Suspension design decisions on scratch build...
Discussion
Hi all, Just found this place after spending fruitless hours surfing for info!
I've decided to build a car for myself from scratch, having been involved in a scratch build before and numerous rebuild/kits.
This time I'm moving into a different league though - going for some thing light, fast and powerful. I've read a couple of books on designing my own suspension and in between the massive headaches and general looks of confusion decided it might be best to just take these parts from either an existing design or a production car.
The car design is very much in my head at the moment, as I dont want to start building in CAD until I've sorted out the suspension. I'm also trying to track down schematics of the engine and gearbox to get those in cad but having no joy there either
Size and style it will be somewhere between a Mclaren F1 and a Lambo Murci Predominantly carbon bodied with space frame chassis similar to those found in many lambo style replica's, as that uses materials I'm most comfortable using! Looking at a RPVv6 turbo through a Renault G'box
So... I originally thought about using Lotus Esprit suspension, but dont know anyone with one, let alone that would let me take it apart to measure up, then I thought about just getting the dims and making up some tubular versions myself, though in all honesty though I'd trust my welding for virtually everything I'm not 100% convinced of doing my own wishbones, plus I still cant find the geometery out!
Looking for ANY thoughts and advice really on where to start with this suspension, places I can find accurate dims of existing, or suggestions for production car ones I can take from a donor - anything really!!
Many Thanks
I've decided to build a car for myself from scratch, having been involved in a scratch build before and numerous rebuild/kits.
This time I'm moving into a different league though - going for some thing light, fast and powerful. I've read a couple of books on designing my own suspension and in between the massive headaches and general looks of confusion decided it might be best to just take these parts from either an existing design or a production car.
The car design is very much in my head at the moment, as I dont want to start building in CAD until I've sorted out the suspension. I'm also trying to track down schematics of the engine and gearbox to get those in cad but having no joy there either
Size and style it will be somewhere between a Mclaren F1 and a Lambo Murci Predominantly carbon bodied with space frame chassis similar to those found in many lambo style replica's, as that uses materials I'm most comfortable using! Looking at a RPVv6 turbo through a Renault G'box
So... I originally thought about using Lotus Esprit suspension, but dont know anyone with one, let alone that would let me take it apart to measure up, then I thought about just getting the dims and making up some tubular versions myself, though in all honesty though I'd trust my welding for virtually everything I'm not 100% convinced of doing my own wishbones, plus I still cant find the geometery out!
Looking for ANY thoughts and advice really on where to start with this suspension, places I can find accurate dims of existing, or suggestions for production car ones I can take from a donor - anything really!!
Many Thanks
Sensible approach under the circumstances would probably be to use the entire Esprit backbone chassis, complete with suspension. It's not exactly state of the art, but at least it was developed by people who knew what they were doing.
It does beg the question why you are going to the trouble and expense of building your own car, particularly using a material as expensive as carbon fibre for the bodyshell, if you don't have any definite ideas about chassis/suspension design and you are going to power it with something as mundane as a PRV V6 Turbo, though?
Why not just buy yourself an Esprit V8 and have done with it?
It does beg the question why you are going to the trouble and expense of building your own car, particularly using a material as expensive as carbon fibre for the bodyshell, if you don't have any definite ideas about chassis/suspension design and you are going to power it with something as mundane as a PRV V6 Turbo, though?
Why not just buy yourself an Esprit V8 and have done with it?
well I can cope with the chassis design OK, and frankly the Esprit chassis doesn't fit the bill for the rest of the requirements, it's just down to the suspension. Given the time I could work it out but there are too many unknown variables at the moment to make the start on the design cycle, and being that it's my area of least knowledge was just looking for some suggestions. Cost of carbon needn't be as bad as some make out either if you know the right people... As I say, I'm used to working with it, like working with it, so it's a simple choice. As for building - I'm still working on CAD at this point, hence why I'm asking now rather than later. I have a VERY definite idea of the finished article in my head.
Why the RPV? well personally from the figures I think it has quite a good power to weight ratio - I might be wrong, often am, but at 300bhp plus I dont really think it's tame.
At the end of the day yes I could go and buy an esprit, but then at the end of the day I'd still own a production car that held as much attraction for me as my current get-me-to-work and back that I paid £1200 for - why do any of us build kits and scratch builds when the end item is often far inferior to a cheap production car?!
>> Edited by Davi on Tuesday 13th December 22:56
Why the RPV? well personally from the figures I think it has quite a good power to weight ratio - I might be wrong, often am, but at 300bhp plus I dont really think it's tame.
At the end of the day yes I could go and buy an esprit, but then at the end of the day I'd still own a production car that held as much attraction for me as my current get-me-to-work and back that I paid £1200 for - why do any of us build kits and scratch builds when the end item is often far inferior to a cheap production car?!
>> Edited by Davi on Tuesday 13th December 22:56
If your building a car from scratch then design yourself a space frame and put double wishbones onn (sorry, I don't know what suspension the esprit has).
If you have some good ideas, feel free to email me and I'll be able to put your ideas into a suspension design program and you can see the characteristics of it. It will also give you the xyz points for putting into CAD (what CAD -ackage are you using?).
It depends whether you simply want the suspension to simply work or to look trick and work.
Cheers
If you have some good ideas, feel free to email me and I'll be able to put your ideas into a suspension design program and you can see the characteristics of it. It will also give you the xyz points for putting into CAD (what CAD -ackage are you using?).
It depends whether you simply want the suspension to simply work or to look trick and work.
Cheers
many thanks for the offer nightdriver - I tried to email you but it says you dont accept emails :P Most important for me is it works - i.e. doesn't throw me off the road unexpectedly, but I'd like it to look good too!!!
Pum, I was told that the later models have wishbone front and rear, so must have changed at some point, something else to investigate!
Pum, I was told that the later models have wishbone front and rear, so must have changed at some point, something else to investigate!
It's possible to work through traditional suspension calcs per Staniforth's suspension book in which he lists all the relevant equations in an easily spreadsheet-able format.
That of course depends on if you actually think roll centres are important!
There are rumors that those in the know regard the traditional suspension design approach as rubbish. The assumption being that a cornering force acts at the cars centre of gravity and that the car rolls according to the loads reacted at the contact patches, just as if you were calculating the loads on a simply supported block in "O level" physics. The roll centres then become irrelevent. The position in space of the effective swing axle centre for each individual wheel remains as relevent but only to calculate a secondary jacking force effect on the wheel movement.
Anyone in the know like to comment on this?
That of course depends on if you actually think roll centres are important!
There are rumors that those in the know regard the traditional suspension design approach as rubbish. The assumption being that a cornering force acts at the cars centre of gravity and that the car rolls according to the loads reacted at the contact patches, just as if you were calculating the loads on a simply supported block in "O level" physics. The roll centres then become irrelevent. The position in space of the effective swing axle centre for each individual wheel remains as relevent but only to calculate a secondary jacking force effect on the wheel movement.
Anyone in the know like to comment on this?
I regard the position of the roll axis as very important. It affects how the lateral weight transfer is apportioned between the front and rear axles in cornering, which has a fundamental effect on the car's handling. It also affects how much body roll occurs, which has a fundamental effect on wheel geometry.
GreenV8S said:
I regard the position of the roll axis as very important. It affects how the lateral weight transfer is apportioned between the front and rear axles in cornering, which has a fundamental effect on the car's handling. It also affects how much body roll occurs, which has a fundamental effect on wheel geometry.
I've read about it, understood it and spreadsheeted it. None of that makes it true however!
The alternative theory was one I came across on the locostbuilders.co.uk site by a member claiming that roll centre theory wasn't taken seriously by major car companies or serious consultancies any more, except for a bench mark with past suspension designs.
Remember that the only reason we think we know that roll centres and the associated roll calculation approach is right is because we read it somewhere!
I'm genuinely intrigued that there may be an "industry insider only" theory.
The best way to do it is, as mentioned, using the forces to determine the roll characteristics of the car. Unfortunately this cannot be done by hand so you need some form of software to work it out for you.
It doesn't mean the roll centre is rubbish but you actually have a force roll centre rather than the traditional style, its much more accurate and is a truer representation of what is actually happening.
It doesn't mean the roll centre is rubbish but you actually have a force roll centre rather than the traditional style, its much more accurate and is a truer representation of what is actually happening.
NightDriver said:
The best way to do it is, as mentioned, using the forces to determine the roll characteristics of the car. Unfortunately this cannot be done by hand so you need some form of software to work it out for you.
It doesn't mean the roll centre is rubbish but you actually have a force roll centre rather than the traditional style, its much more accurate and is a truer representation of what is actually happening.
Why can't you work it out by hand?
Surely, using this theory, the tyre contact loads can be determined by simply taking moments about a contact patch and resolbing loads vertically. There will be a secondary jacking effect which will depend on an individual wheels suspension geometry.
Are there any "force roll centre" articles on the net that explain the true situation?
If you're designing wishbone layouts you really want to avoid large roll couples. Roll is the enemy of wishbone suspensions as it creates a situation whereby you have to design some camber control in roll whilst limiting large bump steer variations. If you can isolate camber from roll you're onto a winner.
De-dion rear ends are a lovely idea .. a titanium beamed and hubbed set-up would be great, you might recreate that on the front too .. I'd love to do something lightweight with de-dion.
Mercedes had a demo car that went further than isolating camber and roll, in fact it added camber in cornering .. and i think one of the kit-car manufacturers had a mechanical system that did broadly similar things.
In essence roll centres needn't matter if you can stop roll compromising other suspension characteristics. In reality you probably need to pay them close attention.
De-dion rear ends are a lovely idea .. a titanium beamed and hubbed set-up would be great, you might recreate that on the front too .. I'd love to do something lightweight with de-dion.
Mercedes had a demo car that went further than isolating camber and roll, in fact it added camber in cornering .. and i think one of the kit-car manufacturers had a mechanical system that did broadly similar things.
In essence roll centres needn't matter if you can stop roll compromising other suspension characteristics. In reality you probably need to pay them close attention.
trackcar said:
i think one of the kit-car manufacturers had a mechanical system that did broadly similar things.
DAX used an extremely cute moving wishbone system which took several hours to work out. As far as I can remember, it acted a lot like a de-dion but was much easier to package.
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