loco-gt
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| posted on 9/10/11 at 07:21 PM |
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inboard coilover geometry
Newbie alert! ...lots of questions and haven't even turned a spanner.
After years of excuses not to I'm finally ready to embrace the Locost experience.
Project idea: enclosed wheel GT/targa/roadster (not finalised yet but not a big fan of the elements when driving expecially as I live in reportedly
the wettest county in England ...Lancashire) based on modified Haynes 7 chassis (i'm 6'5", 17 stone with size 12 boots) with MX5
single donor. As it happens after months of looking at my options and arriving at this basic outline I just saw Autocar run a road test on
Ginetta's new roadgoing version of their G40r ...just served to concrete my idea as the best way to go, only hope mine does't cost the
£28K Ginetta charge.
Question1: which springs
On the basis that the Haynes 7 has a finished curb weight of approx 550kg with the recommendation of 350 Lb per inch front/200 Lb per inch rear and an
estimated weight gain of 200kg?, for chassis mods and full bodywork.
What would the spring rates have to be upgraded to (for a road car)?
Q2: suspension travel
Same Haynes 7 recommends coilovers with travel from 13" to 9" = 4".
How much of this travel is used just to absorb the standing curb weight of the car and how much remains for actual suspension movement to handle all
of Englands roads wonderfull potholes?
Q3: Inboard mounted coilovers (yeah, the novice wants to go the high tech engineered route)
If I were to go this route utilising pushrods and bellcranks what affects would different orientations of the coilover offer (pros/cons)? eg: would a
coilover mounted horizontally following the triangulated direction of the tunnel to the opposing corner help reduce torsion effects.
What if the coilover was inclined towards roof of the vehicle, or angled down to vehicle centre point at ground level ...would either have any affect
on roll centre height?
Anyone built an inboard system and care to share specs?
Think that should do for now - appreciate any guidance/feedback
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Sam_68
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| posted on 9/10/11 at 09:41 PM |
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Question 1: If you want to keep things comparable to the 'book' design, work out the suspension frequencies in cycles per minute for the
'original' design, then use these frequencies to calculate whatever spring rates you need for your own design... but don't forget
that spring rate is very different to wheel rate, particularly if you're using a pushrod system, so you'll need to take into
account the leverages to get to the wheel rates, before you can calculate the frequencies.
Question 2: As above; you can calculate these yourself, once you know the wheel rates. 'Static deflection' in itself is directly related
to wheel rates and sprung mass, so if you keep the same static deflection, you'll also be keeping similar suspension frequencies; the late,
great Arthur Mallock used 'stactic deflection' as one of his basic suspension design/tuning criteria for this reason.
Question 3(a) The main advantage of pushrods/pullrods is that you can make the leverages do pretty much what you want them to in terms of wheel
deflection: damper compression ratios, rising rate, etc. There is an aerodynamic advantage on open wheel cars, too, but it's pretty trivial on
something as crude as a traditional 'Seven' style car. Against these advantages you have to balance the additional weight and complexity
of the set-up.
Question 3(b) Torsional effects/benefits: negligible. Chassis, road springs and tyres acts as springs in series with one another (road springs are
damped, chassis is undamped and tyres aremostly undamped, except for the limited self-damping characteristics of the tyre carcase) ; torsional
effects are therefore almost entirely related only to the basic dimensions (wheelbase/track) between the tyre contact patches versus the rates of
these three sets of 'springs'. There as sometimes limited benefits to be gained in terms of finding structurally convenient load paths as
a result of how you react the loads from the coilovers and bellcranks, but there's limited effect on the overall torsional stiffness.
Question 3(c) Positioning of the coilover has no effect on the roll centre: the geometric roll centre is dictated only by wishbone geometry.
Get yourself a copy of Staniforth's 'Competition Car Suspension', if you haven't done so already: it gives you the information
you need to answer all of the above questions (and the formulae to calculate spring frequencies).
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loco-gt
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| posted on 10/10/11 at 07:05 PM |
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Thanks for such a comprehensive answer, though must admit am rather disappointed to hear there are so few benefits (in terms of a low budget build for
the road rather than professional track car) for the cons of weight and complexity and the added cost.
will have to reconsider wether apropriate for this build.
Will lookout for that particular book title - Thanks again.
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