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DB3 Mantra Handling
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vort28



Joined: 22 Mar 2010
Posts: 2194
Location: Northwest , UK

PostPosted: Fri Aug 19, 2011 11:11 am    Post subject: Reply with quote

Robin.
Can't really comment on the Mantra, but I do have a slightly lightened DB4, which is basically the same as a mantra , slightly steeper head angle, 23 as apposed to 24, but of course a bit more wieght over the front end due to clip-ons to counter act this.
It does have to be said that although I would not call it flighty , you do have to allow it its own head. Trying to go down a dual carriageway / motorway in a straight line is just impossible, it just weaves all over. I have found the best thing is either just relax to allow it or only use A-roads. The A-road option is obviously the best.
I must also say the first thing I do with any bike is have a good ride , then reset all suspension back to delivered state and compare, then generally start as stated with static sag etc.
It does also have to be said I find the RSU43's are not very good and when I get around to it the DB4 will be getting a fully reworked (valves/springs etc) set of Tin coated showa's that are sat in the garage, waiting.
Also what are you comparing it with ? Bimota's are never going to feel as planted as a jap retro bike.
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robin



Joined: 15 Nov 2009
Posts: 55
Location: London

PostPosted: Sun Oct 16, 2011 4:06 am    Post subject: Stuff I've tried Reply with quote

Hi

I've had another go at the Mantra handling. So far I have

checked head bearings - fine

checked tyre pressures - Dunlop qualifiers 2.5 front 2.9 rear - good condition, wheels well balanced

raised front by putting forks down yokes to flat (from standard 30mm out)

raised rear by increasing shock length (adjustable)

played with static sag on rear (increased preload)

checked front forks - these are in great condition, look new, and have good damping, though no ride height adjustment.

None of these made a difference! I reversed the fork change so it's back to standard.

Then.. I refitted the standard Paoli spring on the Nitron shock. Nitron shock seems to have a stronger spring. I think this is coz they built it for my then weight (120kg). I now weigh 80kg, so figure I don't need the big spring. Sadly, if anything, made it worse.

So, next I will double-check the wheel bearings front and rear, and the swingarm pivot through the engine. I might try making the rear as high as possible via shock length and preload, see if that's better.

On the front, I could try lower bars, but when I try crouching over the front as if I had clipons it makes no difference. The small fairing I have is handlebar mounted, but the screen will if anything give a little downforce, so not sure if that's it. I have great big mirrors (KTM), as the bike is meant to be a usable town bike. I might try taking those off to see if that makes any odds.

Other than that I am stumped, and when I have time this winter I'll make a trek to a suspension guru I've heard of in Suffolk and let him have a play. I hope he'll see it as a challenge!

Meanwhile I slowly refine the bike. The only thing I can't eliminate so far is the carb flooding if the tap is left on when parked. What I need is an in-line vacuum tap, but I've never seen one for sale.

I've seen a Mantra special with one front disc - mine is quite over-braked for the weight, and quite capable of a stoppie, so perhaps I'll try that. It would reduce the unsprung/sprung weight ratio at the front. I notice the bike 'bobs' front to rear over some bumps, which I think is a feature of low weight.

Will update again if I make any progress.

Robin
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oily



Joined: 05 Apr 2008
Posts: 4788
Location: worcestershire

PostPosted: Sun Oct 16, 2011 9:19 am    Post subject: Re: Stuff I've tried Reply with quote

robin wrote:

I've seen a Mantra special with one front disc


would this be it?

http://i52.tinypic.com/2ilhoo5.jpg
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robin



Joined: 15 Nov 2009
Posts: 55
Location: London

PostPosted: Sun Oct 16, 2011 9:57 am    Post subject: Mantra Special Reply with quote

Hi

Yes, that's the one. I've seen it at the Ace Cafe a couple of times, and spoken to the guy who built it. He's an engineer, and so puts effort into every little bit. It's a beauty!
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oily



Joined: 05 Apr 2008
Posts: 4788
Location: worcestershire

PostPosted: Sun Oct 16, 2011 10:09 am    Post subject: Reply with quote

Well next time you speak to him, ask him why he thinks it's a doochati? Laughing

I agree, it's a nice tidy, well built bike. Almost makes me want a doochati engined bike Laughing
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GeeKay



Joined: 29 May 2009
Posts: 1767
Location: West Yorkshire

PostPosted: Sun Oct 16, 2011 4:45 pm    Post subject: Reply with quote

if the bike handling is skittish at the front, I think you went the wrong way with the front forks. Lifting then through the yokes puts more weight on the rear, which would make the problem worse.
You need more weight on the front tyre, so you should drop the forks further through the yokes - go back to standard (30mm?) then drop them another 5mm. If you have to go further than 10mm, you have problems.
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Pompey



Joined: 31 Jul 2008
Posts: 2311
Location: Marlborough

PostPosted: Sun Oct 16, 2011 8:01 pm    Post subject: Reply with quote

Geekay is right. It is what I suggested earlier in this post...
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Pomps
Bimota's Db2, Db5 ,Ducati's 851' 92, 888' 93, Honda blade' 93, Triumph speed triple' 07, kawasaki zxr 750 k1
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robin



Joined: 15 Nov 2009
Posts: 55
Location: London

PostPosted: Sun Oct 16, 2011 8:18 pm    Post subject: I'd do anything, but I can't do that! Reply with quote

Hi

Perhaps I should have mentioned that the handlebars prevent me dropping the forks any further than the standard 30mm through the yokes. With all due respect, since changing the front from 30mm through the yokes to 0mm (and back) made no difference, I don't think 10mm more would have done it.
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2bims



Joined: 03 Apr 2010
Posts: 7289

PostPosted: Mon Oct 17, 2011 12:56 pm    Post subject: Reply with quote

Sounds like you've tried most things one way or another. You say wheels well balanced, can't see any wheel balance weights on your shiney rims though, but assume they are there? Presumably you have new tyres and they are matched front and rear? What sizes have you gone for though, have you overdone the back width, is it 170 or 180, and what profile. Other than that have you checked your front disc calipers aren't binding on at any point and are the discs running true and not warped?. With the front wheel off the ground you should be able to free spin the front wheel, without the pads binding, and pull on the brakes and then respin, the calipers should release and the wheel spin freely again (Head stand needed or a piece of 4x2 under the deadstock) This way you can also check the front wheel for play on the bearings
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GeeKay



Joined: 29 May 2009
Posts: 1767
Location: West Yorkshire

PostPosted: Mon Oct 17, 2011 4:09 pm    Post subject: Re: I'd do anything, but I can't do that! Reply with quote

robin wrote:
Hi

Perhaps I should have mentioned that the handlebars prevent me dropping the forks any further than the standard 30mm through the yokes. With all due respect, since changing the front from 30mm through the yokes to 0mm (and back) made no difference, I don't think 10mm more would have done it.


with respect it made no difference because you went the wrong way!
you will not believe the difference dropping the forks 2 - 5mm makes - 3mm drop made my trackday RGV a front-end demon and 5mm cured a mates wayward trackday CBR600.
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Pompey



Joined: 31 Jul 2008
Posts: 2311
Location: Marlborough

PostPosted: Mon Oct 17, 2011 5:24 pm    Post subject: Reply with quote

ahhh, I see ... when we say drop the forks what we mean is push them back up through the yoke so they stick out more at the top, not the other way. It has 3 effects, it increase the angle by pulling the forks back underneath the bike. This makes the wheelbase shorter, and it pushes the centre of balance further forward and down. All of which puts more weight through the front which increases contact and grip through the tyre. To much of it will bottom out your suspension which is equally bad because it can wash out the front tyre under load as you enter the apex prior to powering out. BUT, it is the quickest way to stop the front from being flighty and weaving... Where is the bike?
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Pomps
Bimota's Db2, Db5 ,Ducati's 851' 92, 888' 93, Honda blade' 93, Triumph speed triple' 07, kawasaki zxr 750 k1
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GeeKay



Joined: 29 May 2009
Posts: 1767
Location: West Yorkshire

PostPosted: Mon Oct 17, 2011 6:48 pm    Post subject: Reply with quote

apologies - inappropriate use of our language. Guess it should be "drop the yokes down the forks" but thats too simple for us Yorkshire folk. Very Happy
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deebee4



Joined: 08 Jan 2011
Posts: 266
Location: Netherlands

PostPosted: Mon Oct 17, 2011 6:59 pm    Post subject: Re: Stuff I've tried Reply with quote

robin wrote:
Hi

I've had another go at the Mantra handling. So far I have

checked head bearings - fine

checked tyre pressures - Dunlop qualifiers 2.5 front 2.9 rear - good condition, wheels well balanced

raised front by putting forks down yokes to flat (from standard 30mm out)

raised rear by increasing shock length (adjustable)

played with static sag on rear (increased preload)

checked front forks - these are in great condition, look new, and have good damping, though no ride height adjustment.

None of these made a difference! I reversed the fork change so it's back to standard.

Then.. I refitted the standard Paoli spring on the Nitron shock. Nitron shock seems to have a stronger spring. I think this is coz they built it for my then weight (120kg). I now weigh 80kg, so figure I don't need the big spring. Sadly, if anything, made it worse.

So, next I will double-check the wheel bearings front and rear, and the swingarm pivot through the engine. I might try making the rear as high as possible via shock length and preload, see if that's better.

On the front, I could try lower bars, but when I try crouching over the front as if I had clipons it makes no difference. The small fairing I have is handlebar mounted, but the screen will if anything give a little downforce, so not sure if that's it. I have great big mirrors (KTM), as the bike is meant to be a usable town bike. I might try taking those off to see if that makes any odds.

Other than that I am stumped, and when I have time this winter I'll make a trek to a suspension guru I've heard of in Suffolk and let him have a play. I hope he'll see it as a challenge!

Meanwhile I slowly refine the bike. The only thing I can't eliminate so far is the carb flooding if the tap is left on when parked. What I need is an in-line vacuum tap, but I've never seen one for sale.

I've seen a Mantra special with one front disc - mine is quite over-braked for the weight, and quite capable of a stoppie, so perhaps I'll try that. It would reduce the unsprung/sprung weight ratio at the front. I notice the bike 'bobs' front to rear over some bumps, which I think is a feature of low weight.

Will update again if I make any progress.

Robin


Hi Robin,

Sure seems as the more you change the more you get off the way to better handling. Sad

So better try to make it as much as possible stock Mantra in shock lenght, pre load, fork position in yokes etc. If you have pre-load adjusters in the front fork screw them in full and back off 10 mm. same goes with damping regulating screws. Turn in full and then back off 10 clicks. This is now your base setting. Now you can make small alterations at the time and when they don't work go back to the base setting, because that wasn't the solution (nor the cause of the problem;-).

At first I would suggest to take off that fork mounted fairing and ride with your base settings. On a Mantra, db4, bb1 etc nothing is mounted to the front fork except bars with switches (and on a Mantra the mirrors). If this doesn't make a difference you can put that on your list of no problem causes.

Then see what happens with the instability if you move your body all the way to the front (push down on the triple clamps) and to the rear. If it gets better when you move to the front there's too little weight in the front end. On a Mantra there's A LOT of weight in the front (fuel tank/headlamp assy, 2 batteries). If it gets worse when you move backwards it definately is the weight distribution that has been disturbed by your modifications. But that can be solved with lower bars and maybe another frame-(headstock) mounted headlamp enclosure.

I remember that front fork/handlebar mounted fairings caused instability at higher speeds in the 70's & 80's on normally good handling bikes, like Moto Guzzi's. That was my experience too.

You wrote that a lighter spring in the rear made it worse. That could indicate a weight distribution problem.

I find the tire pressures quite high for a 80kg rider.

Did you check the headstock bearings just for play or also for a tight spot in the center (straight) position? The fork should move freely and fall from straight to the side without any play. Overtightened headstock bearings can create erratic handling too, you know?

Worn wheel bearings and swing arm pivot bearings would give a funny feeling at lower speeds and in turns too, so I don't think that's it.
Warped disks? Don't think so, that would be noticable when you apply the front brake and not riding straight out. Except a massive warp what would be noticable from 50 km/h up.

"....I notice the bike 'bobs' front to rear over some bumps...." Please clarify what you mean with that. Question

First test for suspension setup is that when you push the bike down on the fuel tank both sides should compress at the same time at the same amount. I suspect yours will stay stiff at the front end... (OMG what am I writing....Wink Embarassed

I have a track modified db4 with tuned engine and no headlight assy and nothing in the front except the front fairing stay. I can ride at speeds around 230 km/h+ with no steering damper and no signs of instability. Only difference I noticed when lightening the front end was shaking in the front and more wheelies when getting out of sharp turns. Exiting! Razz Razz

Keep me posted!

Cheers!

Riny
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deebee4



Joined: 08 Jan 2011
Posts: 266
Location: Netherlands

PostPosted: Mon Oct 17, 2011 7:31 pm    Post subject: Reply with quote

vort28 wrote:
Robin.
Can't really comment on the Mantra, but I do have a slightly lightened DB4, which is basically the same as a mantra , slightly steeper head angle, 23 as apposed to 24, but of course a bit more wieght over the front end due to clip-ons to counter act this.
It does have to be said that although I would not call it flighty , you do have to allow it its own head. Trying to go down a dual carriageway / motorway in a straight line is just impossible, it just weaves all over. I have found the best thing is either just relax to allow it or only use A-roads. The A-road option is obviously the best.
I must also say the first thing I do with any bike is have a good ride , then reset all suspension back to delivered state and compare, then generally start as stated with static sag etc.
It does also have to be said I find the RSU43's are not very good and when I get around to it the DB4 will be getting a fully reworked (valves/springs etc) set of Tin coated showa's that are sat in the garage, waiting.
Also what are you comparing it with ? Bimota's are never going to feel as planted as a jap retro bike.


Some of your statements I have to disagree. The RSU43 are great froks provided they are well setup, well maintained and serviced frequently. The internals are aluminium in contrast to most of the other forks with steel internals. That calls for frequent oil changes. Unsprung weight is less than on USD forks too.

When your db4 weaves all over the place at high speeds there's more than just the quality of the forks to blame. Maybe they just need to be serviced and setup well.
The db4 was one of (if not THE best) handling bikes when they came on the market. Never have read reports about instability other than a "nervous" or "lightfeeted" handling due to the short wheelbase and track orientated geometry.

My track db4 doesn't have any stability issues all the way up to top speed, is ridden hard, has no steering damper and sees off a lot of faster bikes in the turns. Whatever happens (crash or gravel or highside) I cannot blame the bike. It's perfect for track riding (just give her some extra HP's and you'll be fine) Wink

If you want a super stable planted bike in long motorway sweeps, get a 250 kg (wet) Jap retro bike or a Doochati and sacrifice a lot of fun when the turns are heading your way...

Cheers!
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robin



Joined: 15 Nov 2009
Posts: 55
Location: London

PostPosted: Tue Oct 18, 2011 7:31 am    Post subject: More info Reply with quote

Hi

Some of you have asked questions that are answered in my original post - worth having a look. The tyres are 120/70 and 180/55 Dunlop qualifiers (original type). They are balanced and at the recommended pressure of 2.5 front 2.9 rear. These are the recommended sizes, yes?

Wheels and discs are true and don't bind.

I've checked head bearings, they are fine.

I will try taking the fairing and mirrors off to eliminate these.

I have been given a handling manual that has some interesting points on sprung vs. unsprung weight. I've made the bike lighter, but the wheels and brakes are standard, meaning there's a lot of unsprung weight. I will try a single disc arrangement, as that's the only cheap way I can reduce the weight of the front wheel.

As I can't drop the front any more (forks have no preload, and no space to extend further through the yokes) I will also try raising the rear as much as possible. The shock has ride height adjustment, and I will increase the preload. Not sure if it will help, but I'll try it.

All the experience of other bikes is fascinating, but I really want to know how a Mantra handles. Does anyone actually have one?

ps the bike is in London. I get to the Ace regularly, if anyone wants to have a look.

Robin
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