|
Bimota Forum Forum for Bimota Owners and Riders
|
View previous topic :: View next topic |
Author |
Message |
robin
Joined: 15 Nov 2009 Posts: 55 Location: London
|
Posted: Tue Jul 12, 2011 7:26 am Post subject: DB3 Mantra Handling |
|
|
Hi
I've got my heavily-modified Mantra up and running - pics to follow. It has most of the bodywork removed, upright handlebars, lowered foot pegs, a light alloy tank, and is generally much lighter than standard. Currently running Ducati Sport Classic silencers, though these will change. Apart from a new shock, it still has a standard chassis.
At anything above 50mph, and getting more serious with speed, it is quite unstable at the front. Going over a bump gives a wobble, and if I start a wobble at the bars, it feels very unstable. Has anyone else experienced this? I still have the forks up through the yokes about one inch, but this is the same as the standard bike, for the clip-on bars.
I've checked tyre pressure, wheel bearings seem fine, I can't see any steering head bearing movement, and nothing seems loose. Any ideas?
I'm pleased with the power and gearing, which make it plenty fast enough for a naked, and the round-town manoeuvrability is great. Brakes are good too, with a radial master cylinder at the front. |
|
Back to top |
|
|
Pompey
Joined: 31 Jul 2008 Posts: 2311 Location: Marlborough
|
Posted: Tue Jul 12, 2011 9:47 am Post subject: |
|
|
yep, you have raised the bars and lowered pegs pushing more of your body weight backwards. The light front end feel is caused by a light front end, made worse at speed and under acceleration by loading the rear shock down and the wind pressure on your upright body pushing even more leverage down at the the back. Solution should be simple enough, you need to tip the balance forward again by a number of options : Raise rear ride height, drop front forks through the yoke further, increase rear compression damping. These are quick fixes to work on but I would consider better long term solutions such as making sure you have a strong enough rear spring for you body weight and matching it to the front springs and oil in forks. make sure your sag both front and rear is correct then start to work on the damping settings. It will be worth the effort, good luck _________________ Pomps
Bimota's Db2, Db5 ,Ducati's 851' 92, 888' 93, Honda blade' 93, Triumph speed triple' 07, kawasaki zxr 750 k1 |
|
Back to top |
|
|
Pompey
Joined: 31 Jul 2008 Posts: 2311 Location: Marlborough
|
Posted: Tue Jul 12, 2011 9:49 am Post subject: |
|
|
yep, you have raised the bars and lowered pegs pushing more of your body weight backwards. The light front end feel is caused by a light front end, made worse at speed and under acceleration by loading the rear shock down and the wind pressure on your upright body pushing even more leverage down at the the back. Solution should be simple enough, you need to tip the balance forward again by a number of options : Raise rear ride height, drop front forks through the yoke further, increase rear compression damping. These are quick fixes to work on but I would consider better long term solutions such as making sure you have a strong enough rear spring for you body weight and matching it to the front springs and oil in forks. make sure you sag both front and rear is correct then start to work on the damping settings. It will be worth the effort, good luck _________________ Pomps
Bimota's Db2, Db5 ,Ducati's 851' 92, 888' 93, Honda blade' 93, Triumph speed triple' 07, kawasaki zxr 750 k1 |
|
Back to top |
|
|
Evilchicken0
Joined: 12 May 2010 Posts: 2996 Location: London
|
Posted: Tue Jul 12, 2011 11:56 am Post subject: |
|
|
Rather than trying to raise the rear or drop the front straight away reset the static sag for what you have. Chances are it won't be a silver bullet for the problem but you need to know what you have is working right before you change it.
After this raise the back etc.
It might be better to get the bike to a suspension specialist, they can probably diagnose the problems quite quickly. RaceLab in Bournemouth set my SB6 up but I'm sure there are others. _________________ Don't read everything you believe |
|
Back to top |
|
|
2bims
Joined: 03 Apr 2010 Posts: 7292
|
Posted: Tue Jul 12, 2011 9:30 pm Post subject: |
|
|
You say you have replaced the rear shock, presumably same length as previous paoli unit, but does it have a dual rate spring? As the linkage is direct off the swingarm without any cantilever brackets the dual rate spring is needed, i.e. a spring with different spaced coiling wide and narrow. All the Db's 2,3, and 4 have a dual rate spring to counter the direct linkage off the swingarm. Without the dual rate you'll get more squish and the back will sit lower, as the others say making the front higher and lighter.
Presumably you have refurbed the front forks at some stage as presumably the forks are over 10 years old, new oil will make a big difference, with new seals. Both my Bims when bought had been sat for 10 years and had no action in the forks due to leaking seals and no oil and thus were sitting very low on the front. Front sag on the forks, i.e. when you sit on the bike with feet off the ground should be about 20-25mm. Attach a cable tie to one stanchion and ush doen to the bottom, and then sit on the bike nice and level, get off the bike and see how far the cable tie has moved up the fork stanchion. if more than 20-25mm then tighten down the pre-load adjusters if you have any until the level is right. Try to set up the rear the same using a yard stick at the bike of the bike and a friend |
|
Back to top |
|
|
robin
Joined: 15 Nov 2009 Posts: 55 Location: London
|
Posted: Tue Jul 12, 2011 10:24 pm Post subject: db3 measurements |
|
|
Hi
Thanks for comments so far. Does anyone have a standard DB3 they can give me some measurements from? I guess I need
Front - distance forks through yokes
- static sag
- rider sag (tell me your weight)
Rear - height at the highest point of the rear seat
- static sag
- rider sag (tell me your weight)
Also front and rear weight with fuel
As for dual-rate spring, I have the original Paoli unit - it didn't look like a dual-rate to me, but I'll check in the morning.
thanks
Robin |
|
Back to top |
|
|
2bims
Joined: 03 Apr 2010 Posts: 7292
|
Posted: Wed Jul 13, 2011 12:05 am Post subject: |
|
|
I'm sure GeeKay can help you with measurements on this forum if you pm him. He has just finished over 6000miles around Italy on his Mantra so his suspension seems sorted. On his breakdown pictures of his Mantra under Spring Time header on Bimota chat it looks like a dual rate spring on the back of his bike, a Paoli unit. Hes a helpful guy with measurements. Only other thing to check is whether your front discs are running true and not warped and that the caliper pistons aren't binding.
If you hve a front stand prop the bike up off the front wheel and spin the wheel to check the discs are running true. Apply the brakes and then re spin the wheel, it should run clean without binding, with the calipers releasing |
|
Back to top |
|
|
robin
Joined: 15 Nov 2009 Posts: 55 Location: London
|
Posted: Thu Jul 14, 2011 10:11 pm Post subject: Mantra in latest form |
|
|
Hi
Here it is with Sport Classic silencers... |
|
Back to top |
|
|
GeeKay
Joined: 29 May 2009 Posts: 1767 Location: West Yorkshire
|
Posted: Mon Jul 18, 2011 5:22 pm Post subject: |
|
|
Wanders off into garage armed with trusty tape-measure again.....
forks through top yoke by approx 3cms - difficult to measure with those wanky "clip-ons" fitted.
front static sag- how do you measure this?
front rider sag - have to admit I do have a bit of a belly.........
2.5cms / 85kgs
rear static sag - same as above
rear rider sag - arse cheeks have dropped due to age..........
couldn't get a zip-tie on the damper rod - sorry.
But I still weigh 85kgs.
front & rear weight - would have to wait until MOT time as they weigh them for the brake test. But if I'm passing I'll nip in and weight it, though I won't know how much fuel it has onboard.
Rear shock spring on my Paoli unit isn't dual rate - but as said if you have fitted a replacement, make sure it is for a direct-operating suspension system as the spring weights are massively different (direct-operating systems have a much stronger spring rate than a rising-rate system). If it had a soft spring, the rear would squat under acceleration, making the front go light.
I have new 10 weight oil in my forks and I twiddled the pre-load adjusters and turned the adjusters at the bottom of the fork legs towards "hard". Though probably not by the same amounts.............
My Mantra is great solo, but with a pillion the front goes very light.
I think your problem is those bloody great silencers - if you are leaving them on, a sack of coal on the front mudguard should help counteract the problem - I assume you fitted them as a joke? |
|
Back to top |
|
|
2bims
Joined: 03 Apr 2010 Posts: 7292
|
Posted: Tue Jul 19, 2011 10:01 am Post subject: |
|
|
Knew you would help out where you could GeeKay.
Front static sag is when on a headstand. Measure from bottom yoke to top of stanchion, then sit the bike on its own weight. |
|
Back to top |
|
|
GeeKay
Joined: 29 May 2009 Posts: 1767 Location: West Yorkshire
|
Posted: Tue Jul 19, 2011 11:05 am Post subject: |
|
|
cheers Steve!
although I believe you meant to say "measure from the bottom yoke to the top of the slider"?
don't have a front stand so can't measure the static sag either.
How do you adjust static sag - wind the pre-load up or down to suit?
Where do you get static sag values from - surely they differ bike-to-bike?
And if you set static sag to an agreed value, then measure rider sag and adjust that, doesn't it screw up the static sag setting?
So many questions - must go lie down for a while.............. |
|
Back to top |
|
|
2bims
Joined: 03 Apr 2010 Posts: 7292
|
Posted: Wed Jul 20, 2011 11:22 am Post subject: |
|
|
Yeh Gary, to top of slider. To help Robin, the height when the bike is on its sidestand would do, or when sitting under its own weight upright, from underside or top of stanchion to top of slider. If the balance of his bike is off to the rear then this gap would be longer on the front than yours, which would be adjustable by lowering the compression. Not so sure with your bike if it has preload adjusters on the top of the forks, they were available as an option, so maybe fitted? The hex nuts on the top . Mine shouldn't have had them but seeing as the forks were changed for 51mm Paolis it does have them. And each leg has a compression and rebound adjuster also, whereas the standard had one adjuster for compression on one leg and the other leg was rebound, strange but thats the way they were
The red ally billet hex nuts are the preload adjusters for static sag when the rider is on the bike, and mine has the seperate compression and rebound clicks on top and bottom of each unit.
I've only had a front end wobble the once but it was on a long uphill straight double carriageway when overtaking an HGV at the crest when cranked over to one side so I can excuse it for that |
|
Back to top |
|
|
Pompey
Joined: 31 Jul 2008 Posts: 2311 Location: Marlborough
|
Posted: Mon Jul 25, 2011 8:03 pm Post subject: |
|
|
any news on the handling front? _________________ Pomps
Bimota's Db2, Db5 ,Ducati's 851' 92, 888' 93, Honda blade' 93, Triumph speed triple' 07, kawasaki zxr 750 k1 |
|
Back to top |
|
|
robin
Joined: 15 Nov 2009 Posts: 55 Location: London
|
Posted: Mon Jul 25, 2011 10:59 pm Post subject: Progress |
|
|
Hi
I've been a bit busy. The best advice I've had was 'get the bike to a suspension specialist'. I haven't raised the bars - the Mantra comes with raised bars anyhow.
I spoke to a guy who's made a very different, but lovely, special out of a Mantra - clipons not raised bars, even lighter than mine, single front disc, with the standard shock, and he mentioned that the steering was prone to wobble. When I get it sorted I'll post what it was. BTW, the silencers currently on the bike weigh the same as the stock Mantra silencers. |
|
Back to top |
|
|
Corona
Joined: 26 Jun 2008 Posts: 118 Location: Norfolk
|
Posted: Wed Aug 17, 2011 5:46 pm Post subject: |
|
|
Hi Rob, I did have similar issues on my Magnum 2 but at higher speeds. Turned out to be the front fairing which was mounted to the forks and not the frame and having higher bars which put wind pressure on my shoulders so the tighter I gripped the more it weaved Try leaning forward a bit and make sure your arms and shoulders are relaxed and see if it helps. Suspension setup guru's are also well worth the effort. |
|
Back to top |
|
|
|
|
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
The Bimota Forum has no official connection to Bimota S.p.A.. We just ride or are interested in their motorcycles and support the brand. All trademarks are acknowledged
|