Wheel frequencies VS track surface

[johnny comelately]

Interesting couple of posts, and a healthy reminder that everything I have contributed assumes that a vehicle has four wheels...

I know nothing about two wheelers, though I did get to sample the Millikens' remarkable simulator, and I often wonder how the tyres remain attached to the road in turns. I read once that it is common to have control of rebound damping in one, and compression damping in the other, of the front forks - how does that work, I con't believe that the structure is that stiff?

Yes that is right and the structure is rigid enough.
Also there is the newish air forks (https://transmoto.com.au/air-forks-sort ... t-fiction/).
Often the tyres are not attached

[Tommy Cookers]

for many years (the 'good old days') motorcycles normally had rebound-only front damping
the front (and sometimes the rear) suspension was entirely the makers design ie no bought-in bolt-on units
and influenced by established hydraulic damping technology in aircraft, where killing initial rebound is vital

and some (bad and good) had no front hydraulic damping - remember there's friction in those sliding tubes
250s eg Ducati and Royal Enfields got good results from friction via the whole overlap in contact (bare slider, no bushes)

2 way damping was driven by off-road competition (rebound-only could clearly pump down)
generally a design improvisation by the makers arranging for oil trapping by bushes already there for mechanical reasons
this was rather unreliable as trapping depended critically on bush fit and conveniently ignored the effect of wear

ok now car dampers and motorcycle dampers are more or less the same
and they've stopped making the Citroen 2Cv/3Cv with its mass (+ a bit of friction ?) damping

the modern motorcycle (like cars) is structurally an order of magnitude (torsionally) stiffer without change of basic layout
having done everything a Mr P C Vincent advised 50 years ago
but race motorcycle design is again looking into a blind alley
the structure is now being fiddled to do some of the suspension's job because the suspension can't do it leaning at 60 deg
as a Mr J Surtees had in mind

[johnny comelately]

for many years (the 'good old days') motorcycles normally had rebound-only front damping
the front (and sometimes the rear) suspension was entirely the makers design ie no bought-in bolt-on units
and influenced by established hydraulic damping technology in aircraft, where killing initial rebound is vital

and some (bad and good) had no front hydraulic damping - remember there's friction in those sliding tubes
250s eg Ducati and Royal Enfields got good results friction-only via the whole overlap in contact (bare slider, no bushes)

2 way damping was driven by off-road competition (rebound-only could clearly pump down)
generally a design improvisation by the makers arranging for oil trapping by bushes already there for mechanical reasons
this was rather unreliable as trapping depended critically on bush fit and conveniently ignored the effect of wear

ok now car dampers and motorcycle dampers are more or less the same

the modern motorcycle (like cars) is structurally an order of magnitude (torsionally) stiffer without change of basic layout
having done everything a Mr P C Vincent advised 50 years ago
but race motorcycle design is again looking into a blind alley
the structure is now being fiddled to do some of the suspension's job because the suspension can't do it leaning at 60 deg
as a Mr J Surtees had in mind

Well described Tommy.
My reference above goes back to the 70's when they first had mono shocks at the rear and two speed valving, basically atf oil, no nitrogen or cooling, bad materials on bikes that still did 205 MPH.

[johnny comelately]

Had a smile today , even Mark Webber was commenting on a wheel problem where it was vertically loaded vector something or other, isnt that just bottomed out?

[johnny comelately]

DaveW, i wouldnt mind your opinion of these tyre / suspension peculiarities of the right rears in the photos i posted please - viewtopic.php?f=1&t=27067&hilit=comelately&start=1290

[DaveW]

DaveW, i wouldnt mind your opinion of these tyre / suspension peculiarities of the right rears in the photos i posted please - viewtopic.php?f=1&t=27067&hilit=comelately&start=1290

I think I would prefer a tyre person to comment on that. I am reasonably sure, however that the rear suspension design is compromised for aero/weight gains, and that inflation pressure restrictions don't help. The tyres also leave something to be desired, perhaps.

There is also the rear tyre "instability" I noted last year in this post viewtopic.php?f=6&t=24166&p=715402#p715402. The gif from that post is here. I also saw the instability during the coverage of the Australian GP.

[johnny comelately]

DaveW, i wouldnt mind your opinion of these tyre / suspension peculiarities of the right rears in the photos i posted please - viewtopic.php?f=1&t=27067&hilit=comelately&start=1290

I think I would prefer a tyre person to comment on that. I am reasonably sure, however that the rear suspension design is compromised for aero/weight gains, and that inflation pressure restrictions don't help. The tyres also leave something to be desired, perhaps.

There is also the rear tyre "instability" I noted last year in this post viewtopic.php?f=6&t=24166&p=715402#p715402. The gif from that post is here. I also saw the instability during the coverage of the Australian GP.

Thanks for the reply Dave, I saw the tyres part as secondary with and without their own contribution. i saw the geometry and suspension design factors as being primary. but either way it is a shortcoming particularly under power. the other observation is it must be low speed??

[Rustem 1988]

Then dyamically, Mu*D*D(Xu) = (Ks+Cs*D)*(Xs-Xu) + (Kt+Ct*D)*(Xu-Xr).

The question of changing the load during compression due to the additional weight. Let the additional weight be the same at compression, but the ratio Kt / Ks for a quarter of the car model will be different. If the sum of Kt and Ks remains constant and the additional weight is also constant, but the ratio Kt / Ks changes in the direction of increase, then can the product C * (ẋs -ẋu) also increase? Or does the ratio Kt / Ks have little effect on the ratio (ẋs / ẋu)? I know that for a simple spring, when the spring stiffness increases by 2 times, the maximum velocity decreases by √2 times , but I do not know how much this can affect the load on the wheel. I also think that with an increase in Kt / Ks, the average acceleration of unsprung mass will decrease.

[DaveW]

Let the additional weight be the same at compression, but the ratio Kt / Ks for a quarter of the car model will be different.

I don't really understand the statement, but I suspect you are equating mass with force, and (possibly) confusing static response and dynamic response... What are your equations?

[Rustem 1988]

Let the additional weight be the same at compression, but the ratio Kt / Ks for a quarter of the car model will be different.

I don't really understand the statement, but I suspect you are equating mass with force, and (possibly) confusing static response and dynamic response... What are your equations?

My question is for a static response on a flat road. The reaction of the suspension when the weight on the wheel increases and the suspension is compressed. If the ratio of Kt / Ks (tire stiffness / spring stiffness) increases, but the sum of Kt + Ks remains the same, how much will ẋs / ẋu (or ẋs-ẋu) change or will it not have a significant effect on the wheel load?

[DaveW]

My question is for a static response on a flat road.

That implies that vertical motion of the masses will be zero, so both ẋs, ẋu and therefore ẋs-ẋu will be zero....
.

... a significant effect on the wheel load?

It follows that tyre stiffness, spring stiffness, and damper settings will have no effect on wheel load...

[Rustem 1988]

So the stiffness of the springs and shock absorbers on a flat road does not affect the grip?

And still question for dynamic response.

In order to tune for maximum grip, you want the lowest transmissibility possible- as the body is
bouncing around, the forces on the springs are changing, decreasing the grip.

http://www.optimumg.com/docs/Springs&Da ... _Tip_4.pdf page 3.

Do we always need the lowest transmissibility for maximum grip?

[DaveW]

So the stiffness of the springs and shock absorbers on a flat road does not affect the grip?

That isn't the question you asked...
.

Do we always need the lowest transmissibility for maximum grip?

No...

[Rustem 1988]

So the stiffness of the springs and shock absorbers on a flat road does not affect the grip?

That isn't the question you asked...
.

Do we always need the lowest transmissibility for maximum grip?

No...

Thank you.
I wanted to ask. Is the stiffness of the suspension on a flat road for grip of the car?

[DaveW]

I wanted to ask. Is the stiffness of the suspension on a flat road for grip of the car?

A hypothetical question, because flat circuits don’t really exist. There are two axles in most vehicles, and they are likely to have different spring stiffness.

There are five main factors that affect spring selection: vehicle characteristics, track surface, driver preference, lateral balance of the vehicle, and tyre temperature.

Even “proper” race vehicles may have features that require stiff springs to reduce suspension deflection (e.g. changing motion ratios). The ability to manage lateral and longitudinal inertia forces is also a factor.

Track surface can be issue, particularly through corners e.g. driving round a feature (stiff) or driving through it (soft).

I suppose every driver would prefer to drive a stable platform (with stiff springs), but too stiff and he will give up contact patch load control, and may over-work the tyres. Drivers who can manage a soft set-up are often quicker than those who cannot.

It’s fair to say that, in my view, spring stiffness selection is not that important, but bar and damper settings are (and should match the springs)….