PDA

View Full Version : R90S Info - Newbie

mjy78onR90S
03-31-2008, 11:45 AM
Hi
I bought an R90S.
But, as I've been BMW-less for maybe 15 years (insert sigh here) I am not current with specs and such...used to ride a 72 R75/5(5 spd).
How is a R90S different than an R90/6 or R100 ? I've heard compression ratio...
lkchris
03-31-2008, 11:52 AM
Compression, carbs, fuel tank, seat.

The R100 is a /7, i.e. a whole different generation BMW. They were not sold concurrently.
DarrylRi
03-31-2008, 12:03 PM
Dual front disk brakes and the gas tank are different vs. the R90/6. These became standard on the /7s and beyond.

The R100s have the front brake master cylinder directly on the handlebar, rather than being actuated by a short cable to the reservoir under the tank.

The Dell'Orto pumper carbs are different from everything else. I believe the cams are different.

Depending on which year R90S you have, the switch gear is different from either the /5 or the /7 -- the '74s have /5 switches, the '75s and '76s have the switches that were also on the /7s.
20774
03-31-2008, 12:37 PM
The R100 is a /7, i.e. a whole different generation BMW.

But still within the "farm implement" generation, right??:stick
bikerfish1100
03-31-2008, 12:58 PM
[QUOTE=DarrylRi;311412]

The R100s have the front brake master cylinder directly on the handlebar, rather than being actuated by a short cable to the reservoir under the tank.

QUOTE]


not in '78- the mc was still under the tank.
HP was up on the R90S models compared to the /6 std. The cool fade/smoke paint jobs were S variants only.
Did they also have different damping rods (# of holes) in the front end? What about hydraulic steering damper, wasn't that a R90S feature (like it was on the 100S models)?
Braddog
03-31-2008, 12:58 PM
The R100s have the front brake master cylinder directly on the handlebar, rather than being actuated by a short cable to the reservoir under the tank.


Don't I wish!

I've got 2 R100's, both of which have the reservoirs under their respective tanks. They're both 77's, so I assume the transition to the handlebars happened sometime after that. Thankfully, removing the tank is no big deal for purposes of flushing the brakes and such.
r60us
03-31-2008, 05:05 PM
Don't I wish!

I've got 2 R100's, both of which have the reservoirs under their respective tanks. They're both 77's, so I assume the transition to the handlebars happened sometime after that. Thankfully, removing the tank is no big deal for purposes of flushing the brakes and such.

The switch to the handlebar master cylinder coincided with the introduction of the Brembo calipers for the 1981 model year.

Camshaft: The R90S used the same camshaft (308 degree) as the R75's, R90/6, and the R100's. The R90S used 9.5:1CR pistons compared to 9.0:1 for the R90/6 which would account for some of the increase in performance over the standard R90/6.

Regarding switch/lever mounts: the 1974 only mounts are great for retrofit on a /5 or a /2 conversion as they use the sturdier /6 /7 mirrors and you get a front brake light switch for the /2 conversion.
swall
03-31-2008, 06:01 PM
'74 R90S's had 10mm flywheel bolts, compared to the 11mm bolts on the '75 and '76 models. The smaller bolts can be problematic.
mjy78onR90S
04-01-2008, 10:35 AM
Mine is a 75, the last year wasn't it ?
Sorry I said R90S vs. R100, I meant R90S vs. R90...
So the cams are the same...
The pistons are different...
What about rear axle ratios ?
I remember my R75/5/6 being overrev'd IMHO, with a 32/11 I think it was.
For differences, so far we have: pistons, paint,
mjy78onR90S
04-01-2008, 10:35 AM
One other thing...
How different or the same, are the R100S and the R90S ?
ccolwell
04-01-2008, 04:27 PM
Mine is a 75, the last year wasn't it ?
Sorry I said R90S vs. R100, I meant R90S vs. R90...
So the cams are the same...
The pistons are different...
What about rear axle ratios ?
I remember my R75/5/6 being overrev'd IMHO, with a 32/11 I think it was.
For differences, so far we have: pistons, paint,

1976 is the last, and the best, so "they" say. I think the cams are different, but others will know better.
8ninety8
05-14-2008, 02:38 PM
When I pulled the orginal cam out of my '73 R90S #4071406, it was definitely different than the 336 I was putting in. By looking at the lobes from the end you could see the lift and overlap increase.

I have always wondered after hearing, and now observing myself, how much better the 336, why that cam was not installed as matter of course in the R90S as part of its upgrade??

If you've got a few bucks laying around, it is well worth the upgrade. There are so many upgrades from stock that make the bike a better ride. Problem, most are no longer available.