Home › Forums › Motorcycle Tech Talk › 1991 R100RS head shake at 70-85 km/h on certain roads

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  • Anonymous
    Inactive
    16/01/2011 at 1:31 am
    Post count: 9
    #3538

    I own a great '91 R100RS that I use for everything from commuting to short tours, I use Metzeler Lazertec tyres, and keep them inflated to the specs found on the bike (under the seat). The current ones are pretty new but the problem has been around since I got the bike (about 5 years ago). The wheels are balanced.The problem is that around 70-85 km/h on particularly smooth roads the bike develops a 'shimmy' or 'head shake' which is pretty disconcerting. If I ride through that speed, the problem goes away but come back as I slow down. It is only a problem on certain smooth roads - the problem doesn't seem to be there on 'normal' roads. Mostly it isn't a problem but happens enough to make me alert for it and it does spoil the ride a bit.Sometimes it gets really bad (bike shakes a lot), and other times not so bad (on the same stretch of road). Always seems less if I have my lady on the back.And a couple of times, it nearly threw me off. But it only happens on certain smooth roads (Cobham Drive and the Road from the airport is where I mainly experience it 'cos I drive to work along there every day) but I experienced it in Napier once on a trip and a few other places.I have mentioned it to Michael at Motorad but he can't find anything wrong and says that this sometimes happen on older bikes - which is true. However, I wondered if anyone had any ideas on what I should adjust / change / modify to stop it happening. Interested in the 'collective wisdom' out there. Maybe it's some thing that I need to live with (as I have done) but worth asking the question.Any thoughts, team?Cheers,Terry

    Dave Ross
    Moderator
    16/01/2011 at 3:52 am
    Post count: 2310
    #14582

    I am sure Michael would have checked the steering head bearings and possibly the wheel bearings. Other than that it could be a swing arm bearing or the shocks. Have you ever serviced your forks?

    Anonymous
    Inactive
    18/01/2011 at 5:21 am
    Post count: 9
    #14583

    Thanks for that. I had the fork oil changed when I first got the bike but nothing since then. Perhaps it is time for another oil change. Other than that, no, I haven't had the forks serviced.The swingarm seems pretty solid (no sideways movement at all).Cheers, and thanks for your response.Terry

    Dave Ross
    Moderator
    18/01/2011 at 5:33 am
    Post count: 2310
    #14584

    Bryan and I recently took the forks on his K bike apart to change the oil seals. One of the bushes was worn. Alex

    Anonymous
    Inactive
    19/01/2011 at 1:04 am
    Post count: 84
    #14585

    Terry;  I had a 'shimmy' on my 1997 K1100LT which was most pronounced on smooth roads.  The shimmy developed at certain speed ranges after an innocuous T-bone incident with a car's rear wheel.  Nothing obvious could be seen, and 'experts' declared the forks/steering head/frame etc were all OK.  I finally set up a micrometer (borrowed from work) to detect a 3mm buckle/run-out in the front rim.  A mag wheel repair firm (Brugar Engineering) in Barry's Point Rd., Takapuna trued the wheel (no balancing subsequently needed!) and Voila! no more shimmy!    (Note: somewhere in Clymer is a warning not to attempt to straighten cast wheels, but Brugar Engineering said “no problem, we do them all the time!).

    Dave Ross
    Moderator
    19/01/2011 at 4:24 am
    Post count: 2310
    #14586

    That's useful information. Gerda's bike has problems with warped discs. It could well be that the wheel on her bike is warped as well.I will have to check that.

    Anonymous
    Guest
    19/01/2011 at 5:54 am
    Post count: 2134
    #14587

    I used to get this on my K100LT on smooth roads with a Metzeler, but never with a Michelin tyre.  And it was more pronounced once the tyre was getting a bit worn.Trial and error found a couple more psi in the front made it go away, but that might just have been because the K100 was so heavy on the front.

    Anonymous
    Inactive
    19/01/2011 at 6:15 am
    Post count: 9
    #14588

    Thanks for that. It certainly seemed to go away for a while when the tyre was very new but did come back. That's interesting indeed.I might look at a different brand next time. Probably can't hurt. I've always used Michelins on my previous bikes with no problem. Food for thought.And I'll try another couple of PSI and see if that helps. Terry

    Anonymous
    Inactive
    20/01/2011 at 10:22 am
    Post count: 72
    #14589

    Hi Folks Your not so bike Guru here. I believe the phenomenon of feeling shakes on smoother surfaces, is the less resistance the wheel has to turning. I have felt more delaminations of tyres on smooth stetches than the rougher chip. By this i mean the wheel can  "walk" left/right/left, for whatever the reason, delam, dynamic or couple inbalance or resistance like disc warp. The speed is related to the wheel mass. Its natural frequency speed. So I conclude resistance or couple unbalance or warpage is likely reason . Delamination occurs noticably at slow speed.

    Anonymous
    Inactive
    24/01/2011 at 11:00 pm
    Post count: 9
    #14590

    Thanks team,As Bwucie's thoughts were the easiest to implement, I tried adding a couple of PSI into both front and back tyres and have noticed an improvement. It hasn't gone away but is noticably better. So that is definitetly food for thought, and maybe a brand change next tyre change. I'll let you know.Thanks for all the advice. Definitely one of the advantages of belonging to the register.Cheers,Terry

    Anonymous
    Inactive
    27/01/2011 at 6:19 pm
    Post count: 1
    #14591

    Hi Terry,I had a similar problem with my R100RS, particularly with hands off bars. It was rectified by replacing the front tire which looked new but obviously had issues.Campbell.

    Richard Kuysten
    Participant
    27/01/2011 at 9:18 pm
    Post count: 623
    #14592

    Followed this with interest and finally thought i might add a few words from my own experience with the “head shimmy” phenomena.  Years ago in the 80s I bought my first new bike which was a Kawasaki GT 750.  Shaft drive with a small handle bar fairing.  At just over 6 grand it was a bargin and I loved it.  However sometimes (particularly on the bridge down hill side) I noticed a slight shimmy at the bars and worried about this checking everything I could to no avail.  Eventually someone suggested I take it to Vince Sharpe over town.  Now Vince is second to none when it comes to setting up bikes particularly suspension/tyres etc.  Vince gave it a really good going over and could find nothing wrong with it saying “Just ride it and forget about it, it'll go away” and sure enough it did.  Put nearly 50 k on it with two or three sets of Metzler Lazers on the front and nairy a problem was had.  Wise words from Vince eh.  Of course there's “shimmys and there's SHIMMYS”  and I'm not suggesting anyone doesn't take them seriously.Good luck guys.

    Dave Morris
    Participant
    28/01/2011 at 6:02 am
    Post count: 615
    #14593

    In 1995 I got given a new K1100TIC and it used to shake its head at about 185kmh and then be alright up to 225kmh. I just ignored it. 3TKC was the plate number

    Anonymous
    Guest
    28/01/2011 at 6:53 am
    Post count: 2134
    #14594

    Probably shook like that 'cos you was using a handheld cellphone like the lawman in the Herald this morning.  By 225 kph you had probably hung up and put the phone back inya pocket, so you could hold on wiv both hands.

    Richard Kuysten
    Participant
    28/01/2011 at 8:31 am
    Post count: 623
    #14595

    I don't suppose you're old enough to remember the 750 Yammy's that carried their own weight in extras then Steve? I recall Roger Freeth telling me about testing them at Puke for the traffic department to see if he could figure out why they didn't handle and kept throwing the pilots off.  Loaded up with radios, lights etc etc etc.  Roger said they had no bloody idea.

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