The chassis con
Discussion
Did you know you were been had about the the abilities of those spaceframes out there?
I do structural analysis for a living and I've done a lot of finite element analysis on chassis designs for some projects of my own that I'd like to get round to starting one day.
The vast majority of spaceframes are not better than ladder frames! They are simply not triangulated in the right way to get the stiffness advantages they claim.
Take the following example.
Lowcost chassis to the book has a stiffness of 1200ftlbs per degree of twist and 160lbs weight for the welded steel structure. Considering the variations in the book the siffness and weights could vary a bit either way. An X braced ladder frame of 4x2x14gauge for the same size of car could have about 1350ftlbs per degree stiffness and weight of 130lbs. Even allowing for the extra structures needed to support the body work this is still likely to equal the Lowcost book chassis.
There are a few makers of spaceframes out there that could be a lot better off for cost, weight, complexity, stiffness and engine access if they had a decent ladder frame instead.
Some midengined kits are pretty floppy and some of the simple ladder frames under Cobra replicas are actually very stiff even if they do weigh a lot.
For an equally well designed spaceframe and ladder frame the difference for a complete car is probably about 5% in most cases. So for equal stiffness the ladder frame car is 5% heavier or for equal weight the spaceframe is 5% stiffer. For a race car this might be a big factor in favour of a spaceframe. On the other hand the lower cost, reduced complexity and better engine access might be reasons to look at a ladder frame.
Lotus had no qualms about putting a very sophisticated ladder frame monocoque hybrid under the Elise though I reckon this is as far as a ladder frame can go before it actually crosses the line and becomes more accurately described as a monocoque.
Some kit car spaceframes are OK. Caterham make one. Theirs is more than double the stiffness of some of its rivals and is very well put together.
Take a look at ultimav12.ca for a list of chassis mods on the Ultima. This site shows how simple design changes make big changes in chassis stiffness.
The next time someone tells you how amazing their spaceframe is just remember that most of them are less amazing than a decent ladder frame!
Regards,
Chris
I do structural analysis for a living and I've done a lot of finite element analysis on chassis designs for some projects of my own that I'd like to get round to starting one day.
The vast majority of spaceframes are not better than ladder frames! They are simply not triangulated in the right way to get the stiffness advantages they claim.
Take the following example.
Lowcost chassis to the book has a stiffness of 1200ftlbs per degree of twist and 160lbs weight for the welded steel structure. Considering the variations in the book the siffness and weights could vary a bit either way. An X braced ladder frame of 4x2x14gauge for the same size of car could have about 1350ftlbs per degree stiffness and weight of 130lbs. Even allowing for the extra structures needed to support the body work this is still likely to equal the Lowcost book chassis.
There are a few makers of spaceframes out there that could be a lot better off for cost, weight, complexity, stiffness and engine access if they had a decent ladder frame instead.
Some midengined kits are pretty floppy and some of the simple ladder frames under Cobra replicas are actually very stiff even if they do weigh a lot.
For an equally well designed spaceframe and ladder frame the difference for a complete car is probably about 5% in most cases. So for equal stiffness the ladder frame car is 5% heavier or for equal weight the spaceframe is 5% stiffer. For a race car this might be a big factor in favour of a spaceframe. On the other hand the lower cost, reduced complexity and better engine access might be reasons to look at a ladder frame.
Lotus had no qualms about putting a very sophisticated ladder frame monocoque hybrid under the Elise though I reckon this is as far as a ladder frame can go before it actually crosses the line and becomes more accurately described as a monocoque.
Some kit car spaceframes are OK. Caterham make one. Theirs is more than double the stiffness of some of its rivals and is very well put together.
Take a look at ultimav12.ca for a list of chassis mods on the Ultima. This site shows how simple design changes make big changes in chassis stiffness.
The next time someone tells you how amazing their spaceframe is just remember that most of them are less amazing than a decent ladder frame!
Regards,
Chris
Chris,
Some good points, but I am not entirely sure what your point is. People usually rave on about space frames as they are stiff (which you have agreed with) and they are practical for racecar and kit car manufacture.
A good ladder frame design produces a good stiff structure for the chassis to be bolted to and should in theory lead to greater handling characteristics.
Most space frames are designed for more than just stiffness, but also added packaging constraints. Generally speaking the occupants are inside the frame not on the outside of it, which leads to better crash protection.
If you take a look at a ladder frame, generally the body is produced in a much stronger (and heavier) way to give the occupants some sort of protection, and hence the weight goes up hugely.
You mentioned an Elise, although this does have a ladder frame giving it longitudinal stiffness, it has a very sophisticated bonded aluminium structure around it which is where it gets around the packaging/safety issues. I woul dnot consider it to be a ladder frame design though....
A TVR, which has a heavy and very strong ladder frame set up, has a very heavy body to incase the occupants and give the required crash protection.
I agree with a lot of your statements, but I don't consider it to be a con as you put it, the space frames that Westfield, Caterigvan, Sylva, Locost use are there for stiffness, low weight and practicality. I do agree that with a degree of FEA, these designs could be improved hugely. I also think that Caterham have done a lot already in conjunction with Universities around Britain.........
Where do you work by the way?
Some good points, but I am not entirely sure what your point is. People usually rave on about space frames as they are stiff (which you have agreed with) and they are practical for racecar and kit car manufacture.
A good ladder frame design produces a good stiff structure for the chassis to be bolted to and should in theory lead to greater handling characteristics.
Most space frames are designed for more than just stiffness, but also added packaging constraints. Generally speaking the occupants are inside the frame not on the outside of it, which leads to better crash protection.
If you take a look at a ladder frame, generally the body is produced in a much stronger (and heavier) way to give the occupants some sort of protection, and hence the weight goes up hugely.
You mentioned an Elise, although this does have a ladder frame giving it longitudinal stiffness, it has a very sophisticated bonded aluminium structure around it which is where it gets around the packaging/safety issues. I woul dnot consider it to be a ladder frame design though....
A TVR, which has a heavy and very strong ladder frame set up, has a very heavy body to incase the occupants and give the required crash protection.
I agree with a lot of your statements, but I don't consider it to be a con as you put it, the space frames that Westfield, Caterigvan, Sylva, Locost use are there for stiffness, low weight and practicality. I do agree that with a degree of FEA, these designs could be improved hugely. I also think that Caterham have done a lot already in conjunction with Universities around Britain.........
Where do you work by the way?
Be careful with ultimav12.ca for info.
The guy was putting in a non standard v12 engine (much larger and more awkward to fit than a v8) and proceded to chop important bits from his chassis before taking torsional stiffness measurements
That said, the Ultima chassis CAN be stiffened even more, but it is a question of whether it is necessary for intended use. Adding more to the chassis adds weight, and does it need to be much stiffer for fast road use?
The guy was putting in a non standard v12 engine (much larger and more awkward to fit than a v8) and proceded to chop important bits from his chassis before taking torsional stiffness measurements
That said, the Ultima chassis CAN be stiffened even more, but it is a question of whether it is necessary for intended use. Adding more to the chassis adds weight, and does it need to be much stiffer for fast road use?
Avocet said:
Is a torsionally stiff chassis necessarily a good thing?
Yes it is – for at least three reasons. The primary role of the chassis is to hold all the elements of the vehicle together. If the chassis is not stiff then it will bend when an attempt is made to twist it (bit obvious really).
The times when a vehicle is so twisted are when:-
a) under acceleration (due to the torque applied by the engine).
b) under cornering.
c) bumps in the road surface are encountered.
The effects of the above (using the same letters) are (or can be):-
a) Torque steer, which means that putting your boot down affects steering. Usually noticed as a ‘twitch’ requiring steering correction when you take the power off suddenly. Note that this can be induced by other means such as cream crackered suspension bushes.
b) Doors flying open unexpectedly.
c) Scuttle shake or rattles and noises from the bodywork or both.
On a more general note, the precision of handling will be compromised IF the chassis, rather than the suspension, is ‘moving’.
Hope that helps.
Den.
but all chassis twist and bend to a lesser or greater degree.
there are some fine handling cars out there that bend and twist.
ok if its purely by the numbers then a non tist chassis would be best but in the real world a non twist chassis isnt anywhere close yet.
yes ill accept that my locost does twist but is this a problem?im sure it twists on a par with the other 7 alikes as it performs on a par with other 7alikes(i mean similar power/weight =similar track times)
so what?
there are some fine handling cars out there that bend and twist.
ok if its purely by the numbers then a non tist chassis would be best but in the real world a non twist chassis isnt anywhere close yet.
yes ill accept that my locost does twist but is this a problem?im sure it twists on a par with the other 7 alikes as it performs on a par with other 7alikes(i mean similar power/weight =similar track times)
so what?
I believe that Chris was simply stating that alot of people are going to alot of trouble in "designing" a space frame chassis that is overly complex when you compare the results achieved with the space frame vs ladder frame.
Im sure he will be the first to agree a properly designed space frame, ultimately, will be hard to beat in terms of both weight and rigidity, and especially when occupant safety is concerned.
Just my $0.02 worth.
Andy
Im sure he will be the first to agree a properly designed space frame, ultimately, will be hard to beat in terms of both weight and rigidity, and especially when occupant safety is concerned.
Just my $0.02 worth.
Andy
Heres a little game I found, its free, but be warned...its VERY addictive.
Its called Bridge Builder. You are required to build a trussed bridge over different senarios.You analyse the stresses in the members, and then try and drive a train over the bridge youve designed...and see if it holds.
You will learn about triangulation very quickly.
After passing a few levels, you find yourself going back and trying to remove as many redundant members as possible. Heaps of good ,clean fun
www.bridgebuilder-game.com
Enjoy!
Andy
Its called Bridge Builder. You are required to build a trussed bridge over different senarios.You analyse the stresses in the members, and then try and drive a train over the bridge youve designed...and see if it holds.
You will learn about triangulation very quickly.
After passing a few levels, you find yourself going back and trying to remove as many redundant members as possible. Heaps of good ,clean fun
www.bridgebuilder-game.com
Enjoy!
Andy
Dom
I don't think this guy was having a go at the Ultima chassis but pointing out a site where step by step mods are recorded with the gains they did or didn't make to a chassis documented. To me it read as if it was just an example.
Also your not quite right, the site clearly shows the measurements before the original engine mounts were cut away and after.
Saying that I think my Ultima will out handle our Boxter S on or off track, so I'm not that worried anyhow.
I don't think this guy was having a go at the Ultima chassis but pointing out a site where step by step mods are recorded with the gains they did or didn't make to a chassis documented. To me it read as if it was just an example.
Also your not quite right, the site clearly shows the measurements before the original engine mounts were cut away and after.
Saying that I think my Ultima will out handle our Boxter S on or off track, so I'm not that worried anyhow.
I'm obviously missing something here! I thought spaceframes were chassis made out of tubes (broadly speaking) and a "true" spaceframe would be one where all the tube centrelines at any particular joint passed through the same point in space and every bay was a triangle- i.e. a chassis where, if it was all "rose" -jointed instead of welded, it wouldn't be any less stiff. On that supposition, a TVR chassis is a spaceframe (albeit not a "true" one) that has a backbone - and a Caterham is a spaceframe that has a small backbone and side members. I daresay there are other "spaceframes" that have no "backbone" but have tubular structures as sills instead?
I can also understand Kitcarman's reasons for wanting a stiff chassis but having seen a Morgan (arguably the floppiest ladder chassis known to man!) kick ass once or twice in the odd sprint and hillclimb, I'm not sure having a stiff chassis is essential for good handling / roadholding. I'm beginning to think that torsional stiffness only becomes important when you have a suspension set up that has very different roll-stiffnesses at each end of the car. I have to completely agree about the scuttle-shake and general rattles argument though!
I can also understand Kitcarman's reasons for wanting a stiff chassis but having seen a Morgan (arguably the floppiest ladder chassis known to man!) kick ass once or twice in the odd sprint and hillclimb, I'm not sure having a stiff chassis is essential for good handling / roadholding. I'm beginning to think that torsional stiffness only becomes important when you have a suspension set up that has very different roll-stiffnesses at each end of the car. I have to completely agree about the scuttle-shake and general rattles argument though!
doc_fudge said:
Heres a little game I found, its free, but be warned...its VERY addictive.
Its called Bridge Builder. You are required to build a trussed bridge over different senarios.You analyse the stresses in the members, and then try and drive a train over the bridge youve designed...and see if it holds.
You will learn about triangulation very quickly.
After passing a few levels, you find yourself going back and trying to remove as many redundant members as possible. Heaps of good ,clean fun
www.bridgebuilder-game.com
Enjoy!
Andy
Oh how addictive is this!!!!
Nearly caused a sacking and a divorce so far!!!
Avocet,
I completely agree with your observations that floppy chassis can and do outperform those that purport to be better (ie stiffer). The Morgan is something of the exception that proves the point.
This leads to another interesting observation which is that it’s not simply a matter of what type of chassis, but how well it’s designed. Quality isn’t restricted to stiffness (ask the misses
) but covers a lot of other aspects. For example if the steering geometry or roll centres are wrong the chassis will be as good as useless in respect to handling.
I think that Chris was saying that a well designed ladder frame can be as good as a poorly designed space frame. However, I don’t think there’s any dispute that a good space frame design is unbeatable except perhaps by a hi-tech composite monocoque. Others have also made the point that a space frame adds numerous benefits such as occupant protection and places to solidly mount things such as seat belts, steering column and everything else above ankle height.
Finally, the Ultima chassis like anything else mechanical could be made stronger. However, it’s clearly strong enough just as it is. It really would have been more constructive IMHO if Chris had chosen, as an example to illustrate his point, a space frame chassis that was genuinely lacking stiffness.
Den
I completely agree with your observations that floppy chassis can and do outperform those that purport to be better (ie stiffer). The Morgan is something of the exception that proves the point.
This leads to another interesting observation which is that it’s not simply a matter of what type of chassis, but how well it’s designed. Quality isn’t restricted to stiffness (ask the misses
) but covers a lot of other aspects. For example if the steering geometry or roll centres are wrong the chassis will be as good as useless in respect to handling. I think that Chris was saying that a well designed ladder frame can be as good as a poorly designed space frame. However, I don’t think there’s any dispute that a good space frame design is unbeatable except perhaps by a hi-tech composite monocoque. Others have also made the point that a space frame adds numerous benefits such as occupant protection and places to solidly mount things such as seat belts, steering column and everything else above ankle height.
Finally, the Ultima chassis like anything else mechanical could be made stronger. However, it’s clearly strong enough just as it is. It really would have been more constructive IMHO if Chris had chosen, as an example to illustrate his point, a space frame chassis that was genuinely lacking stiffness.
Den
Mark B, TVR's of the Chimaera generation are space frame. I've been to the factory a couple of times and it's remarkable how similar to kit cars the construction is. Just make sure Danielle is taking you around the factory when you visit (if your ever decide to), let's just say she's a bit 
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