Imagine a car where the front suspension was designed so that the wheels stayed exactly perpendicular to the road regardless of body roll. Now
imagine the rear suspension is pure trailing arms such that if the car rolls 5 degrees, so does the wheel camber. Let's say the front has a
stiff anti roll bar and firm damping so that the car rolls fairly slowly. And you made a sudden turn of the steering wheel then held it in the turned
position, what affect would you expect to see from the rear suspension?
[Edited on 22-6-2014 by smart51]
Are you saying that the front is so stiff there is zero suspension travel, and on very stiff springs with a stiff front arb?
No, not at all. I'm saying that the front wishbones are designed to have equal camber gain to roll so that the wheels remain perpendicular to
the road, regardless of suspension stiffness. I'm suggesting that the damping is fairly stiff so that the body roll is slow. With slow body
roll, the effect of roll can be felt as a change in the car's attitude to the road as the car rolls.
[Edited on 22-6-2014 by smart51]
The outside wheel is always going to be loaded up more than the inside but if you stiffen the front too much then I'd say there would be terminal
under steer. Especially in the slower corners!
I think the monoshock cars like the force etc are designed so that roll and damping are separate so you can adjust one without the other. They tend to
be set up very stiff with. Very little or zero droop
i think the thing you would notice would be increased understeer
I'm a bit out of touch with F1 suspension design,and not sure if they still run a pull rod front set up for neater packaging. But I saw some thermal imaging of the current cars going through the tunnel an Monaco GP and they were getting more heat on the inside front right tyre going round the Right handed. And they run servers neg camber.
I'm thinking of the effect of the rear wheel camber as the car rolls. When the initial turn in is made, all the wheels are close to vertical. Slowly, the car rolls so slowly the rear camber rolls with it. What will you see happen as it rolls?
It's difficult to understand exactly what you want to know but when the car is sat stationary then the wheels are not close to vertical. The
camber is built in to give the best results when loaded up!
The only way to get the wheels to be vertical when cornering is to have massive tyre pressures,rock hard sprints and mega stiff damping. With the
front set like thisthr rear is irrelavent as like said you will have terminal under steer.
I set my radical at exactly the same damping front/rear with rebound damping 1 click Stiffer at front. But I run a front arb and leave the rear one
off.
Some circuits I run 7-9 clicks all round. Shorter tighter places 4-5 clicks. It makes a massive difference when your chasing hundredths of seconds!
Rear camber is also less than front and so is tow. I run my rears at 0deg toe (parallel)
quote:
Originally posted by daniel mason
It's difficult to understand exactly what you want to know but when the car is sat stationary then the wheels are not close to vertical. The camber is built in to give the best results when loaded up!
The only way to get the wheels to be vertical when cornering is to have massive tyre pressures,rock hard sprints and mega stiff damping. With the front set like thisthr rear is irrelavent as like said you will have terminal under steer.
I set my radical at exactly the same damping front/rear with rebound damping 1 click Stiffer at front. But I run a front arb and leave the rear one off.
Some circuits I run 7-9 clicks all round. Shorter tighter places 4-5 clicks. It makes a massive difference when your chasing hundredths of seconds!
Because the radical is so low (40mm) and there is very little roll anyway. Setting the front too stiff just makes it an Under steering pig!
Have a look at the dax camber compensation system, that may give you some clues.
It's quite fun to play around with the suspension to actually see what's happening. Jack up the front an play around with it!
I think 100% camber compensation is meant here...
So your role pole (not point) is on the other upright.
This will lead to great jacking forces and with the front sole responsable for the roll stiffness
+ has stiff dampers...
understeer city will be the result.
And this is on a biljart track...
Any bump will give a lot of camber gain (you can compensate a bit with very log wishbones)
And thus will upset the car.
Same for the dax system, things get really weird really fast 
OK, you all seem to be missing the point here. The difficulty of achieving the hypothetical situation I described is not what I'm looking for.
Let me put the question another way.
If a car has trailing arm rear suspension, then as a car leans in a corner, the rear wheels will lean by the same amount. This camber gain will
affect the steering of the car. What effect will this have? With the steering wheel held steady, Will the car turn in more as it rolls, turn in less
as it rolls? Will it have no effect or will it do something else?
quote:
Originally posted by smart51
OK, you all seem to be missing the point here. The difficulty of achieving the hypothetical situation I described is not what I'm looking for. Let me put the question another way.
If a car has trailing arm rear suspension, then as a car leans in a corner, the rear wheels will lean by the same amount. This camber gain will affect the steering of the car. What effect will this have? With the steering wheel held steady, Will the car turn in more as it rolls, turn in less as it rolls? Will it have no effect or will it do something else?
quote:
Originally posted by FazerBob
I agree with Daniel, to maintain 0 camber at the front is virtually impossible.
Yeah, OK, rear end grip would reduce if you were on the limit. I'm thinking more about the effect of the camber.
I'm told that a wheel that leans over wants to run round in a circle about the point where the axis points to the ground. If your car is
turning left, it leans to the right. Becase your rear wheels are leaning to the right, they want to turn to the right. This would make the car turn
quicker, which would increase body roll, with would make the rear wheels want to turn round a tigher circle, which would make the car turn even
faster... I was hoping to tease out whether you think this would be a noticable effect or not.
As mentioned above, biggest effect IMO would be the reduction in rear contact efficiency vs the front, which would lead to oversteer pretty
quickly.
Sounds similar in a sense to a peugeot rear trailing arm bearing failing - even slight cornering leads to ass-clenching as the rear takes the
commanded input and adds positive feedback. Horrible to drive.
It all depends on the weight distribution and roll couple distribution. On a lot of small fwd cars such as the real Mini anhd Metro or Golf were
designed with either plain trailing arms or a torsion beam to give a ground level roll centre and zero camber gain this is used to counter understeer
without having excessively stiffen the rear springs or resort to a rear anti-roll bar.
Interesting Rootes did the exactly the opposite on the front of the Hillman Imp which used a front swing axle with a relative high roll centre (at
least on the Mk1) to do induce terminal under steer. The rc was higher than intended by the desiggners because of interference at a late stage by Lord
Rootes, but is was put right on the Mk2 and the rcs were lowered even more on the coupe and sport models to give near perfect balance.
quote:
Originally posted by smart51
OK, you all seem to be missing the point here. The difficulty of achieving the hypothetical situation I described is not what I'm looking for. Let me put the question another way.
If a car has trailing arm rear suspension, then as a car leans in a corner, the rear wheels will lean by the same amount. This camber gain will affect the steering of the car. What effect will this have? With the steering wheel held steady, Will the car turn in more as it rolls, turn in less as it rolls? Will it have no effect or will it do something else?
quote:
Originally posted by britishtrident
quote:
Originally posted by smart51
OK, you all seem to be missing the point here. The difficulty of achieving the hypothetical situation I described is not what I'm looking for. Let me put the question another way.
If a car has trailing arm rear suspension, then as a car leans in a corner, the rear wheels will lean by the same amount. This camber gain will affect the steering of the car. What effect will this have? With the steering wheel held steady, Will the car turn in more as it rolls, turn in less as it rolls? Will it have no effect or will it do something else?
Not camber gain ? I think you mean camber loss as on the rear of a BL Mini or Rover 100 or a 2CV?
I sense a PistonHeads 'plane on a conveyor belt' scenario developing here... 
quote:
Originally posted by smart51
Yeah, OK, rear end grip would reduce if you were on the limit. I'm thinking more about the effect of the camber.
quote:
Originally posted by smart51I'm told that a wheel that leans over wants to run round in a circle about the point where the axis points to the ground.
quote:
Originally posted by britishtridentThe rc was higher than intended by the designers because of interference at a late stage by Lord Rootes, but is was put right on the Mk2 and the rcs were lowered even more on the coupe and sport models to give near perfect balance.
quote:
Originally posted by Sam_68
quote:
Originally posted by britishtridentThe rc was higher than intended by the designers because of interference at a late stage by Lord Rootes, but is was put right on the Mk2 and the rcs were lowered even more on the coupe and sport models to give near perfect balance.
Since drifting off-topic would probably be merciful for this thread and the sanity of all involved...
As an ex-Imp fanatic, my understanding is that it wasn't interference by Lord Rootes, but rather that they realised that they had to raise the ride height of the car slightly to comply with headlamp height regulations. The very early Mk1 Imps had front springs of 10.07" free length (manufacturer's spec., not mine... I think it's safe to assume that they didn't measure the springs to a production tolerance of a hundredth of an inch!), but it was found that when they settled in service, the headlamp height fell below the legal minimum. As a stop-gap solution, they changed early in the Mk.1 production run to 10.35" free length springs, which raised the ride height (and hence roll centre height) by about 3/8".
This was addressed by a revised wishbone mounting bracket design on the Mk.2, however, to drop the pivot position back down to where it should have been.
The coupe and sport models in standard form weren't any different to the standard Mk. 2 saloon brackets and springs, but a common aftermarket modification was to fit 'Monte Carlo' springs of 9.57" free length that lowered both the ride height and RC still further (and increased negative camber), or to fit aftermarket brackets that lowered the pickup positions without much affecting the ride height (but which required you to adjust the rack height, too, to avoid bump steer).
I went the 'Monte Carlo' route with my Imps/Stiletto and it transformed the handling.
quote:
Originally posted by Sam_68drifting off-topic would probably be merciful for this thread and the sanity of all involved...
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.
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.
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?
Sam_68 - 23/6/14 at 05:45 PMquote:
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]
Dave Ashurst - 23/6/14 at 07:18 PMC,
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
Dave Ashurst - 23/6/14 at 07:27 PMPS 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]
Sam_68 - 23/6/14 at 07:42 PMquote:
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]
smart51 - 23/6/14 at 08:35 PMThe 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.
smart51 - 25/6/14 at 07:36 PMI 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
smart51 - 5/7/14 at 10:07 AMTaking 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.
