# ideas for solid rear axle suspension



## galderdi (Nov 17, 2015)

I am slowly gathering my thoughts for my next project. It will be another car specifically for the track. It will differ from my last project because it will only be used in motorkahana (think autocross but even tighter and more technical).

This time around I am aiming to be sub 300kg (minus driver). The car would be slightly bigger than a gokart but with car sized wheels and tyres.
Another key difference from last time where I had a gearbox, I am thinking of running a chain from the motor to a solid / live rear axle (hopefully with a ratio of around 5:1). I think I will have trailing arm suspension. The motor will be mounted with the centre of the motor aligned to the front mounts of the trailing arms. The idea being the distance from motor to the drive shaft should remain constant and chain tension won't be an issue. 

That all works if both sides are under equal strain. IE left and right tyre swing up at the same rate. But that won't be the case. What I am struggling with is how to allow for vigerous cornering where one wheel has all the weight and the other almost none. I haven't thought of a good way to allow for that twist. Normally by running the motor through a diff the diff takes care of the twist. But running the motor via chain to the drive shaft means that any uneven suspension travel would result in a twist in the chain. 

A couple of my unconvincing thoughts include:
1.Somehow separating the whole back end so the back end (including the motor) can twist as much as needed. then have extra shocks/springs to restrict the twist and retain some cornering control.
2.Another thought is to not allow the twist at all. Only allow both sides of the suspension to lift and drop at the same rate. This sounds like an achievable option but it would impact my level of grip.

Anyone spotted any builds / suspension designs similar to my description?


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## galderdi (Nov 17, 2015)

Another idea is to have a stationary central axle held in place with some bearings. Then at both ends of the central shaft it can attach to some CV shafts out to the hubs. In hindsight I think this is sounding like a likely option. 

Any thoughts?


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## Duncan (Dec 8, 2008)

Hi galderdi

I will answer with another question
How smoooth are the tracks you are aiming for?

A live axle is great on a smooth track - and gets progressively worse as the track gets bumpier

Your second solution is standard independent rear suspension - the diff is central and bolted to the chassis with the driveshafts to the hubs

Unless you mean a De-Dion set up which has all of the advantages of the live axle without the unsprung weight penalty

Trailing arm rear suspension - like a mini - is a BAD idea 

Your suspension should be aiming to keep your tyres flat onto the road - with trailing arms as the body rolls so do the tyres - bad

Most suspension systems - like dual wishbone - operate to give more negative camber as the car rolls in a corner - this counteracts the effect of the roll and helps keep the tyre square onto the road
The flip side of this is that as the car squats under braking (or power) the camber increases and reduces the grip

It is always a compromise - I designed my old mini to have optimum tyre contact at 0.5G cornering with 0.25G acceleration

I always wanted to build a lightweight track car with beam axles front and rear - de-dion at the back


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## Tomdb (Jan 28, 2013)

look around for some formula student builds, quite some interesting attempt at different suspension setups. 

Plus hill climb racers or Locost's http://www.locostbuilders.co.uk/

http://locostusa.com/forums/

https://www.youtube.com/watch?v=LZwPwaWOpQ8

Dig through some of those project logs and you will be amazed. I would personally build one based on a mazda mx5 rear subframe, scrapping everthing expect axles diffs and spindels plus brakes. Then building the rest out of tubing and converting to a chain diff. 

https://wiki.ece.cmu.edu/ddl/index.php/Chain_drive_opportunity

http://www.locostusa.com/forums/viewtopic.php?t=4983

Small question, how much does your motor,controller and battery weigh at the moment? Completely off topic question, any rules permitting All Wheel Drive?


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## galderdi (Nov 17, 2015)

Tomdb said:


> look around for some formula student builds, quite some interesting attempt at different suspension setups.
> 
> Plus hill climb racers or Locost's http://www.locostbuilders.co.uk/
> 
> ...


 In the current car:
motor = 65kg, controller = 10kg, battery = 70kg, gearbox = 50kg

In the new car:
motor = 50kg, controller = 10kg, battery = 50kg, gearbox = 0kg

Now that I am tending towards having CVs out to the driven wheels I am swinging back towards front wheel drive. For motorkhana FWD is cheaper on tyres.

All wheel drive is specifically banned for specially built motorkhana cars.


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## Tomdb (Jan 28, 2013)

Which batteries do you want to run? 
Got a torque graph of the motor so I can crunch some numbers and doodle a little in cad?
You could go solid rear axle, however I believe having it unsprung will make it handle/hop around corners quite badly with the slightest bump or body roll. 

What kind of wheelbase to track width ratio are you looking at achieving? Depending on how twisty the courses are the closer you might want to go to near kart like ratios of around 1.1:1. I believe that leaning towards a "palatov d1" design might be a great option http://dpcars.net/
Motor center/slight offset to the right of chassis coupled to a diff with a shaft to an axle, Batteries on the the right side of chassis and Driver on the left.This would give you a very compact package that should be easy to build an frame/cage around.

Small edit, my CAD itch got too big had to doodle with some components I already had. 1.6 meter wheelbase with 17 inch wheels, tires are a tad on the small width. However good enough to give you an idea.


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## galderdi (Nov 17, 2015)

Awesome diagram. Quite close to my thinking. Although I would flip it so the driver is on the right. Because I we drive on the right here so it would feel more natural.

The minimum wheelbase is 1.8m in the rules for motorkhana cars. The width is up to me but I am thinking it would be great if it would fit in the tray of my truck and squeeze between the wheel arches. That would make it 1.2m wide. If that were a problem I could make it 1.2m in the back and wider in the front. Then back it on to the truck.

At the moment I run Chev Volt batteries, but they will be staying in the existing car. I would be looking for another battery pack for this new project. Its realisitically about 2 years away so hopefully the battery options will increase in that time.

This will be a pure motorkhana car. Motorkhana is very tight and the top speed is only around 40 mph. Acceleration is way more important and that is why I was looking for around 5:1. My original though was to runa diff but I think I would struggle to find anything around 5:1 ratio. Plus it adds more weight. At these sorts of speeds having a solid axle / locked diff doesn't hurt too much. 

But I figure if I end up with CVs to the driven wheels anyway I might as well go front wheel drive.


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## Duncan (Dec 8, 2008)

Hi Galderdi

I like the idea of the "Kart" - and you could use a solid axle - without the diff you lose the massive unsprung weight

Check your regs about minimum suspension movement - if you can get away with just rubber mounting the axle and calling that suspension then you could run what is effectively a Kart

If your suspension does not move very much then you don't need to worry about the camber change problem - which is why a lot of the old British sports cars were very stiffly sprung - crap suspension geometry so don't let it move


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## galderdi (Nov 17, 2015)

My latest idea is to take the splines out of the diff and somehow attach them directly on either side of the sprocket hopefully with a bearing either side so I can attach it somewhere. Then the CVs can just insert into the spline as they normally would in a diff.


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## galderdi (Nov 17, 2015)

here is one I whipped up with CAD on my "Notebook"


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## Tomdb (Jan 28, 2013)

Want to get really cheapo.

Get the inboards that have a stub that slots into the diff, a pipe that does not fit over them. Machine the pipe inside and outside diameter (bearing fitment) on both sides and weld on a sprocket hub. Assemble everything, Press in the axle inboard cv's and drill and pin them.

Is you want to get fancy, get bolt on center cv's and turn a nice alloy center section.


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## galderdi (Nov 17, 2015)

Now you are really thinking along similar lines to me. That's my sort of solution.


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## Duncan (Dec 8, 2008)

Hi Galderdi,

If you don't have a heavy old diff I would not bother with the CV joints - just run a "beam" axle - like a Kart


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## Tomdb (Jan 28, 2013)

Yes that is a option, when you decide to stay RWD. 

The whole thing would look like an oversized kart. any rules stipulating certain criteria? Because you could go super light with an kart style frame and axle.

The ride will be very punishing if there is the slightest amount of uneven race surface.


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## galderdi (Nov 17, 2015)

Thanks guys. I think I have decided to stay front wheel drive as they have a slight edge in my form of motorsport. So I am now tending towards a chain from the motor down to a sprocket bolted between two spline receivers and then a short CV drive shaft from each side out to the front hubs. Should be fairly simple to achieve.


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