118 replies to this topic

[SukiDawg]

Most seem to come in around the 500 to 600kg mark.

My weight target is 500Kg, so far according to my spreadsheet of component weights etc I'm a little over that already, but there are still a lot of estimated parts in there.. plus quite a few that are missing altogether (rear suspension mostly because they are not designed yet - hence my weight split is a little to the front).

With driver and passenger (and my passenger come co-driver come mate come garage assistant is quite a big lad) I'm all in at 670Kg. Really hoping to get this down though.

A good addition to your project would be to create a spreadsheet like this to get yourself a weight estimate up front of the finished design. Its straightforward enough, list all the parts of the car, and weigh those that you can, estimate those that you can't. The total will give you an estimated car weight, which is useful for lots and lots of things while you are designing. Not least of which suspension design i.e. specifying your spring rates etc.

Also, if you take (for example) the rear axle centreline as your datum, have a column for the distance between each component of the car and the rear axle centreline, by taking moments about the rear axle, you can come up with an estimate for the position of the centre of gravity - i.e. the front rear weight split. I've also got a column for the height of each component, and this allows me to get a an idea of the height of the centre of gravity (used to calculate roll moment, and therefore predicted roll angle in cornering). This helps with specifying roll bar stiffness and the like. It also gives you an idea of where to put battery, fuel tank etc to manipulate your distribution. About the most significant mass is your driver / passnger, and small movements can make a big difference when it comes to the seats.

To do a good engineering job there should be as few mysteries as possible, and capturing this kind of information avoids the unknowns.

Here's a screendump of what I am using.



The colours of the cells are a guide to whats a fact and whats estimated. Green cells are actual weights which I know are correct, amber cells are estimates, and anything in red is missing. It helps me to see at a glance where I am and how trustworthy the information is. Still very amber though!

This screendump gives you an idea of the number of parts involved:



Which University are you studying at?


Dave.

Edited by SukiDawg, 30 October 2009 - 01:47 PM.

[basher]

whos making the casing for the diff for you? and how much is it costing if you dont mind me asking?

also you say its not going to be sva tested, if its going on the road surely you'll be dam lucky to get away with it, as when you send your log book back with changes it usually triggers a test if a couple of things are changed i believe, i think a lad who bought a car which had been zcars converted is going through this problem, but he has zcars to help him and even they said he will be lucky.

[Sean12]

dave, very good points and yes all those things are to be considered. had many many lessons on stress and dynamics, including bending moments and forces in all directions. A spread sheet is a very good idea and something i had planned to do along the line of the project. I have already got a word doc (on going) with weights/rough weights of parts to be put in the car. And it is good to see you have done yours very thoroughly, even to the point of putting the weight of the intercom etc lol i will be doing somethin along this line, but maybe not that precise

basher, the diff case is being made by a friend of ours who owns an autograss engineering firm and has come across many of these builds before an has many of his own (including a very nice twin blackbird engine - frighteningly fast ). Not sure of the price as may be mates rate. Will post pics when i get it back. I am now just looking into getting a few sprocket sizes to be ordered by him for me, ideally one more acceleration focused and the other top end.

And oh yeah my bad on the typo on the reverse post what i meant to say was..... i will not be using a mechanical reverse as the car will not need one to be SVA'd. The car will have an electric reverse using a flywheel and starter motor, powered by a foot switch. As this will meet the new sva standards (well if we want to be technical,the now IVA standards). how long that reverse last's is another story lol

[SukiDawg]

Well, strictly speaking the techniques used are less advanced than stress or bending moment calculations, just a plain load analysis - but if you think you are confident with that kind of stuff this shouldn't be a problem.

You're right - something like an intercom is a small mass on its own, but 5 things like that add up to 5kg - you wouldn't leave something that weighed 5kg off so why not capture everything? Beauty of excel is you only need to enter a formula once, so its as easy to add many things as it is to add one. Perhaps we could trade a few weights for standard bits and pieces... Accuracy is the key to success, and assumption is the mother of all cockups.....

Where are you studying?

Edited by SukiDawg, 31 October 2009 - 11:50 AM.

[Sean12]

sukidawg, i would say im pretty confident on the bending and stress analysis side of things (well i hope so or i should prob quit my course now lol) so should hopefully be ok-ish lol, and if i am not lol i have a whole faculty of teachers willing to help me, including motorsport engineering teachers (one of which happens to be my mentor for the project). I also have access to lots of stress software such as algor etc. I have already started CAD modelling a frame (worked in CAD for 5 years before uni), that is an on going process, which i will run through algor to test it against all the stresses the car will come under. The frame will then be built and non destructively tested against these stresses, where strain gauges will give me an idea of the stresses in certain areas of the frame. This will give me an idea if anywhere needs modifying. As part of my coursework i will be doing a large literature review on chassis design and will incorporate this into the design of the frame, along with advice from motorsport lectures and also experience from friends.

as far as weight, you are very true it does all add up. you going to be using the intercom for competetive racing/rally/hillclimbs ? or just social use lol dont think im going to bother not really ever needed one on track days so think i will give it a miss, less weight lol anythin i dont really need will be swiftly taken out lol however the girlfriend has already put in a request for ear defenders lol also for the weight issues we quite handily have a set of corner weight scales at uni so this will be heavily used i imagine lol. When the frame is in i will use it to balance the weight up and for the setting up of the suspension, adjusting the coilovers and shocks etc

i am studying Mechanical Engineering and Bristol UWE University.

p.s been watchin your progress on the R1 mini, its coming along very very nicely, really like the work !! some of the CAD is looking very nice, must have been fairly time consuming. hopefully worth it in the end though. Going to be a sweet motor !

Edited by Sean12, 03 November 2009 - 02:34 PM.

[SukiDawg]

I wouldn't get too focused on the fact there is an intercom in my list. I was just brainstorming out all the parts that may or may not be on the car.

Tarmac rallying is a possibility of course.

Yes - my project is inching along. I'm an engine specialist really, so a lot of the vehicle related stuff is new ground - certainly keeps it interesting. I'm currently getting distracted easily though - guess its because I'm on the hard bit (suspension)!


What CAD package are you going to be using? Solidworks at a guess?? That's what most academic institutions seem to have access to these days.

Keep it up chap!

[Sean12]

Ive been using AutoCAD mainly, but will be running my stess analysis through Algor or Ansys. Although i have been thinking about maybe re-modelling it using Solid Edge v20. But not sure yet, we will see.

What program have you been using ?

[Sean12]

spoke to the guy making my chain converted diff today, just talkin about progress. He is in the process of sealing the open area of the diff, incoroporating a bolt into the casing to allow access for grease etc. bearings are on order along with seals, orings etc. Also some race grade aluminium is being used for the billets holding the bearings, these will also be designed to allow them to double up as a chain tensioner. The main problem with this diff is that the output shafts are held in by circlips inside the diff, this will be a complete pain to ever remove if needed as the void will be sealed, so he is going to remove the clips and machine the casing so that the circlips can be put on the outside, to allow access to them for if they are ever wanting to be removed.

the next area we talked about was inboard brakes, i have really come round to this idea and think i will deff try an do this, its a little different to most rear bike engined minis out there so could be quite cool, plus has its advantages (reducing unsprung mass etc). I will be using motorbike disks (as light/lightened as much as poss) and callipers (4 pot). This will also have the advantage of not needing to run brake lines down the swing arms and can be kept primarily to the frame so nice an neat (and will hopefully look fairly smart lol).

he also has a spare starter motor and ring gear from a chevvy. He was saving this for his next project but kindly said i could have it so i thank him for that. It is from an automatic, which apparantely is more ideal as it is susposed to be lighter !?! (which i dunno but im goin to take his advice )

last thing was talking about sproket sizes and what size to order, goin to be looking at my spread sheet in detail in the next couple of days to figure out a good size, maybe even two, for top end/crusing and one for acceleration. From when i first looked i think the sprockets were around 14 to 54 but i will check. The sprockets will be split sprokets, again for ease of changing for track use etc and not having to take the whole thing apart.

while i am on the topic does anyone have any wheel dimensions for superlites with tyres (ideally YOKOHAMA A539 175/50X13 but any tyre dims will be greatly recieved), i am after the outer diameter of the tire? any info would be fantastic !

cheers guys, will update with photos soon.

[bikem1ke]

Outer diameter of 175/50/13 is 505.2mm sean.
basically as the tyres are 50% profile height you get 175/2=87.5mm is the height of the sidewall. But as you want the overall diameter the height of both sidewalls will be 175mm, then 13in in mm is 330.02mm so 330.02+175= 505.02mm. Simples.

[Sean12]

nice shout mike, i like it. cheers bud

[SukiDawg]

Regarding your ring gear, if from an automatic transmission then the statement "it will be lighter" will be because its a torque plate rather than a flywheel. As you know I guess, an automatic transmission doesn't use a clutch and flywheel arrangement like a manual transmisson, it utilises a torque convertor. The torque plate that the convertor mounts to usually carries the ring gear for the starter, and is a lot thinner than a flywheel as the rotational inertia of the TC makes up for it.

Regarding your idea of inboard brakes and making use of motorcycle calipers, be a little bit careful. I'm not saying it won't work, but the rotational inertia of your rotating parts (driveshafts, wheels, diff etc) are going to be a lot higher than a motorcycle. Your using two calipers of course, but the vehicle mass is also higher, so you may get beyond the capability of the motorcycle bits.

As you say, its part of a project for your course as well as something for yourself - so I'd reccomend justifying your choice with a bit of science to maximise your marks. Carroll Smith's book "Tune to Win" (ISBN: 0-87938-071-3) has a good section on braking system design and matching front to rear balance - you can probably order it from your University library on inter-library loan if they don't have it already. Its also got masses of other great information for building race cars, so a good read of it would help in lots of ways.


I know you have lots of advice from your lecturers, but you might ask yourself - if they know everything how come they are teaching and not making their money in professional racing teams?

[bikem1ke]

Well bike brakes from say a hayabusa on the front are designed for stopping it with 2 people on it, it will have been designed for 180kg of passenger plus the rough weight of the bike being about 220kg so thats 400kg that those front engined bikes are meant to take. A rear engined mini will be say 780kg with 2 passengers. So if you could acheive 50/50 brake force split then the front hayabusa calipers would be suitable for use as rear brakes. But in reality your more likely to be looking at a 60/40 brake split making the force needed on the rear less. But you do need to think about cooling etc of bike brakes, those discs on the front of the bike are getting high airflows which you won't have in the middle of your car....

[SukiDawg]

I agree with the cooling point...

Its a very simplistic view you've presented though. Of course, I don't speak from the vaulted position of an undergraduate student its true, but I'm also not saying it won't work - just suggesting a note of caution and the value of a reasoned approach.

You have to consider rotational inertia of the parts in question as well as the mass of the vehicle, and here that will be very significant given the two different drive arrangements between bike and bike engined car. You don't just have to slow the speed of the vehicle in a straight line - you also have to slow the rotational speed of the drive bits. There's quite a bit in the diff, and what about a bike rim and tyre vs. a car rim and tyre?

Your 60/40 approximation of brake force distribution is also a bit questionable - I did suggest your mate Sean read the Carroll Smith book for a more accurate view of this sort of engineering problem.... How effective your rear brakes are depends greatly on the anti-dive characteristic of the suspension, the weight distribution (as I was harping on about before), suspension geometry in droop and several other factors too. Its just as dangerous (perhaps more) to have too much braking on the back as not enough.

Sorry - I don't mean to be confrontational, but - if it helps - I hold 1st class MEng in Automotive Engineering from Loughborough, I've already built several cars, and I've been 10+ years working as a designer in professional motor sport. Not grass tracking - the stuff you watch on the telly Sunday afternoon... :*

If you want I'll let you crack on, but actually I'm just trying to give you a few pointers to help.......

PS: What's a front engined bike?

[bikem1ke]

I think your input is great Sukidawg and it's a real help towards many projects and showing people how things should be done properly, and yes sean needs to analyze and justify everything he is doing if he wishes to do well in his Project and gain marks and also end up with a safe car at the end of it all. Front engined bike is a bad bit of typing by me, i apologise i was just trying to make the point that there are bike calipers out there that would be up to the job if chosen correctly. Who do you work for anyway? Ilmore? MB HPE?

[SukiDawg]

I'm freelance these days....