1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Steering geometry - confirmation bias

Discussion in 'Norton Commando Motorcycles (Classic)' started by acotrel, Dec 26, 2017.

  1. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    I'm posting this topic because I need help. Over a period of time, I have raved on about steering geometry and it's effect on handling. In the old days, I always had bikes which had a taper on the top of the fork staunchions, so there was no way the yokes could be loosened and adjusted to change the rake and trail of the bike's steering - as can be done with modern bikes
    I have come to the conclusion, that reducing the trail at any given rake, makes the bike more stable with the tendency to run wide in corners. And increasing it increases the tendency for the steering to become quicker and more self steering when the bike is gassed coming out of corners and the rear end squats.
    There are several guys on this forum who have obviously had experience with this and I'd really like to know what they think. - Fast Eddie, Dances and SteveA all seem to know what they are talking about. If they can set me straight, I would be very grateful.
     
    Tags:
  2. Brooking 850

    Brooking 850 VIP MEMBER

    Joined:
    Oct 3, 2011
    Check for the set up by Doug MacRae on his Commando frame, same could be used to set up a Seeley frame.
    There are pics in his thread
    Regards Mike
     
  3. o0norton0o

    o0norton0o

    Joined:
    Apr 27, 2015
    Yes, you raved in my thread about how changing to an 110/90/18 rear wheel could,.. as you put it scientific terms, "throw me on my head as it had done to one of your mates." To which, I replied that lowering the rear end of my bike would increase both the rake and trail which makes the bike slower turning, but more stable in a straight line....

    ...... which is exactly opposite to the conclusion you have come to, so I think you have it backwards. So, INCREASING trail makes a bike more stable, but slower to turn (and therefore more apt to make you run wide) DECREASING trail makes the steering faster, but less stable... (decrease it too much and you get the shopping cart effect)

    Think of how much trail a chopper has... quick steering or more likely to run wide?? You're welcome
     
  4. lcrken

    lcrken VIP MEMBER

    Joined:
    Mar 15, 2009
    Yes, you do have it backwards. For a given rake, increasing the trail provides more self centering force on the wheel, and makes the bike more stable at speed. It also increases the force required on the bars when counter steering to start the turn, making it feel slower and heavier to turn in. Too much of that and it will want to run wide in turns unless you keep up the counter steering force, and that's not a comfortable feeling. Decreasing the trail has the opposite effect, making it easier and quicker to turn in, but less stable at speed. You might think that the answer is to have even less trail, to make the bike really steer like a GP bike, but there's a pitfall there. As you lean the bike and get on the side of the tire, you have a smaller effective tire diameter, and consequently less trail. If you have too little trail, it is possible to end up with zero or negative trail when seriously leaned over, as many modern bikes are capable of. Once you get into negative trail, the bike is inherently unstable, and a high side crash is almost guaranteed. On race bikes this is a real issue. On street bikes, not so much, unless you make really major changes to steering geometry.

    For anyone who wants a good understanding of all this, I really recommend Tony Foale's book, "Motorcycle Handling and Chassis Design". There are a few other books with decent explanations of the physics of motorcycle handling, but Tony's is my favorite.

    Ken
     
  5. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Does increasing the yoke offset increase the trail or does it reduce the trail if the rake is 27 degrees ? When the rear end of the bike squats, the rake increases. The JPN Monocoque, ordinary Commandos and all Seeleys have 27 degree rake on the steering head. The replicaJPN Monocoque seems to be more stable than self-steering, according to an article I read yesterday. Tony Foale says those two situations are the two extremes of bike handling when you change the yoke offset. I know that one of our local guys has a Minnovation Seeley which is difficult to get around a tight circuit.
    My problem is that I have arrived at a solution by sheer luck and if I change anything the wrong way, I might crash. The variation of the rake between accelerating and braking is virtually nothing, but it has a big effect on whether the bike becomes stable or self steering. Have you ever ridden a bike around a corner with the front brake dragging ? It can steer you off the road.
     
    Last edited: Dec 26, 2017
  6. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    I was wondering about the negative trail situation. That is what I might have encountered before I changed the fork yokes, when my bike stood up and turned the wrong way during braking. It threw me completely off-balance and I only got out of it by accelerating to try and get a gentle crash. Then I decided to reduce the yoke offset and the bike improved out of sight. Trouble is, I didn't measure the trail before making the change. After the change, from memory the trail is now 92mm. Do you have any numbers for the trail, to say what might be best ? Do you think I should contact Minnovation and ask them what trail they use ? At least I would then know what direction to proceed.
     
    Last edited: Dec 26, 2017
  7. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    Ken. Since I made my last post, I have been away and come back - had time to think. If what you are saying is that increasing the yoke offset increases the trail and makes the bike more stable, it actually makes sense in the situation I had with the Seeley. Originally a friend of mine had the frame with a Laverda SFC motor fitted. The motor and gearbox were much heavier than Commando bits. He still had trouble keeping it on line. When I got it, I thought it would be OK. It had Ducati 450 Ceriani forks fitted to it which had a lot of yoke offset. It might have been the situation where the bike became TOO STABLE and when I braked hard, the rake changed and made the situation worse. Whenever you try to brake any bike into a corner, it tends to run wide rather than stay neutral. I think the Seeley Commando was just doing that in the extreme and very quickly. With the yokes I now have fitted, the trail is probably much reduced and the handling has moved in the other direction - self-steering on the throttle. It now tightens it's line in corners.
     
  8. ludwig

    ludwig

    Joined:
    Aug 30, 2006
    Amazing read ..
     
  9. SteveA

    SteveA VIP MEMBER

    Joined:
    Dec 20, 2011
    My last race meeting was at Brands Hatch, which some think of as a tight circuit. I was in the garage next to Minnovation who ran several Seeleys, Roger Titchmarsh framed. All with pretty much the same suspension components, even if set up differently for rider choice. There were race wins and a broken bone from the races that were run. I ran close to a couple of there bikes in early laps, I didn't notice anyone in difficulty due to anything other than poor track conditions! The effects of which stopped racing Saturday, and then Sunday. Some rode Avons, with a 120 front or a 90 and others ran Contis. His issue may be as simple as tyre choice/pressures.

    These are some of the best prepared bikes you will see anywhere, built from the best components, and even if one or two of the Minnovation team have a reputation of being a little abrasive (they are Yorshiremen after all)!, there is little doubt they are knowledgeable. Assuming the suspension is as delivered and suitable for his weight, and the tyre combo is good and to his liking, it shouldn't be a million miles away, tight or open circuit. Certainly major issues like basic geometry should not be the source of the problem unless he has made radical changes.

    In the UK Minnovation prepared bikes are regular race winners with a number of riders of varying ages and experience. Your local contact needs to talk more to Minnovation and perhaps others who ride bikes based on their products, which includes Kenny Cummings.

    If he is still having difficulty after going back to a known base setup and nothing is broken, it is most likely based on a conflict of set up and riding style, and is soluble by changing one or both. First action drill may be relax and stop fighting it, find out what it does when you let it do it's own thing!

    At a guess, he rode it, it didn't suite how he rode, he changed it and is now lost! I think several of us have been there over the years.
     
  10. SteveA

    SteveA VIP MEMBER

    Joined:
    Dec 20, 2011

    Yup, it is amazing we are here again, but oh look, I may have given it oxygen!
     
  11. Danno

    Danno

    Joined:
    Feb 7, 2010
    Braking coming in to a turn compresses the front and allows the rear to extend unless some trail braking is used. The steeper head angle quickens the steering and reduces straight-line stability, Coming out, acceleration causes the front to extend and the rear to squat, increasing stability, slowing steering and makiing the machine run wide. MotoGP riders speak about "balance" when referring to keeping the machine level, the suspension in the middle of it's travel and attempting to keep braking and acceleration from altering geometry as much as possible. So much easier with electronic suspension sensors and controls.
     
  12. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
  13. o0norton0o

    o0norton0o

    Joined:
    Apr 27, 2015
    NO, just the opposite REDUCING the offset increases the trail. (For all rake angles, not just 27 degrees)

    I have the early commando frame with the 27 degree rake and yokes with LESS offset which increases the trail of my model. Later frames went to 28 degree rake with GREATER offset in their yokes so both bikes have about the same amount of trail... but with different offsets and rakes.
     
  14. ludwig

    ludwig

    Joined:
    Aug 30, 2006
    Not necessarely so .
    Rear suspension can be set up for antisquat , giving better traction .
    When I made an alloy engine craddle for my Commando , I took the opportunity to move the swing arm spindle up 5 mm to reduce squat under acceleration :
    More grip , less chance for spinning in the rain .. ( light bike , always solo ..)
     
    Last edited: Dec 27, 2017
  15. Holmeslice

    Holmeslice VIP MEMBER

    Joined:
    Sep 27, 2009
    Indeed.
     
  16. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    I am under the impression that neutral steering gives stability when braking and a tighter line when accelerating out of corners (slight self-steering ) . I once rode a very original 1961 30M Manx and that is how that handled. My own bike is a lot beyond that and inspires a lot of confidence. It tightens it's line a lot in corners and tends to stay more vertical. However you need to anticipate where it will go, well before you enter a corner. I have never ridden another bike with which I could be so heavy-handed.
     
  17. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    The guy who rides the slow steering Minnovation Seeley is a very competent rider. He was an A-grader when I was racing in the 70s. He usually races his own G50. The Seeley is owned by somebody else. I think that with my Seeley, I have found the two extremes of yoke offset. Originally Mk3 Seeleys were fitted with Metal Profiles forks which must have had yoke offsets somewhere in between the two extremes. I think that is where the Minnovation Seeleys also must be. If they tightened their line in corners as much as mine does, many guys would be crashing them. You need to be sensitive to what is happening when you ride it. I like it, but I think it might be dangerous.
     
  18. Brooking 850

    Brooking 850 VIP MEMBER

    Joined:
    Oct 3, 2011
    Alan, sounds like you may need to put a really competent rider/racer on it to give you good feed back.
    Other wise you may end up spinning your wheels but getting or going nowhere so to speak!
    Regards Mike
     
  19. Danno

    Danno

    Joined:
    Feb 7, 2010
    Mitigation of the forces that seek to unstabilize the machine can sometimes result in other unkoown consequences. This is how racing has improved the breed since Hildebrand met Wolfmuller. I should have said "tends" to make the front end extend. The part about balance is the mitigation.
     
  20. acotrel

    acotrel

    Joined:
    Jun 30, 2012
    The Ducati yoke offset I was using when the bike stood up and turned the wrong way was 66mm. The TZ Yamaha yokes I now use have 40mm of offset. I suggest any offset greater than 55mm would make the bike too stable.
     
Loading...