Page 2 of 2 FirstFirst 1 2
Results 16 to 24 of 24

Thread: F 750 GS Engine nature

  1. #16
    Registered User
    Join Date
    Aug 2018
    Location
    Dunlap, TN
    Posts
    146
    It has always been my understanding that the Rotax engines, like the F800 and F800st and GT motors had a 360 crank, where both pistons rose and fell together, like an old Triumph. Lots of potential for vibration with that design. The new China-supplied 750/850 has a 90 degree crank offset, which should make for a smoother engine than the earlier designs.

  2. #17
    Registered User
    Join Date
    Mar 2003
    Location
    Okanagan Valley, BC
    Posts
    634
    Quote Originally Posted by motormike View Post
    It has always been my understanding that the Rotax engines, like the F800 and F800st and GT motors had a 360 crank, where both pistons rose and fell together, like an old Triumph. Lots of potential for vibration with that design. The new China-supplied 750/850 has a 90 degree crank offset, which should make for a smoother engine than the earlier designs.
    You have overlooked something important. The F800s are in fact smooth running because of a Rotax innovation - an ingenious counterbalancing pivoting connecting rod mounted on the crankshaft between the two piston rod journals. The connecting rod moves in the opposite direction of the two pistons. The first order forces are 100% compensated for and the secondary forces are 70% compensated for. The Rotax motors are in fact smoother that the Loncin engines.

    Conterbalance.jpgEngine cross section.jpg

  3. #18
    The 1993 Ducati Supermono incorporated a similar dummy rod counterbalancer system, which predates the Rotax design. Still pretty clever, though.
    -Live as fully as you can as long as you can-

  4. #19
    #13338 PGlaves's Avatar
    Join Date
    Oct 2003
    Location
    "Big Bend" TX
    Posts
    22,799
    The principle of counter balancing rod/piston assemblies goes all the way back to steam engines at the turn of the 20th century.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  5. #20
    Registered User
    Join Date
    Mar 2003
    Location
    Okanagan Valley, BC
    Posts
    634
    From what I have read in various articles the Rotax counter-balancing scheme is less "power-sapping" than others.

    https://www.motorcyclistonline.com/p...-drawing-line/

  6. #21
    Registered User
    Join Date
    Aug 2018
    Location
    Dunlap, TN
    Posts
    146
    Interesting. I've been looking at the F800GT's but everything I've ever read screams about the handlebar vibrations from that engine. I would say it's the number one complaint, bar none. (is that a pun?) The tester of the F800 in this Visor Down video would disagree (1 min in and again at 2:08) with you: https://www.bing.com/videos/search?q...DFF6&FORM=VIRE

  7. #22
    Registered User
    Join Date
    Mar 2003
    Location
    Okanagan Valley, BC
    Posts
    634
    Quote Originally Posted by motormike View Post
    Interesting. I've been looking at the F800GT's but everything I've ever read screams about the handlebar vibrations from that engine. I would say it's the number one complaint, bar none. (is that a pun?) The tester of the F800 in this Visor Down video would disagree (1 min in and again at 2:08) with you: https://www.bing.com/videos/search?q...DFF6&FORM=VIRE
    We have two F800s. Total of 240,000 kms. Vibration isn't objectionable and in fact I can't remember even thinking about it.

  8. #23
    Quote Originally Posted by PGlaves View Post
    The principle of counter balancing rod/piston assemblies goes all the way back to steam engines at the turn of the 20th century.
    Yeah but i believe Ducati was still the first to use the double connecting rod counterbalance system on the Supermono, although they only made a very few of those engines. I think it works better on a 90 degree angle (for reasons too detailed to discuss here), but the Rotax 804 can run pretty smooth, especially if the fueling is right. I readily admit i do not understand why fueling is a factor, but at least it was on my bike.

    https://www.cuoredesmo.com/the-histo...ermono-engine/
    -Live as fully as you can as long as you can-

  9. #24
    Quote Originally Posted by motormike View Post
    Interesting. I've been looking at the F800GT's but everything I've ever read screams about the handlebar vibrations from that engine. I would say it's the number one complaint, bar none. (is that a pun?) The tester of the F800 in this Visor Down video would disagree (1 min in and again at 2:08) with you: https://www.bing.com/videos/search?q...DFF6&FORM=VIRE
    I have never ridden an 800ST but did experience some high frequency, low amplitude “buzz” on the handlebars of a 700GS, but was able to eliminate most of it with some modification.
    -Live as fully as you can as long as you can-

Similar Threads

  1. 1972 750 engine question
    By kantuckid in forum Airheads
    Replies: 7
    Last Post: 05-19-2009, 12:03 AM
  2. Despite my fastidious nature . . .
    By flgoff in forum Campfire
    Replies: 5
    Last Post: 01-20-2008, 01:01 AM
  3. Trashing Out Mother Nature Needs Analysis
    By redclfco in forum Campfire
    Replies: 17
    Last Post: 04-20-2007, 04:38 PM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •