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Post by hayasa on Mar 9, 2010 20:58:11 GMT
I am in the process of designing a gravity racer but, though much of the concept is sketched out, I have dilemma…do I need suspension or will rigidly mounted axles work just as well?
My limited motorsport experience taught me to make the chassis as stiff as possible so that the suspension can work as intended. If I take the rigid route do I just ‘design’ some flex into the chassis to cope with camber changes etc?
From the pictures and videos I have seen, cars like Night Train and Centa Bavaria appear to be rigid, others like Cairngorm have full wishbone setups yet their performance is similar. I realise there are many more factors that determine a cart’s performance but, never having driven either type of gravity cart, I would be interested to hear your views.
Thanks,
Steve
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Post by woody on Mar 9, 2010 22:10:38 GMT
My part finished build is rigid frame , took it out last weekend for a rolling test and any slight camber change and it is a 3 wheeler ! , also i used 15 degrees KPI and as the steering turns one wheel rises the other drops ,so again 3 wheeler ! i need to do a proper downhill test to see if this is going to be a problem , if so maybe i will need some sort of suspension
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Post by leew on Mar 10, 2010 7:49:49 GMT
I was faced with this decision when building my cart. In the end I went for ridged frame. I'm using 18 degrees of KPI and 10 degrees of caster and have noticed no issues with handling. The chassis does have a small amount of passive flex. One big issue with suspension is the drag caused by the wishbones and all the suspension gubbins sticking out of the body. If you have a look at this picture you can see that I have kept bits outside the body to a minimum, I have placed the track rods and brakes directly behind the box section to keep them out the airflow. picasaweb.google.com/djmogy/Cadwell2009?feat=directlink#5429861139848329810Also unless you use someform of anti-roll bar then your cart will have a tendency to tilt to the outside of a turn and make it more likely to turn over. If you look at pictures of cadwell you will notice that all 3 of the top 3 cars (LG, crockle and TES) have no suspention.
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Post by hayasa on Mar 10, 2010 9:38:43 GMT
Woody
Thanks for the cautionary tale! I presume that it is the outside front wheel that is lifting as you turn in? If so, I can see you having some serious understeer issues when you get up to speed. Thinking about your problem last night I remembered that farm tractors have a rear axle and ‘chassis’ as one piece and the front beam axle has a central pivot to give articulation. Not sure how you could incorporate it into your design but it’s an idea if you don’t want to bother with suspension.
Lee
The suspension I have designed uses beam axles like yours but with radius rods close to the side of the car and the rest of the hardware is inside the bodywork so the aerodynamics are fairly clean. I have yet to come up with a satisfactory anti roll bar setup though which is an issue. My biggest concern is all the extra work involved in making the parts and setting it up if it can handle perfectly well without it. Like the old saying goes; ‘Keep It Simple Stupid!!’
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Post by peasnbarley on Mar 10, 2010 10:25:57 GMT
Hayasa asked do we need suspension :- This is a really interesting question and now you have started the ball rolling I bet the posts run on and on a bit like the Great Weight Debate. Cadwell Park is a racetrack smooth as a babies bum while Cairngorm is a access road to a mountain subjected to extremes of weather and the surface is anything but smooth. In the main Carties are running on 20" BMX or similar wheels and anything which lessens the stress loadings have got to be a good thing. The ideal and of course most expensive is fully compensating suspension which none but the mad could afford. Simple spring type suspension has its drawbacks because what goes into an undamped spring has to come out again with equal force. The result being bounce transmitted to the cart body. The wheels however have been protected and bye and large remain in contact with the road which is where they are supposed to be. Without any built in suspension the tyres, rims and spokes are effectively your suspension. Handling is an issue and without built in suspension there will be numerous times during a race where your cart effectively turns into a three wheeler as your front wheel lifts in the turns. It might not be noticeable but the effect is there. The thing is to go with what you think is right for you and what you think is doable and affordable.
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Post by Scottish Cartie Association on Mar 10, 2010 11:01:28 GMT
I fitted soft tail swing arm suspension to the Flying Ferret a few years ago, but I only did it because I picked up some midi moto shocks on eBay and wanted to use them. I also thought it'd look cool. I am easily impressed by shiny things. I think you need a small amount of give in the cartie, either due to natural flex in the chassis or from suspension, in order to keep all your wheels on the ground despite uneven road surface and steering jack effect. Be careful not to make it too soft, though, as that can be equally problematic (check out the effect of the body roll on the lead cartie's wheel camber!) Of the top four at Cairngorm last year, two were full suss and two were no suss.
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Post by andy on Mar 10, 2010 12:09:40 GMT
we built our cart (centa bavaria) a few years ago and tried to keep it as simple a possible. The chassis was designed to be strong but not over braced and there is an amount of flex/ twist in it .There are no camber or caster angles so we dont get any stearing lift. The cart generally rides pretty well and goes more or less in the direction it is pointed. Tho as the speeds increase it does get a bit twitchy. Steve, The boys on here who have already posted know their stuff and will (I'm sure) help you out as much as they can.............where abouts in Yorkshire are you?
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Post by hayasa on Mar 10, 2010 20:35:37 GMT
Firstly, many thanks for the great response. As some of you have said, the main objective is to keep as many wheels as possible in contact with the road for as long as possible. Especially when braking and entering a corner. I guess coming out of a corner, one wheel in the air might reduce rolling resistance a bit I’m liking the simple route of a fixed rear axle and a damped pivot in the middle of the front axle that occurred to me last night. (Thanks Woody!) If only for the ease of construction. That would at least stop the 3 wheeler effect! I suppose it would be good to have a little negative camber on the outside wheels when cornering, if only to put the loading into the wheels a bit nearer where it was designed to go. But, without suspension that camber would be fixed. Would it then result in increased rolling resistance on the straights? I’m not sure changes in camber are too important for grip when your contact patch is only an inch wide. As can be seen on Catterlinecarties’ picture of Cairngorm where, despite the positive camber, he still has more than enough grip to deform the outside front wheel! In fact, on closer inspection, I think some of that apparent camber is the deformation of the wheel. It would be interesting to see a picture of Night Train at the same point in the corner to see how much grip he was generating. Andy, Are you saying that your kingpins are vertical? The way I understand it, caster is what creates the self-centring effect. Please explain! By the way, I live near Wetherby so we’re almost neighbours!!
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Post by Scottish Cartie Association on Mar 10, 2010 21:24:41 GMT
Look more closely - he isn't deforming the wheel at all.
Look at the angle of the wishbones. The outside one is nearly level, but the inside one is pointing down. What's happened is that the outside suspension has loaded up and the body has rolled outwards. The geometry of the double wishbone setup has caused both wheels to camber in the same direction - i.e. negative on the inside and positive camber on the outside.
You should see the state of the tires now - they're down to the canvas.
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Post by hayasa on Mar 10, 2010 21:51:22 GMT
Yes I noticed the positions of the wishbones and the bodyroll but I was looking at the relationship between the rim and the hub. The bottom of the rim looks to be out of alignment with the hub. The only way we'd know for sure was to stop the action 'Heroes' style and go and ping the spokes!!!
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Post by andy on Mar 10, 2010 23:59:21 GMT
yes everything on the steering is vertical and horizontal. It is a very straight forward system . Not a hint of ackerman either. It works well tho the text books suggest it shouldn't !! But it's simple and makes the cart easier to set up. Wetherby is 'just up the road' from us...if you want to have look at our set up first hand, let me know and we can sort something out.......
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Post by peasnbarley on Mar 11, 2010 13:05:16 GMT
Hayasa wrote :- It would be interesting to see a picture of Night Train at the same point in the corner to see how much grip he was generating. You can see all the carties on that corner and others by following the link. I was quite chuffed when I looked at the picture of our cartie Runfree No16 and noted the lack of any noticeable lean, or deformation on the wheels. www.action-heroes.co.uk look under Charity and then Cairngorm
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Post by Scottish Cartie Association on Mar 11, 2010 13:20:58 GMT
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Post by hayasa on Mar 11, 2010 19:41:36 GMT
As you say Andy; conventional wisdom says it should handle like a pig but, the fact that you won at Belchford last year through the chicanes suggests it must handle pretty well, or you’re a hell of a driver. Or both!! Thanks for the invitation. Yes, I would be interested in seeing it first hand, at your convenience. If only to get my head around that steering system!
Thanks for the links guys. Very interesting. 16 does look nice and stable. The pics of Cairngorm are just scary but I bet it’s a comfy ride over the bumps! The sequence of the red bare framed cartie with rear suspension passing Night Train around the outside at the hairpin in the semi finals was interesting. Now, was that because you can corner faster with suspension or is it due to the bravery/skill/insanity of the guy in the yellow helmet?!
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Post by peasnbarley on Mar 11, 2010 21:12:36 GMT
Without even looking at the pictures I can say the Guy in the yellow helmet is Colin, or squirter as I call him. He seemed to be able to take the hair pin flat out while squirting the opposition with jets of water. Really pissed me off the first time because I was taking everything so seriously. He is great fun, very competitive in a nice way and Cairngorm would not be the same without him. Hayasa asked if the great overtaking was down to suspension, or the bravery/skill/insanity of the yellow helmet driver? Cairngorm is a gravity race and those from distant planets have secrets they don't share.
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Post by Scottish Cartie Association on Mar 11, 2010 21:44:12 GMT
Now, was that because you can corner faster with suspension or is it due to the bravery/skill/insanity of the guy in the yellow helmet?! Definately the later. ;D Check out his on-board videos at www.youtube.com/user/harrismagnum04
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Post by Scottish Cartie Association on Mar 11, 2010 21:57:25 GMT
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Post by andy on Sept 28, 2010 16:49:24 GMT
Just to open this one up again !!!!!
After suffering a very "interesting moment" the day Before Belchford. We were testing on Tetford hill.
At about 50mph I went through a fairly gentle left hander. On the apex I hit a dip which pitched me 6 feet to the other side of the road. Fortunately the car that was coming up the hill didn`t appear until I had got things back under control....but it could have been a different story if the car had been coming round the corner at the same time as me.
Our cart is very stiff and up until recently worked well. As the speeds are rising, I am finding things a little bit more entertaining in the cockpit. This is not a good thing !!
So suspension ??
Any ideas ?
What is the best solution?
Or am I just being a JESSY.
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Post by leew on Sept 28, 2010 19:03:05 GMT
Best solution is to add zero scrub radius (or close to zero) steering. From what you describe it seems like one of your front wheels hit an imperfection in the road and the wheel was pushed backwards jerking the steering to one side. By getting the KPI angle correct you can eliminate bump steer and brake steer.
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Post by team-art on Sept 28, 2010 20:45:07 GMT
Hi Andy, you've got a great fast cart, if you change it change only one thing at a time! At least that way you can revert to the previous set up, which has served you so well, if need be.
Altering the scrub radius might help as might a bit of caster/trail (your cart sometimes looks a bit tail happy?) and some caster might help that? But remember you might be altering some of the very things that make your cart fast!
If we are just talking suspension, I have the same dilemma and have considered a few options and have still not made up my mind.
Some of my beliefs: Tyres make very good suspension (damping) but we don't want damping because that is lost energy. Rubber is good at absorbing energy but not so good at giving it back.
So from the above no rubber mounted axles ,or low tyre pressures..
Springs theoretically return a high proportion of any input energy, so for carts should be quite efficient, control the cart (keep wheels on the ground) and not waste too much of the input energy?
You're an engineer so you understand steel, so build as you have done,"keep it simple" and try a flex steel front axle (I believe the rear should be much stiffer).
If you tree the axle (just support the middle) and used CDS tube size "x" you could then go 1 gauge up or down at a time. Provided you could readily change the axle! Make more than one, bolt in/out axles that fit your kingpins??
I have considered using fiberglass/carbon fibre (think skis) or use skis! As it would be easy to add more material. But I still believe steel has the best properties! Ignoring the weight factor!
Cheers Rich
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