Front Suspension - inboard or outboard

[SkyUK]

What are the pro's and con's of inboard vs outboard front suspension? I noticed this car has inboard and wondered which is better: http://www.woodcotesportscars.com/car-sales/detail...

[Sam_68]

Inboard pros:

• Slight reduction in aerodynamic drag
• Ratio of wheel deflection:spring/damper deflection can be arranged to give pretty much whatever you want, so more favourable damper ratios and rising rate on springs can be obtained.
• Can assist with packaging, 'cos you can put the springs/dampers wherever you want them, within reason.
• People think they are sexy 'cos they've seen them on proper racing cars.

Inboard cons:

• Extra (unsprung) weight, cost and complexity.
• Dampers can overheat and fade if you don't arrange sufficient cooling air.
• Much more difficult to set-up accurately.
• Can place high leverages on mounting points, so care is required to feed the loads into the chassis correctly.

[Alistair1537]

I'd think it would less unsprung weight???? More sprung weight yes but you know what they say - a saved pound of unsprung weight is worth 10 of sprung weight!!

[Sam_68]

The weight of the pushrod and (roughly) half the weight of the rocker is on the unsprung side of the spring.

[SkyUK]

Sam,

Thanks for the clear & concise explanation. And for the clarification on the sprung/unsprung query - I also thought briefly you were explaining sprung as wheel side rather than chassis side

Presumably there are chassis manufacturers/suspension tuners that have done better jobs at matching variable rate compression to spring/damper combinations. Could you recommend one in the Reading area, or is it more complicated than that?

Cheers, Rich

[Mike Biddle]

It's more than a technical thing, having changed mine from outboard to inboard I can advise that the ride and handling is much better with inboard, the same Freestyle system as on the car you mentioned.

Unfortunately Freestyle is no longer in business, but if you are thinking of buying the car, don't worry about that too much about that, replacement parts are available.

[mickrick]

"Can place high leverages on mounting points, so care is required to feed the loads into the chassis correctly. "

That's a valid point, as I've seen a couple of examples of the fulcrum bolts shearing.
You would need to be very particular about the quality of bolts, and where the grip/thread interface sits in relation to the double shear of the fulcrum/mounts.

"Extra (unsprung) weight, cost and complexity."

I doubt the rod and fulcrim would be heavier than a spring and damper?

The car in the link could be changed back to the standard caterham set up very easily.

[Sam_68]

mickrick said:

"Extra (unsprung) weight, cost and complexity."

I doubt the rod and fulcrim would be heavier than a spring and damper?

Actually, coming back to this thread with my brain in gear, there was an error in my previous statement, but it's in the other direction. Forget the bit where I said half the weight of pushrod and rocker: I must have been half asleep and thinking in terms of the rule-or-thumb that half the coilover is unsprung, and half sprung.

It's all the weight of the pushrod and rocker that's unsprung: everything 'downstream' of the half of the damper that's bolted to the chassis is has inertia when the wheel hits a bump, hence it needs to be controlled by the dampers.

[damdy-cash]

Mike Biddle said:

Unfortunately Freestyle is no longer in business

Does anyone know whether the Freestyle inboard set is somewhere else available?

Cheers Volker

[SkyUK]

Sam_68 said:

Actually, coming back to this thread with my brain in gear, there was an error in my previous statement, but it's in the other direction. Forget the bit where I said half the weight of pushrod and rocker: I must have been half asleep and thinking in terms of the rule-or-thumb that half the coilover is unsprung, and half sprung.

It's all the weight of the pushrod and rocker that's unsprung: everything 'downstream' of the half of the damper that's bolted to the chassis is has inertia when the wheel hits a bump, hence it needs to be controlled by the dampers.

So inboard imposes a slight increase in unsprung weight due to the pushroad and rocker assemblies, but if set up properly offers the benefits of non linear spring compression?

[Sam_68]

Yes, amongst the other benefits and disadvantages I listed originally.

It's probably fair to say that with a 'Seven' (which has about the dirtiest aerodynamics you can get, short of attaching parachutes, but doesn't have any packaging issues), there's probably little or no overall advantage in theory, but the Freestyle set up does have the reputation of being competent. I've never driven one, though, so I can't really comment on whether that particular set-up works better than a well set-up stock arangement or not.

One further slight disadvantage of the Freestyle installation, you will notice from the photographs, is that it raises the car's centre of gravity slightly by placing the coilovers quite high, and having additional high-level framework to support them. Personally, when designing inboard systems, I always try to place the coilovers as low as possible. It has a fairly small effect, but every little bit counts, I think.

[SkyUK]

Thanks
Rich

[Mike Biddle]

You can get the fabricated parts from Arch motors, they made them for Freestyle.

Nor sure about the Fulcrum, but all you would need to do is measure one and get it machined up, there are lots of fiendly owners on here and blatchat who would be happy to help, me included.

After that, all you need is springs and shocks.

[mickrick]

I still can't see how a pushrod and half the aluminium fulcrum will be heavier than a damper and half the spring?

[Farlig]

mickrick said:

I still can't see how a pushrod and half the aluminium fulcrum will be heavier than a damper and half the spring?

+1

[Sam_68]

mickrick said:

I still can't see how a pushrod and half the aluminium fulcrum will be heavier than a damper and half the spring?

You're assuming that the pushrod and rocker replace the unsprung mass of the damper and spring? They don't: they're in addition to it... the half of the damper that's not bolted to the chassis, and half the mass of the spring, is still unsprung weight.

And any additional unsprung weight on a car as light as a Seven is a Very Bad Thing.

The pushrod and rocker are on the 'unsprung' side of the spring, so whether they weigh more, less, or exactly the same as the damper and half the spring, they are additional unsprung weight that creates inertia when the car hits a bump and which has to be controlled by the spring and damper.

[mickrick]

Sam_68 said:

mickrick said:

I still can't see how a pushrod and half the aluminium fulcrum will be heavier than a damper and half the spring?

You're assuming that the pushrod and rocker replace the unsprung mass of the damper and spring? They don't: they're in addition to it... the half of the damper that's not bolted to the chassis, and half the mass of the spring, is still unsprung weight.

And any additional unsprung weight on a car as light as a Seven is a Very Bad Thing.

The pushrod and rocker are on the 'unsprung' side of the spring, so whether they weigh more, less, or exactly the same as the damper and half the spring, they are additional unsprung weight that creates inertia when the car hits a bump and which has to be controlled by the spring and damper.

Ooh er! I think you're right! Thanks, I hadn't thought of it that way.

[Farlig]

Sam_68 said:

mickrick said:

I still can't see how a pushrod and half the aluminium fulcrum will be heavier than a damper and half the spring?

You're assuming that the pushrod and rocker replace the unsprung mass of the damper and spring? They don't: they're in addition to it... the half of the damper that's not bolted to the chassis, and half the mass of the spring, is still unsprung weight.

And any additional unsprung weight on a car as light as a Seven is a Very Bad Thing.

The pushrod and rocker are on the 'unsprung' side of the spring, so whether they weigh more, less, or exactly the same as the damper and half the spring, they are additional unsprung weight that creates inertia when the car hits a bump and which has to be controlled by the spring and damper.

Hmmm not sure I see it your way - we're getting weight & mass mixed up here...

In terms of mass, I agree that anything moving with the suspension, relative to the chassis, will add it's own inertia but you also need to consider / contrast the leverage ratios... With a shock & spring hanging out in the wind, a rather substantial amount of it's weight is unsprung and it's mass is acted on by a leverage ratio of approximately 2:1 (I think)

With the shock inboard, all of it's weight is sprung and it's mass is acted upon by a larger fulcrum ratio effectively reducing it's inertia.. (the shock moves less for a given wheel travel) Yes you have to take into account some of the weight of the pushrod and the fulcrum on the other side of the lever (and circa half the mass due to the same 2:1 ratio as the "old" shock had (wheel-to-fulcrum ratio) but there's more than aerodynamics to be gained by moving that shock weight inboard given a quality shock & the right setup of course ;-)

Edited by Farlig on Monday 23 April 22:14

[mickrick]

I thought the shock moved more with the inboard system. Hence the term rising rate?

[Sam_68]

Farlig said:

Hmmm not sure I see it your way - we're getting weight & mass mixed up here...

I think what we’re actually getting mixed up is mass and inertia/momentum.

I do tend to use the terms mass and weight interchangably (because nobody I know talks about ‘unsprung mass’ and most of us live on a planet where the force of gravity is pretty much a constant). I also have a bad habit of using ‘inertia’ and ‘momentum’ interchangeably (because they’re two sides of the same coin), but never the less – it’s not the weight (sorry, mass) that’s a problem in itself… it’s the momentum caused by it being deflected by a bump which has to be reacted against the inertia of the sprung mass.

Certainly some bits of the unsprung weight (sorry, mass) will have more momentum than others, because they’re moving faster – I think the concept you were trying to explain is that of moments of inertia – but they all have mass, they are all caused to move by a suspension deflection, therefore they all have momentum that has to be reacted against the inertia of the sprung mass.

Look at it this way: if your approach was correct, all you’d need to do would be to fit a rocker arm with a weight on it that exactly counterbalances the weight of the upright/hub/ brake/wheel, etc. on the other side, and all our ride and damping problems would be over?

mickrick said:

I thought the shock moved more with the inboard system. Hence the term rising rate?

You can arrange the bellcrank so that it magnifies the wheel movement into larger movement of the damper, but it's not an essential feature of a pushrod system - you could also just as easily arrange it so that there is a 1:1 wheel:damper movement ratio, or a reduction ratio.

And rising rate is a bit different... its when you arrange the pushrod/bellcrank geometry so that the ratio of wheel:damper movement is not fixed, but gradually increases the further the wheel is deflected.

Edited by Sam_68 on Tuesday 24th April 18:43