Power Losses when retarding Ignition
Discussion
LeeSpeakman said:
Gavin, drivetrain / suspension is not my department, but im sure the guys will be doing all they can to optimize the mechanical grip / squat. At last years event none of the cars seemed to get out of 2nd gear. I hope we use a shorter gear ratio this year. As well as the TC the system will automatically shift the gears at optimum points in each gear if I get my way!
Im not sure if we are going to use a locked axle or an lsd. Last years car used an lsd so im assuming we will go down that route again. Do you think there will be much advantage going for a solid rear axle apart from weight? The event is held on a tight go-kart track at brunters.
The correct way to optimise the tyres is to use a pyrometer to determine the pressures based on straight line running, then tune the geometry and anti roll bars etc afterwards. This ensures you get all of the grip out of they tyre and ensure the car is balanced.
As far as LSDs go most people have so little understanding of them and how to tune the plate settings that in many cases you are better off with no diff whatsoever. This works on karts and on a lot of competition cars.
If you are barely using the gears then something is seriously wrong with the gearing choice. My view is that you need two lights for the driver to see, a red and a green. Red says change down and green says change up. Just remember that every change is taking about 0.6 seconds and there is a case for fewer changes if you cannot make the time up by increased acceleration.
I left university long before they did Formula Student, but I try to follow it fairly closely and take a fair bit of interest in the Formula SAE cars here in Detroit. While the cars are interesting, they show a complete lack of prioritisation in what is needed to win. When you look at a kart that is successful what you will find is that they pay attention to a number of details: Seat position to get the lowest polar moment of inertia, tyre position and bumper tightness, which is roll stiffness, tyre pressure, gear ratio and the engine comes from a tuner who extracts every bit of power possible.
In the case of FSAE I reckon it is best to have a crude, lightweight chassis and the minimum of suspension. The tyres should be the grippiest hillclimb tyres available, new every competition. Wheels should be the lightest and widest allowable. You should have two engines, one for the competition and one to practice on, and set the CR to the very highest that yields most power. If you are limited by gear ratio choice choose cam profiles that match the gearing in terms of torque curve. Keeping it very simple and reliable may not look elegant but it can make a win. In terms of the driver you also need somebody of the level who can win a club kart race and give feedback on what the car is doing.
Hope this helps.
Alex - Im at the University of Liverpool. I think last year was our first year back after a long break. Our car was nicknamed "The Beast" weighing in at 300Kg+ dry! We had M12 nuts everywhere.
We are hoping to get the car done early so we can do some testing. The system should be fairly optimized. Once I get some data from our simulation software the engine / drivetrain inertia and the wheel type, I should be able to get the TC setup on Simulink.
I agree it might not be much use in the dry, but the supercharger should give us extra low down torque, and if it rains then it will make a big difference.
Green - The clutches on these things arent very easy to modulate, I think we are using a clutch lever on the end of the gear change so that makes it harder. Its something I will look into though.
Gavin - Your sound like a voice of common sense and experience. We are trying to get our car alot lighter than last years, but I hope we dont try anything 'too trick' to make it faster then it doesnt work. "To finish first, first you must finish." I have confidence in the Mechanical Boys they will do a good job. Just a good proven design thats easy to setup should do us well.
I dont think we have any people that have raced at club level, but I used to go a few times a week at college and consider myself fairly quick on a kart ( Dont we all!) I also used to race petrol Radio Controled cars so I have a good idea how to give feedback and how a setup changes a car. Id be honoured if I could drive at this years event!
Edit: Spelling mistakes
We are hoping to get the car done early so we can do some testing. The system should be fairly optimized. Once I get some data from our simulation software the engine / drivetrain inertia and the wheel type, I should be able to get the TC setup on Simulink.
I agree it might not be much use in the dry, but the supercharger should give us extra low down torque, and if it rains then it will make a big difference.
Green - The clutches on these things arent very easy to modulate, I think we are using a clutch lever on the end of the gear change so that makes it harder. Its something I will look into though.
Gavin - Your sound like a voice of common sense and experience. We are trying to get our car alot lighter than last years, but I hope we dont try anything 'too trick' to make it faster then it doesnt work. "To finish first, first you must finish." I have confidence in the Mechanical Boys they will do a good job. Just a good proven design thats easy to setup should do us well.
I dont think we have any people that have raced at club level, but I used to go a few times a week at college and consider myself fairly quick on a kart ( Dont we all!) I also used to race petrol Radio Controled cars so I have a good idea how to give feedback and how a setup changes a car. Id be honoured if I could drive at this years event!
Edit: Spelling mistakes
Edited by LeeSpeakman on Sunday 1st October 20:08
Few quick helpers for you,
Make sure you get as light as possible, our car was 220kg which was one of the lightest spaceframe cars about, but we still got slated by the judges for being too heavy. Tell your chassis boys not to be too bothered about stiffness, I went all out on ours and used structural sidepods and fully stressed engine and we had about 7000+nm/deg - however the judges werent even that interested, just go for a light minimalistic design. The chassis needs to weight sub 20kg, mine was 26 kg inc front and rear which was the lightest we have ever made at Lancaster, but again the judge (he was a bit of a t
t) said that it was too heavy even though a lot of other car were comfortable heavier than ours as in 40kg+ for chassis alone!
We are hoping to get the car done early so we can do some testing.
This is the most important thing by a huge amount but I will be honest, hardly any uk teams with a smallish team (we had 9) ever finish it early enough for proper testing, it is just such a huge task if you have never done it before. So push everything as quickly as you can as it is absolutely vital that it is finished early.
You using a rotrex supercharger?, we used one 2 years ago and it blew up, take it you are using a single if you are considering supercharging, just dont get too hung up on the complexity as when you get to the competition you will realise that just having a working finished car is more than most teams can hope for!
Again we used a hand clutch 3 years ago and have never used one since as they are notoriously difficult to use, the drivers just end up stalling all the time.
Yep, exactly the right way to look at it, however this is easier said than done when poeple are doing final year projcts and want to get as good marks as possible so they always try and add extra flair here and there.
Our team decided to split the driving up between the members, but too be honest getting a decent driver is the more sensible option and benefits your times a lot more. However we decided that we had devoted so much of our lives to it over the previous 9 months that we would rather drive it which is fair enough really.
Sorry about the longish post but the event took over my life to such a degree last year that I literally didnt think about much else, it is great fun and very rewarding but very very very very very hard.
Good luck
Alex
LeeSpeakman said:
Alex - Im at the University of Liverpool. I think last year was our first year back after a long break. Our car was nicknamed "The Beast" weighing in at 300Kg+ dry! We had M12 nuts everywhere.
Make sure you get as light as possible, our car was 220kg which was one of the lightest spaceframe cars about, but we still got slated by the judges for being too heavy. Tell your chassis boys not to be too bothered about stiffness, I went all out on ours and used structural sidepods and fully stressed engine and we had about 7000+nm/deg - however the judges werent even that interested, just go for a light minimalistic design. The chassis needs to weight sub 20kg, mine was 26 kg inc front and rear which was the lightest we have ever made at Lancaster, but again the judge (he was a bit of a t
t) said that it was too heavy even though a lot of other car were comfortable heavier than ours as in 40kg+ for chassis alone! LeeSpeakman said:
We are hoping to get the car done early so we can do some testing.
This is the most important thing by a huge amount but I will be honest, hardly any uk teams with a smallish team (we had 9) ever finish it early enough for proper testing, it is just such a huge task if you have never done it before. So push everything as quickly as you can as it is absolutely vital that it is finished early.
LeeSpeakman said:
I agree it might not be much use in the dry, but the supercharger should give us extra low down torque, and if it rains then it will make a big difference.
You using a rotrex supercharger?, we used one 2 years ago and it blew up, take it you are using a single if you are considering supercharging, just dont get too hung up on the complexity as when you get to the competition you will realise that just having a working finished car is more than most teams can hope for!
LeeSpeakman said:
The clutches on these things arent very easy to modulate, I think we are using a clutch lever on the end of the gear change so that makes it harder. Its something I will look into though.
Again we used a hand clutch 3 years ago and have never used one since as they are notoriously difficult to use, the drivers just end up stalling all the time.
LeeSpeakman said:
Your sound like a voice of common sense and experience. We are trying to get our car alot lighter than last years, but I hope we dont try anything 'too trick' to make it faster then it doesnt work. "To finish first, first you must finish." I have confidence in the Mechanical Boys they will do a good job. Just a good proven design thats easy to setup should do us well.
Yep, exactly the right way to look at it, however this is easier said than done when poeple are doing final year projcts and want to get as good marks as possible so they always try and add extra flair here and there.
LeeSpeakman said:
I dont think we have any people that have raced at club level, but I used to go a few times a week at college and consider myself fairly quick on a kart ( Dont we all!) I also used to race petrol Radio Controled cars so I have a good idea how to give feedback and how a setup changes a car. Id be honoured if I could drive at this years event!
Our team decided to split the driving up between the members, but too be honest getting a decent driver is the more sensible option and benefits your times a lot more. However we decided that we had devoted so much of our lives to it over the previous 9 months that we would rather drive it which is fair enough really.
Sorry about the longish post but the event took over my life to such a degree last year that I literally didnt think about much else, it is great fun and very rewarding but very very very very very hard.
Good luck
Alex
Edited by bales on Sunday 1st October 22:59
Alex thanks for the advice. I know its going to be really tough. I havent thought about much else all summer. Electronics is really broad based and after doing it for 6 years I still feel im starting from scratch with this project. Our team current has 20+ members so I hope the workload will be less.
I see what your saying about the flair as its some students final year project, there is nothing I can really say about that as its not my department. We have a few Mech Engineers from last years car on the team so Im sure they will have some words of wisdom that can only be gained from being invovled in such a project.
Exciting times! I love racing and hope to do some clubman circuit racing or rallying when I finish uni.
Lee
I see what your saying about the flair as its some students final year project, there is nothing I can really say about that as its not my department. We have a few Mech Engineers from last years car on the team so Im sure they will have some words of wisdom that can only be gained from being invovled in such a project.
Exciting times! I love racing and hope to do some clubman circuit racing or rallying when I finish uni.
Lee
GreenV8S said:
If you going light, remember that you only need torsional stiffness if you're shunting lots of weight transfer from one end of the car to the other. If you're going for relatively even distribution of roll stiffness, you don't need a torsionally stiff chassis.
Dont want to get into a massive discussion about this but thats really not correct. My entire final year project was about design and optimisation of a spaceframe chassis and torsional stiffness is vitally important.
My point was that in terms of the event and how the car will be driven (and judged) it doesn't need to be mega stiff but in terms of the design of the car it is the most important aspect. If the chassis ain't stiff the suspension isn't working against a hardpoint and you may aswell pluck your suspension geometry out of thin air as it will change so much during cornering.
We made the mistake 3 years ago of making a basic twin tube style ladder chassis with moderate triangulation and it had a MASSIVE effect on the handling, it was an absolute pig to drive. All the energy that gets generated when cornering gets transferred into the chassis as twist and then suddenly untwists itself as you leave the corner and you have very snappy handling giving massive understeer or oversteer.
The roll stiffness really doesnt enter the equation as the amount of energy placed into the chassis during lateral acceleration will be constant regardless of the roll stiffness of the car.
The question really needs to be how stiff is stiff enough, and if you read some race car design texts a common figure for a minimum is usually about 4000nm/deg+ for a decent single seater car. However you will NOT get this figure without doing pretty comprehensive triangulation (as in every plane) but whether structural sidepods are needed is perhaps a better question. I would tend to say now that they are not.
Anyway sorry not really on topic, but as this is the one area that I have spent A LOT of time on (read as 200 page dissertaion), and am relatively confident that I know my onions on I thought I should reply.
Alex
Edited to add, however we should really say that stiffness-to-weight ratio is actually the most important factor, but thats still not an excuse for having a floppy chassis
Edited by bales on Monday 2nd October 01:36
It's pretty clear that bales knows what he is on about with respect to designing the chassis. Honestly, I don't think it matters if it is 'overweight' to the degree he mentions but it needs to be stiff.
One of the areas that is most important is to look at what the regulations tell you that you HAVE to do, and then you do nothing more than the bare minimum. That saves weight. Use the lightest wheels and tyres, keep the suspension simple and lightweight, don't worry about driver comfort, lose instrumentation, keep cooling marginal, make the exhaust super light and you will get there.
As far as the driver goes this really is an area where egos should be set aside - it is all about winning so get the best driver.
One of the areas that is most important is to look at what the regulations tell you that you HAVE to do, and then you do nothing more than the bare minimum. That saves weight. Use the lightest wheels and tyres, keep the suspension simple and lightweight, don't worry about driver comfort, lose instrumentation, keep cooling marginal, make the exhaust super light and you will get there.
As far as the driver goes this really is an area where egos should be set aside - it is all about winning so get the best driver.
You need to have the suspension stiffly connected to the mass. That requires some beam stiffness if the mass isn't over the axle. Obviously you need to control the geometry of the suspension, most of the forces that are applied can be resolved against the other end of the axle. I'm not saying you can replace a stiff chassis with a piece of spaghetti without any ill effects. I'm just saying that you don't need much torsional stiffness unless you are trying to move weight transfer around. To take an extreme axample, if you cut your chassis in half and put a hinge in that allows the back to roll relative to the front, but prevents any other relative movement, then the only real difference it makes is that you can't use the chassis to move the weight transfer around. Even if you need to do that, you can usually achieve a lot by carefull suspension of the main masses (engine/trans assembly, and diff) within the chassis. If you design the car to work without torsional stiffness, you don't need it.
GavinPearson said:
It's pretty clear that bales knows what he is on about with respect to designing the chassis. Honestly, I don't think it matters if it is 'overweight' to the degree he mentions but it needs to be stiff.
One of the areas that is most important is to look at what the regulations tell you that you HAVE to do, and then you do nothing more than the bare minimum. That saves weight. Use the lightest wheels and tyres, keep the suspension simple and lightweight, don't worry about driver comfort, lose instrumentation, keep cooling marginal, make the exhaust super light and you will get there.
As far as the driver goes this really is an area where egos should be set aside - it is all about winning so get the best driver.
One of the areas that is most important is to look at what the regulations tell you that you HAVE to do, and then you do nothing more than the bare minimum. That saves weight. Use the lightest wheels and tyres, keep the suspension simple and lightweight, don't worry about driver comfort, lose instrumentation, keep cooling marginal, make the exhaust super light and you will get there.
As far as the driver goes this really is an area where egos should be set aside - it is all about winning so get the best driver.
Yep I agree completely with the majority of that, the driver thing is very true but its hard sometimes to let someone else crash it (its happened a few times) when you have spent a long time building something. But I supposes thats just a bit selfish as you say - if you want to do aswell as possible you NEED a decent driver, the level ast FSUK seems to have gone up considerably over the last few years so its no longer enough to just finish if you want a decent position.
GreenV8S said:
If you design the car to work without torsional stiffness, you don't need it.
Sorry but nope!
Incidentally green, I seem to be disgreeing with you on quite a few threads here, I hope you don't mind, I'm not being awkward for the sake of it. You seem to be a pretty knowledgable chap so it is interesting discussing these things with you even if I do disagree.
I suppose that this point on torsional stiffness ultimately comes down to our differing views on load transfer......
Edited by bales on Monday 2nd October 10:25
bales said:
GreenV8S said:
If you design the car to work without torsional stiffness, you don't need it.
Sorry but nope!
Incidentally green, I seem to be disgreeing with you on quite a few threads here, I hope you don't mind, I'm not being awkward for the sake of it. You seem to be a pretty knowledgable chap so it is interesting discussing these things with you even if I do disagree.
I suppose that this point on torsional stiffness ultimately comes down to our differing views on load transfer......
Happy to 'chat' about it, whether you agree or disagree. I find that listening to differing view points is the best way to learn.
I've got some practical experience of vehicles with relatively low torsional stiffness which is why I say it can work - but the less stiffness you have the more work you have to do to get the chassis to work properly. You certainly don't want to make the chassis soft just for the sake of it, but torsional stiffness is not essential to good handling if the chassis is designed accordingly. (If torsional stiffness is important to you then you're probably considering a tetrahedron based X frame rather than a conventional space frame.)
GreenV8S said:
Happy to 'chat' about it, whether you agree or disagree. I find that listening to differing view points is the best way to learn.
I have incidentally got a much greater understanding of weight transfer and roll centres etc.... since joining these discussions than I did at uni as I have to think loads about what I am writing and where the theory comes from. So whether I am right or wrong I am certainly learning a fair bit.
GreenV8S said:
I've got some practical experience of vehicles with relatively low torsional stiffness which is why I say it can work - but the less stiffness you have the more work you have to do to get the chassis to work properly. You certainly don't want to make the chassis soft just for the sake of it, but torsional stiffness is not essential to good handling if the chassis is designed accordingly. (If torsional stiffness is important to you then you're probably considering a tetrahedron based X frame rather than a conventional space frame.)
When I think about a spaceframe I think about a tetrahedon not some abominations that some (actually most) kit car manufacturers seem to call spaceframes. i.e a structure in pure tension or compression
I agree you can get a car to handle well without being intrinsically stiff, but since it is always harder to do this (but in the case of a road car there isnt a huge amount you can do to increase stiffness massively so you have to do the best with what you are given which I guess is your point)
The way I see it is there is no downside to stiffness oo-er! other than perhaps a small weight penalty depending how well designed it is, but it just makes everything else so much easier especially tuning the car to handle how you want and the added ease of adjustability.
This is where our viewpoints differ I guess, you have experience playing with road cars where you do the best with what you are given, and I was tasked with building something from scratch that had a defined purpose. Polar opposites really.
'Stiff' is a relative term and so is 'handling'.
The issue regarding handling is that it is really a measure of chassis balance amongst the peers of the subject vehicle. So when you turn in provided there is sufficient force generated from the tyres to overcome the vehicle inertia it would 'turn in beautifully' and provided there is sufficient grip at the limit from both axles it would 'be really nicely balanced at the limit'.
But we are not talking about a road car, instead we're talking about something where inferior design will show up and ultimate grip is important, or the lap time will be poor.
One of the fundamentals is providing a low polar moment of interia, and the other is making sure the centre of polar mass is optimal for the tyres chosen.
One way to fix this is to use wishbones that are A shaped, italic A, and reverse italic A. By selecting a set you can vary centre of polar mass relative to the wheelbase and cure inherent oversteer / understeer. This is SO important when you use control tyres. The other thing is you can't fix a problem by putting mass at the end of the vehicle, because that creates inertia.
A good Formula car has this right, mostly designed in after years of iterative design. But understanding the concept is important because after one test session you can probably get it right.
If you design a car with wings you would get this part sorted for low speed and then select wings that gave downforce with a centre of pressure befitting the tyres. That isn't necessarily the same as centre of mass, as tyres react differently to load and the grip to load profile may be different for each axle set of tyres.
As far as 'stiffness' goes, you need a combined sidewall / wheel rate such that you maximise contact patch grip and are able to go over bumps without hopping. However, if you make the suspension soft you cannot optimise tyre contact patch. Some people advocate a high roll centre to give the roll stiffness, or a stiff anti roll bar, but both of these have downsides - high roll centres give jacking, which is a disaster on cars with controlled aero packages, and stiff anti roll bars give high single wheel bump rates that don't match the damper rates.
In a tuned road car - say an MG Midget which has the lowest body torsional stiffness I know of, it's lap time is a package, that is aerodynamics (CdA), power, weight, gearing, tyres, wheels, wheel rates. Compared to a lot of cars it has a lot of tuning parts available to optimise everything apart from the wheel rates, and those are just about adequate to make it quite competitive on the track. However if you took the same looking car, made a stiff spaceframe but with existing geometry, mass distribution and weight, the spaceframe would be better placed to get the most out of the tyres from both geometry and because the wheel rate wouldn't be a function of chassis flex.
The issue regarding handling is that it is really a measure of chassis balance amongst the peers of the subject vehicle. So when you turn in provided there is sufficient force generated from the tyres to overcome the vehicle inertia it would 'turn in beautifully' and provided there is sufficient grip at the limit from both axles it would 'be really nicely balanced at the limit'.
But we are not talking about a road car, instead we're talking about something where inferior design will show up and ultimate grip is important, or the lap time will be poor.
One of the fundamentals is providing a low polar moment of interia, and the other is making sure the centre of polar mass is optimal for the tyres chosen.
One way to fix this is to use wishbones that are A shaped, italic A, and reverse italic A. By selecting a set you can vary centre of polar mass relative to the wheelbase and cure inherent oversteer / understeer. This is SO important when you use control tyres. The other thing is you can't fix a problem by putting mass at the end of the vehicle, because that creates inertia.
A good Formula car has this right, mostly designed in after years of iterative design. But understanding the concept is important because after one test session you can probably get it right.
If you design a car with wings you would get this part sorted for low speed and then select wings that gave downforce with a centre of pressure befitting the tyres. That isn't necessarily the same as centre of mass, as tyres react differently to load and the grip to load profile may be different for each axle set of tyres.
As far as 'stiffness' goes, you need a combined sidewall / wheel rate such that you maximise contact patch grip and are able to go over bumps without hopping. However, if you make the suspension soft you cannot optimise tyre contact patch. Some people advocate a high roll centre to give the roll stiffness, or a stiff anti roll bar, but both of these have downsides - high roll centres give jacking, which is a disaster on cars with controlled aero packages, and stiff anti roll bars give high single wheel bump rates that don't match the damper rates.
In a tuned road car - say an MG Midget which has the lowest body torsional stiffness I know of, it's lap time is a package, that is aerodynamics (CdA), power, weight, gearing, tyres, wheels, wheel rates. Compared to a lot of cars it has a lot of tuning parts available to optimise everything apart from the wheel rates, and those are just about adequate to make it quite competitive on the track. However if you took the same looking car, made a stiff spaceframe but with existing geometry, mass distribution and weight, the spaceframe would be better placed to get the most out of the tyres from both geometry and because the wheel rate wouldn't be a function of chassis flex.
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