and desirable. A nice /6 is a very nice bike! For me, the deal killer would be the /5 bits that remained part of that particular years bike. If you want an R90, save yourself some trouble and look for a clean 1976 with dual disk brakes. Thats the one to own!

The R100S has several improvements over the earlier bike, including a somewhat stronger frame, stronger engine cases, dual front disks, a more complete gauge package, the "S" fairing and the updated body work is very pretty and it has an updated transmission.

The R100 engine has an added 100cc of displacement over the /6, meaning that the bike has bigger carbs, more torque, poorer mileage (In my experience anyway.), vastly updated controls, higher gearing and a tad greater fuel capacity. I cant remember whether the 78S had an oil cooler or not (I think it did.) but the R100 engine produces a bit more heat. I like having oil coolers on my Airheads but YMMV... Oh... And the R100S seat can be very, very uncomfortable during long rides but fixes are readily available, if somewhat expensive.

You'll have to look at both bikes before making a decision but with the better brakes, prettier body work and way more possibilities for upgrading and hot rodding using after-market parts, I'd choose the R100S hands down, even if it was going to cost more. All-in-all, despite my loving R90s , (I rode one for 200K miles!) I'd still buy the R100.

no cooler on the S model. Euro RS models would likely have come with one.

I did not think that any /6 came with dual discs, other than the R90S.

bikerfish1100 said:

no cooler on the S model. Euro RS models would likely have come with one.

I did not think that any /6 came with dual discs, other than the R90S.

Click to expand...

I have an R90/6 - 1976, with dual drilled front discs. I ordered it new with that option.

Pi Variables

R100S...stronger frame, ~22 lbs. heavier, better controls, brakes (probably) and more umph. Nifty fairing, but you could fit one onto the /6 too and a seat cowl on it too. The R100S might have slightly higher compression, not that you'd notice it. I think its 9.5:1 vs. the R90/6 9:1. Final Drive ratio could possibly be different too, according to Phil Hawksley's spec site.

R90/6 Specs

R100S Specs

Either would be fine candidates for a rebuild, that you'd not likely regret at least as far as getting an Airhead goes. Lots of good advice in here, sprinkled with typical campfire BS.

R90 vs. R100

I recall reading in Classic Motorbike that the unfaired R90/6 had the highest top speed and the fastest two way standing mile of all airheads tested, including the 77 R100 RS and the R90S.

If top speed and acceleration are considerations, it seems that in the real world, the differences are insignificant.

bikerfish1100 said:

its no contest, the R100 is the better bike, imo.
Frame is slightly upgraed on the R100- extra gussetting was added to the steering neck in '76 or '77. Slightly stiffer chassis, which is good. swingarm and fork tubes are consistent, as are tire sizs, so no other handling differences of note. S bars may feel better than the more upright USA bars of the /6.
switch gear is a bit different- kind of a toss up otherwise. no handlebar kill switch on the /6, iirc.
Brakes are dual disc vs single solid disc. Slight upgrade.
'74 is first year of the 5 speed trans, and has an inherent weakness or 2- notably the detent spring. however, that has likely already been repaired/replaced, as they typically snap in the 30K mile range. '74 has a kicker that the S does not. not that useful for starting , but good for setting valves and timing/pionts.
upgrade on alternator in the '78, iirc.
'74 is the first of the more "modern" airheads for styling (upgraded headlight & instruments from the /5, so both have the same basic visual aspect), so nothing gained by going older in that dept.
more power, more torque on the 100.
a running bike is generaly superior to a non-operative one.
stock paint schemes on the R100 (red smoke?) are nicer than any stock paint on the R90.
i owned both a 1974 R90/6 and a '78 R100S. R100, no contest.

Click to expand...

+1

The '74 had a problematic transmission. Please correct me, but I think many parts for that tranny are NLA. The kick start is more for looks than function.

Your third option is to keep looking, nice bikes are always popping up.

subagon said:

The kick start is more for looks than function.

Click to expand...

I use my kick start frequently. Once warm, the engine starts with a single, easy kick. Saves battery.

Pi Variables

pi variables said:

I use my kick start frequently. Once warm, the engine starts with a single, easy kick. Saves battery.

Click to expand...

The cost of a battery or two is less than the cost of a tranny rebuild! The kicker has been known to fail and create more problems. On the plus side, just think of all the cardio you're getting!

pi variables said:

I have an R90/6 - 1976, with dual drilled front discs. I ordered it new with that option.

Pi Variables

Click to expand...

so let me rephrase-

I did not think that any /6 came with dual discs as stock/standard equipment from the factory, other than the R90S.

felaw said:

I recall reading in Classic Motorbike that the unfaired R90/6 had the highest top speed and the fastest two way standing mile of all airheads tested, including the 77 R100 RS and the R90S.

If top speed and acceleration are considerations, it seems that in the real world, the differences are insignificant.

Click to expand...

i would like to see those comparisons, the results you stated make no sense to me. 32mm carbs vs 38 or 40mm should not give more oomph, nor should the smaller 38 dia. exhaust of the R90/6 vs the 38 or most R100s or the 40 mm of most '77 RS models.
thats not even addressing the increased hp/torque of the later motors;
R90/6- 60hp, 53 lb ft
R90S- 67 hp, 56 lb ft
R100S- 65 hp, 55 lb ft
R100RS- 70 hp, 56 lb ft (i believe this is for the 40mm variants, which includes most '77s).
i have a tough time figuring how a smaller, less powerful engine can give both better acceleration and higher top speed- as you could not get both to be better from just a final drive difference. you might bump accelration via a shorter FD, or increase top speed via a taller FD, but not both improvements in one package. (and transmission gear ratios were basically the same, so no advantage there).
Factor in the wind cheating RS fairing vs a naked /6, and the top speed issue falls by the wayside as well.

however, bottom line is that there is not a ton of a performance diff between a 900 and a 1000 stock motor, tho advantage does have to go the the bigger engine.
oh yeah- the 22 lb gain in weight of the R100 would eat away about 2 hp, still leaving the R100 with an edge of several hp.

apples vs apples?

This is from Bill Stermer's book on the history of the RS, however I think the same engine was on the R100S. Now, if someone can find times and speeds for a R90?

Since the RS is heavier, it should also be slower on acceleration. I would not want to venture a guess about top speed though, I have read that the RS's faring improves top speed?

toooldtocare said:

Now, if someone can find times and speeds for a R90?

Click to expand...

There's a bunch of R90/6 road test articles here:

http://www.beemergarage.com/road_tests.html

Cycle World Feb 1974 - quarter mile 13.45 sec, 97.27 mph
Cycle Guide Jun 1974 - quarter mile 13.49 sec, 96.8 mph
Cycle Test Sep 1974 - quarter mile 13.71 sec, 95.8 mph

Acceleration data from the /6 owner's manual:

http://www.pbase.com/dwerbil/image/74356140
http://www.pbase.com/dwerbil/image/74356147

Cycle (Dec 76) magazine's results on the 1977RS - standing start 1/4 mile- 13.33 secs @ 99.11 mph. That's with a 9.5:1 CR.
fwiw, they also mentioned a reinforced swingarm came into production in late '76.
Cycle Guide did a review in May 1981 on the RS (8.2:1 CR), and reported 1/4 miles 13.48 secs @ 98.36 mps. They calculated a top speed of 127 mph.

bikerfish1100 said:

so let me rephrase-

I did not think that any /6 came with dual discs as stock/standard equipment from the factory, other than the R90S.

Click to expand...

Well said. I believe that you have accurately portrayed the options in 1976.

Pi Variables

20774 said:

There's a bunch of R90/6 road test articles here:

http://www.beemergarage.com/road_tests.html

Cycle World Feb 1974 - quarter mile 13.45 sec, 97.27 mph
Cycle Guide Jun 1974 - quarter mile 13.49 sec, 96.8 mph
Cycle Test Sep 1974 - quarter mile 13.71 sec, 95.8 mph

Acceleration data from the /6 owner's manual:

http://www.pbase.com/dwerbil/image/74356140
http://www.pbase.com/dwerbil/image/74356147

Click to expand...

Gentlemen, I feel that if performance / speed is the issue, then one should not purchase a BMW (of that vintage). There are many options of greater performance. If one has a passion for vintage twins, and that is the driving force behind the purchase, then older is better.

Pi Variables

Guys, this has bee a great comparison education! I think I'm going to pass on the 74, & keep my eyes peeled for a 75/76, or wait until a good R100 comes by. Performance is mot my sole reason for buying such a bike, but I would rather buy a more performance oriented model that one that is not. plus, looks like I must sell my F650CS before jumping in! Gives me more time to look.

Magazine test stats are always interesting to read but these days we have better ways to gather info.

I know that the Garman 550 GPS mounted on my RS read just over 113 while I was blasting through in the desert on HWY-50, during the summer of 2009. Not only that but my 'seat-of-the-pants' meter was telling me that there was still a little bit more speed left in the bike. I attribute that speed to the fact that the RS had a fresh tune-up, fresh tires, a tank full of real "Hi-Test" and I was riding loaded, which probably canceled some rear tire slippage. As far as I know, GPS systems don't lie.

When I was riding my bone stock R90 back in the 80's and 90's, I regularly had my clock cleaned by my buddies, riding their R100 RS's and "S" bikes. The extra torque put out by the larger engines allowed them to exit corners faster and pull away on straights, with no problem what-so-ever. Add in the fact that the later bikes handled better and it was no contest. In fact, the performance gap between the different bikes is what pissed me off enough to build my very modified /6.

If you doubt me, go ride some bikes and prove it to your own self!