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A query on inboard shocks...
David Jenkins - 19/6/07 at 07:37 AM

A fellow forum member and I were standing in front of the MK chassis at Newark, looking at the inboard shocks. We started to discuss why manufacturers have started fitting them to 7's, and struggled a bit... I thought of a couple of reasons, but they're unconvincing...

1. They're out of the rain and dirt - but they're now getting hot behind the radiator, and they probably still get wet and dirty.

2. Reduced unsprung weight - marginal, I would have thought, as the steel pushrod probably doesn't weigh a lot less than the bottom half of a shock absorber.

3. Reduced wind resistance - as the typical 7 has the aerodynamics of a house brick, this won't make much difference.

4. Bling value - possibility!

Anything I've missed, that may justify replacing a very simple solution for something quite complicated?

David

BTW: This is NOT a pop at MK - there's enough of that going on elsewhere - but a genuine question, as other makers seem to be going that way.

JoelP - 19/6/07 at 07:40 AM

i think its entirely bling value, though using pull rods can help you bring the centre of mass lower and reduce the polar moment (ie towards the centre).

IMHO it raises the unsprung mass, but that turned into a can of worms last time so i shalln't argue the point!

tiger7 - 19/6/07 at 07:53 AM

Inboard shocks only remember me of fitting a Pinto engine in my Tiger Cat, a touching issue.

smart51 - 19/6/07 at 07:58 AM

MNR talk of rising rate with inboard springs and falling rate with outboard. I'm not clear on this but I think it means that for small suspension movements, the springs are soft, giving a nice ride, but the more the body rolls (or drops) the firmer it gets, giving good handling.

edit: people on here always say that the seven has terrible aerodynamics as if you'd be mad to even consider improving them. Why? Surely if inboard dampers, wraparound cycle wings and nose cone shape can improve aerodynamics then why not?

[Edited on 19-6-2007 by smart51]

Agriv8 - 19/6/07 at 08:00 AM

All to do with 'rising rate' against 'falling rate ' I belive.

The way I understand it is that the more angle you run your coilover at the less affective it is, You then need to add a beeffier coil spring to prop the front end up, add the desire to get wider fron track ( cornering stability ) you are going to make the problem even worse.

Oh nearly forgot an added bonus is on the MNR system you can change your ride height without haveing to wind the coilover spring in.

To give you an idea of how well it works I am running 125 lbs springs on the front of my inboard V8 ( a little soft for the track ) but perfect for the Cr4ppy roads arround here.

It May be bling but it is function over form sure Marc will be along to put me right.

regards

Agriv8

SaveTheDodo - 19/6/07 at 08:01 AM

Wind resistance plays a significant part in having inboard suspension per se - remember an open wheeled F1 car in 1962 (Lotus 25) did'nt have much better aerodynamics than a 7, and this was stated as the reason for switching to rocker arms. I believe Jeremy Phillips also introduced his rocker arm type suspension for the same reason.

However the switch to pushrods came about because the increased forces in the ground effect and big wings era were bending the rocker arms - I can't see this reason applying to 7alikes!!

Credit where credit is due - the original idea for rocker arm suspension apparently came from the Clairmonte brothers who commissioned a special from Lotus that was originally intended to have the number 7. The wheel has come full circle!!

David Jenkins - 19/6/07 at 08:18 AM

quote:
Originally posted by smart51
edit: people on here always say that the seven has terrible aerodynamics as if you'd be mad to even consider improving them. Why? Surely if inboard dampers, wraparound cycle wings and nose cone shape can improve aerodynamics then why not?


Slightly off-topic - but I don't believe that aerodynamics are relevant on a 7. For me they are about acceleration and cornering rather than top speed.

Other people may have different opinions - it is allowed - but my car is used for blasting around local roads most of the time, so wind resistance is not very important to me.

After all, some of the cars with inboard shocks have a typical 7-style windscreen - you may as well have a parachute dangling from the back of the car!

Now back to in-board shocks!



[Edited on 19/6/07 by David Jenkins]

SaveTheDodo - 19/6/07 at 08:22 AM

David,

At road speeds they may be bordering on irrelevant, but remember these manufacturers are in the business of making track day and race cars - aerodynamics are significant for these.

Cheers

David Jenkins - 19/6/07 at 08:25 AM

Agriv8,

If there is a technical benefit for inboard shocks then that would be a good reason to have them... if they improve the spring function, or increase the adjustability, then that is A Good Thing.

nitram38 - 19/6/07 at 08:48 AM

I will be running compressor driven Airbag spring bilstein shocks on the Motaleira.
I will be able to raise or lower the car from 0-6" with a running height of 3".
Apart from not having a big shock in the airstream, the main advantage to me is ride height adjustment.
The shocks optimum setting is 13.5", so I will mount a dummy rod in place of the shock and make my pushrods the correct length in relation to the running ride height of 3".
I also intend buying carbon csr cycle wings which according to catering van, stops them lifting.
Using wilwood calipers instead of the rover calipers saves 3kg per wheel.
All these "little" things help gain a bit more speed and suspension compliance.
Aerodynamics may not mean much to a road going car, but if you track or compete, then very small gains make all the difference.
I have seen cars with little bits of plastic placed all over the suspension/protrusions to help them win.

Agriv8 - 19/6/07 at 08:51 AM

I have come up with another way to explain it better ( possibly ! ).

On an inboard setup the spring and shock is working through a vertually constant line while the suspension arm travels from full droop to full compresion.

in an outboard setup the shock is working at 'diferent angles' while going from droop to compresion.

There are other ways arround this ( have a look at Dax for instance ) they run the coil over more vertically on an outboard setup by moving the top coilover mount out from the chassis

Just re-reading that I have almost convinced myself that I know what I am talking about

Regards

Agirv8

short track 123 - 19/6/07 at 09:07 AM

There is also the advantage of less unsprung mass with inboard shocks + the aero. And if your design allows you could put them much closer to th C/G.
Every little bit helps.

nitram38 - 19/6/07 at 09:10 AM

quote:
Originally posted by short track 123
There is also the advantage of less unsprung mass with inboard shocks + the aero. And if your design allows you could put them much closer to th C/G.
Every little bit helps.


I have said this before and got shot down in flames! I'll get my coat...................

Agriv8 - 19/6/07 at 09:15 AM

quote:
Originally posted by nitram38
quote:
Originally posted by short track 123
There is also the advantage of less unsprung mass with inboard shocks + the aero. And if your design allows you could put them much closer to th C/G.
Every little bit helps.


I have said this before and got shot down in flames! I'll get my coat...................


I agree

but as a safty precaution.

A My coat is aready on.
B My parashute is strapped to my back
C I have my fireproof Grunndies on



regards

Agriv8

3GEComponents - 19/6/07 at 09:16 AM

I think what Agriv8 is trying to say that with normal outboard mounted units, as the suspension compresses the effectiveness of the shock is reduced.

With inboard, cam actuated systems this doesn't happen, you can tune the rocker to give a rising rate which works wonders for suspension.

One thing that has yet to appear on kit cars, as far as i know, is stable platform dampers, these are big in the off-road scene in the States, and mountain biking, where the shock is stiff to move until a large bump force happens, the point at which this happens is controlled by air pressure and so adjustable to suit needs.

Could work wonders for handling, just think flat stable car where the suspension only moves when it hits a bump, Mmmm..........................

[Edited on 19/6/07 by jroberts]

swalf3 - 19/6/07 at 09:46 AM

Another advantage of pushrod or pullrod suspe n.sion is that when you alter your ride height you don't need to redo the corner weights.The main advantage that has already been mentioned and explained is the adjustment to rising rate .This is all the more important if you widen the track, as a result the angle of the shocker to horizontal is reduced so is the rate of springing in bump. Winston

JonBowden - 19/6/07 at 10:01 AM

quote:

There is also the advantage of less unsprung mass with inboard shocks


This won't make much difference. If the rocker mechanism has a 1:1 ratio, then the moving mass of the damper still has to be moved by the same amount. The only way to use this to reduce the unsprung mass would be to use a rocker mechanism that caused the damper to move by a smaller amount using a stiffer spring and damper. Even then, the the push/pull and rocker mechanism will add to the unsprung mass.

nitram38 - 19/6/07 at 10:36 AM

quote:
Originally posted by JonBowden
quote:

There is also the advantage of less unsprung mass with inboard shocks


This won't make much difference. If the rocker mechanism has a 1:1 ratio, then the moving mass of the damper still has to be moved by the same amount. The only way to use this to reduce the unsprung mass would be to use a rocker mechanism that caused the damper to move by a smaller amount using a stiffer spring and damper. Even then, the the push/pull and rocker mechanism will add to the unsprung mass.


Good job mine is 2:1 then !
1" travel on my shock gives 2" ride height change.

David Jenkins - 19/6/07 at 11:22 AM

oops! I seem to have inadvertently opened one of these...




It was an honest question, though...

Agriv8 - 19/6/07 at 02:08 PM

Naa - Just like everything else in life everbody got their views on a given subject.

Nothing wrong with a little livley debate.

regards

Agriv8

locostv8 - 19/6/07 at 04:13 PM

One nice thing about the rocker system is that you can change your effective spring rate by changing the rocker ratio.

MikeRJ - 19/6/07 at 09:43 PM

quote:
Originally posted by locostv8
One nice thing about the rocker system is that you can change your effective spring rate by changing the rocker ratio.


The downside being you also change the damping at the same time.

JoelP - 19/6/07 at 09:47 PM

quote:
Originally posted by MikeRJ
quote:
Originally posted by locostv8
One nice thing about the rocker system is that you can change your effective spring rate by changing the rocker ratio.


The downside being you also change the damping at the same time.


i was gonna say that but im tired of arguing about inboard shocks!

However, i really like the rising rate aspect, makes a lot of sense.

locostv8 - 20/6/07 at 06:20 AM

I'm using R6 coilovers which are adjustable and rebuildable/rrevalveable and CHEAP. With shipping I have less than $200 in 8 coilovers.

[Edited on 20/6/07 by locostv8]

short track 123 - 20/6/07 at 05:50 PM

If you put a set of scales under the wheel with a block supporting the chassis and remove the bolts from the shock is this unsprung weight ?

If so.
If we repeat this with push/pull rod set up how come we get a lower weight.

I could be wrong but is mass not weight?
I did think that the force it takes to compress the shock is a separate issue.
A crude experiment but it does seem to work.

JoelP - 20/6/07 at 06:41 PM

quote:
Originally posted by short track 123
If you put a set of scales under the wheel with a block supporting the chassis and remove the bolts from the shock is this unsprung weight ?



not to me...

Unsprung mass represents more than just the literal mass that is unsprung, it must also take into account its position in order to represent how much affect it has on the ability of the wheel to move fast.

If you extended your wishbone 10 feet out to the side, a kilo weight would make it very hard to move the wheel. That same weight attached 1cm away from you inner pivot (on the wishbone still) wouldnt affect the wheel at all. An inch of wheel travel would move the weight 10" in the first example but maybe only 1mm in the second, this obviously involves very different amounts of energy.

The same idea applies to the rocker and rods.

For simplicity of comparison, you could break items into a few groups. Stuff at the wheel and moveing with it, ie your wheel itself, tyre, discs, calipers, cycle wings etc. Stuff that is only moving at one end, which you could roughly halve for comparison with the first group. This would be wishbones, brake flexis, your spring etc. Another group would be the things that dont fall easily into the first groups, and this group is where rockers and pushrods would fall. Stuff at 45 degrees like your pushrod would move 70% less than the wheel items, so would be less important. Your rocker would be roughly 35% biased, as its both pivoting and only moving with the 70% of the pushrod.

That gets a bit complex though, which is never a good start.

The most accurate way to measure your unsprung mass would be to apply a force and measure acceleration, as this would automatically include the bias's above. However, you would then also have to exclude gravity, which takes us in a neat circle as to the use of bathroom scales - measure the weight at the wheels, covert to newtons, add balast to round the weight at the wheel to a comparable amount (ie 20kgs), measure the acceleration under gravity, work out the actual mass from that, deduct the balast added, and VOLIA, several hours wasted on an internet discussion.

gazza285 - 20/6/07 at 07:41 PM

I'm not getting invlolved in this one again.

short track 123 - 21/6/07 at 05:55 AM

Do you know of any good books, web site's where i can find info that cover this area of suspension design all the books i have only quote unsprung mass as mass (moments of inertia are covered i.e mass and distance from pivot point) and have nothing about including force of shocks benning included in the un-sprung mass but do cover wheel and spring rates as separate areas.

JoelP - 21/6/07 at 07:10 AM

no idea im afraid, i wrote/made that up myself. With your moment of inertias though you're not far off, works fine for outboard suspension but for inboard it needs tweeking.

[Edited on 21/6/07 by JoelP]