smart51
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| posted on 23/6/14 at 03:51 PM |
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quote:
Originally posted by Sam_68drifting off-topic would probably be merciful for this thread and the sanity of all involved...
It didn't seem this hard when I started off. And yet to attack further your sanity...
The car in question has 3 wheels - a single rear wheel. It has a single trailing arm holding the rear wheel. At speeds up to 50 MPH it handles just
fine. However at higher speeds it is quite twitchy. I'm trying to pin down what is causing it.
If you turn in to a fast corner, it turns in pretty quickly. However as the car rolls you can feel it turn ever more quickly, often needing you to
turn out somewhat for the car to maintiain the desired line. It does feel like the back of the car is steering. Once settled, it grips well but it
turns out of this too quickly as well.
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Sam_68
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| posted on 23/6/14 at 04:03 PM |
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quote:
Originally posted by britishtrident
Sport and Coupe defo had shorter springs than the saloon, Husky and Van were longer. Interestingly all the production springs sets were the same rate
and only differed in length.
Van spring sets were exactly the same as Chrysler Comps RAC springs.
I disagree; part numbers for the later Saloon and Sport/Coupe front springs are identical according to my lists (part no. 7102493).
Obviously, if you'd taken the trouble to re-jig the suspension to keep the headlights at legal height, you're not going to fit shorter
springs on the Coupe/Sport only to make them illegal again...
According to my information, Husky and Vans were shorter (9.94" free length vs. 10.35" for the saloons and coupes) but higher rate
(230lb vs. 195lb for the saloons/coupes).
Again think about it: if you fit stiffer springs to the van/Husky, your static deflection will be less, so if you made them longer (rather than
shorter) as well, you'd end up with a much increased ride height.
The RAC springs were slightly different again from the van springs; 9.8" free length and 250lb, though I dare say that some aftermarket
vendors may have tried to pass the van springs off as RAC springs, since the genuine competition parts (which carried Rootes Competition Department
'CTS'-prefixed part numbers) were harder to get hold of, higher quality and carried a cost premium.
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Sam_68
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| posted on 23/6/14 at 04:22 PM |
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quote:
Originally posted by smart51The car in question has 3 wheels - a single rear wheel. It has a single trailing arm holding the rear
wheel. At speeds up to 50 MPH it handles just fine. However at higher speeds it is quite twitchy. I'm trying to pin down what is causing
it.
If you turn in to a fast corner, it turns in pretty quickly. However as the car rolls you can feel it turn ever more quickly, often needing you to
turn out somewhat for the car to maintiain the desired line. It does feel like the back of the car is steering. Once settled, it grips well but it
turns out of this too quickly as well.
Could be lots of things. Certainly the lack of camber recovery at the rear vs. the front will progressively degrade grip, but other suspects (far from
an exhaustive list, and not in any order) might include:
- The fact that 3-wheelers tend to be tail happy 'cos they've got (roughly) twice the contact patch at the front than they do at the
back
- Damping
- The fact that the single rear wheel wont be following the 'swept path' cleared by other vehicles.
- The trailing arm could be flexing/deflecting under load.
- Relative front and rear spring rates (it's a bit of a myth that all the roll resistance is at the front on a reverse trike: there will be
some diagonal weight transfer causing squat at the rear, which is resisted by the rear springs)
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smart51
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| posted on 23/6/14 at 04:55 PM |
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quote:
Originally posted by Sam_68
quote:
Originally posted by smart51
- The fact that 3-wheelers tend to be tail happy 'cos they've got (roughly) twice the contact patch at the front than they do at the
back
- Damping
- The fact that the single rear wheel wont be following the 'swept path' cleared by other vehicles.
- The trailing arm could be flexing/deflecting under load.
- Relative front and rear spring rates (it's a bit of a myth that all the roll resistance is at the front on a reverse trike: there will be
some diagonal weight transfer causing squat at the rear, which is resisted by the rear springs)
Thanks Sam. I've done a lot of work stiffening up the rear A frame (trailing arm) and its mounting points and it has made some improvement.
Short of using girders, I'm not sure there's much left to gain there. I'm beginning to think about the chassis' torsinal
stiffness though.
When you say damping, are you thinking front, rear or the combination? The ride is firm but not objectionably so. The front dampers can be adjsuted,
as can the spring preload at both ends. The scooter's original springs and dampers are used because the weight on the rear is about the same as
on the donor vehicle.
What's the story with relative spring rates? What would make it better?
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Sam_68
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| posted on 23/6/14 at 05:45 PM |
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quote:
Originally posted by smart51When you say damping, are you thinking front, rear or the combination?
Combination.
It's about relative stiffness front:rear, in the main (although if the damping is radically out of kilter at one end of the car, it will
cause problems regardless).
Apart from its obvious function of damping spring oscillations over bumps (which is dealt with mainly by the high velocity range of the damper
movement), the principal function of dampers in terms of chassis tuning is to temporarily prop up one corner of the car as it enters or exits a
corner, to adjust transient response (by means of low velocity damping characteristics).
Good turn in but corner exit oversteer tends to suggest damping may be too stiff at the front, relative to the rear.
quote:
Originally posted by smart51What's the story with relative spring rates? What would make it better?
Again, it's about relative spring rates front:rear, and where the function of dampers is to manage transient response, that of the
springs (apart from basic ride compliance) is to manage steady-state roll resistance and diagonal weight transfer.
If your rear is lots softer than the front, on a trike it will tend to promote progressive squat as the chassis loads up.
The other thing to be aware of is perception: the human inner ear is remarkably sensitive to changes in attitude, so you have to be careful to be sure
that what you're getting is really progressive oversteer, rather than (or as well as) diagonal pitching motion.
At risk of drifting off-topic, again, a lot of the b*ll*cks spouted by the semi-educated enthusiast on PistonHeads and the like - about how RWD cars
feel 'naturally' different because they consistently oversteer, whereas FWD cars consistently understeer - is nonsense. 99% of the time in
road use RWD cars are understeering too. The difference is that RWD cars are set up to diagonally pitch onto their rear outside corner and will pick
up a front wheel in extremis (so that the driven wheels remain loaded...), whereas FWD cars are set up to diagonally pitch onto their outside
front corner and will pick up a rear wheel in extremis (again, so that the driven wheels remain loaded.
Certainly, there will be a transition to oversteer on RWD, whereas FWD will tend toward a progressive increase in understeer, but a lot of the
perception of differing 'feel' is down to the fact that your inner ear is sensing that you're diagonally leaning in a different
direction.
... but my gut feeling is that you've got more problems than just front:rear spring/damper imbalance.
It's likely to be a combination of several issues, one of which is probably just that reverse trikes are naturally tail-happy little bastards.
Edited so that b*ll*cks isn't auto-corrected to 'nutsack'.
WTF?
I can just about put up the with spell-checker highlighting the correct English spelling for words, but American bowdlerisation of perfectly decent
Anglo Saxon is a step too far...
[Edited on 23/6/14 by Sam_68]
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Dave Ashurst
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| posted on 23/6/14 at 07:18 PM |
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C,
I understand you think the front wheels remain upright at all angles of body roll, but the rear wheel tilts with the body roll and hence steers the
rear end - like a tilted wheel does - but actively directs the rear end out of the bend, tightening the turn-in and increasing roll.
If so then that effect should be controlled by reducing roll? Can you reduce roll to nil as a control? Perhaps with rigid front suspension - replace
the shock absorbers with solid struts perhaps? and pump up the front tyres hard. Does that remove it?
If not:
Does it develop a resonant left-right steering wobble that can only be reduced by slowing down?
Have you checked all suspension and axle bearings for play? (including the front ones) see if tightening them has any effect.
Is the rear wheel assembly torsionally rigid? (it would not have been designed to deal with much lateral force on a tilting motorbike)
Are there any compliant elastomeric bushings in the suspension train left over from when it was a scooter?
Does it do this oversteer effect more to one side? or equally both ways?
Is the rear wheel mounted between two forks or on a single sided cantilever?
Is the spring/shock suspension single-sided?
How much does the tyre distort under lateral force?
How grippy is it? (grippy in the middle, but slippy on the shoulders?)
How does tyre pressure affect the effect?
Have you tried videoing it?
Why don't Morgan 3-wheelers do this? or did they? and then Morgan found a solution?
All questions you've addressed no doubt!
Bring it over to our house next weekend? (political activity and weather permitting of course)
I'd love to see it.
best D
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Dave Ashurst
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| posted on 23/6/14 at 07:27 PM |
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PS is chassis twist adding to the effect?
D
PPS. Just read all the posts above... OK, you have it all covered !!
[Edited on 23/6/14 by Dave Ashurst]
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Sam_68
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| posted on 23/6/14 at 07:42 PM |
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quote:
Originally posted by Dave Ashurst
Why don't Morgan 3-wheelers do this? or did they? and then Morgan found a solution?
If I'm understanding correctly the information given so far, the OP's car uses a scooter drivetrain?
If so, then the weight distribution is going to be a bit different to the Morgan... more weight over the single rear wheel, compared to the Morgan,
which has its engine hung out beyond the front axle, so inherently more oversteery in terms of balance to start off with?
The original Morgan Three wheelers, of course, used a sliding pillar suspension that gave the same camber=roll angle at the front as you get at
the rear (though the skinny, round section tyres probably weren't very camber-sensitive, anyway).
quote:
Originally posted by Dave Ashurst
Is the rear wheel assembly torsionally rigid? (it would not have been designed to deal with much lateral force on a tilting motorbike)
Are there any compliant elastomeric bushings in the suspension train left over from when it was a scooter?
These two are particularly pertinent questions, I think.
With a bike, there is effectively no lateral load to contend with, so no reason that the designers shouldn't have used big, squishy
elastomeric bushings to give good isolation of NVH.
Is the engine and gearbox part of the unsprung mass, as on some scooters, or does the swing arm pivot separately?
[Edited on 23/6/14 by Sam_68]
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smart51
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| posted on 23/6/14 at 08:35 PM |
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The rear wheel is mounted on both sides, each of which has a spring and damper. The original swing arm was ditched and a braced A frame made for the
rear pivot. This increased NVH a bit but did stiffen it up a lot. The engine is mounted in 4 places which should hold it square and without
moving.
Rear laden weight is 51% which is more than I'd like but it isn't too bad. The weight is all very low with the CofG about 350mm above
ground level IIRC. Steady state cornering speeds can be "brave" and at modest speeds it is very chuckable.
There's plenty here to think about.
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smart51
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| posted on 25/6/14 at 07:36 PM |
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I did a test run today with the front dampers turned to minimum. I didn't expect to cure the problem but was curious to see what effect it
would have. At around 70 MPH the car felt less twitchy and there was more time to wind back the steering. But at 50 it felt less stable and there
was a tendency to start a back and forward sway if too roug with the wheel. at low speeds there was more front suspension movement as the dampers
were holding them back less, but not entirely in a peasant way.
I can see now how the front dampers can be used to tune the response of the car, once I've got to the bottom of why it behaves as it does. All
this blasting round at top speed has dropped my MPG into the 60s though 
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smart51
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| posted on 5/7/14 at 10:07 AM |
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Taking advice from Dave Ashurst (thanks Dave!) I stiffened the front ARB. It improves the situation a fair amount. I'm going to look at
stiffening the anti roll further, though taking advice from another thread, not eliminating it altoghether.
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