In the past my team has always used 20" tires for our car but I have wondered if 26" tires might be worth a try. As they are extremely common and have much more support as they are popular with many bicycles. I want to know if anyone has done something similar and if so what results they have had. Any other thoughts or ideas on the subject are also welcomed . My main concerns would be with the issues of need more space in-between the tire and body for turning and the added drag from the larger frontal surface area. 

There is supposed to be a theoretical rolling resistance advantage to larger diameter tires. I am not sure I understand why, something to do with the larger arc.

There is a clear aero drag disadvantage. If your tires are 1 inch wide, then 6 inches difference by two tires means an increase of the frontal area of the car by 12 square inches. That is significant in Electrathon.

We discovered that the biggest challenge was how sloppy the handling became. The longer spokes in the wheels allowed more flex. The wheels would turn but the chassis would try and continue in the original direction. The spokes would reach their limit of flex and then the chassis would snap back and then oscillate back and forth. 

We've started going with kart tyres over in Australia. We haven't won with them yet - our wins have come from cars with 16" fronts and 20" rear wheels/tyres - but they've been promising. Pump them up to 55psi and make sure you've got top notch bearings (our problem in our championship event last year was that we didn't check them before the race - and they were worn horribly) and the rolling resistance will be more than a larger wheel but less difference than you think. Plus you can build a much lower car, get virtually free slightly used tyres from your local kart racers/track, and the wheels will never break at the speeds we go.

I got the idea from the guys who built Atomic Scalpel. It's the world speed record holding gravity racer. They use narrow kart tyres and with nothing other than gravity powering them down hills they topped 100mph.

Like I said we haven't managed to get a win with a kart-wheeled car yet but I'm confident it will come.

I will be interested to hear how the karting tires work out. We were considering them for our next electrathon vehicle but have decided on (stand up) scooter tires instead.

In theory, wider tires have lower rolling resistance than skinny tires, all else being equal. However, we normally run these tires in the 12-16 PSI range on a racing kart which gives even wear across the tire. I am worried that at the higher tire pressures that we use for Electrathon, the middle of the tire will bulge and lose any advantage of the wide tire.

Have you tried different tire pressures to see where the best balance of rolling resistance and grip are or is 55 PSI the limit of what they will take?

Cliff

www.ProEV.com

G'day Cliff,

Not sure if wider tyres provide less rolling resistance - happy to be proven otherwise however. The main reason we use them is more practical. Tyres are basically free if we want them, and brand new tyres are pretty cheap, they last forever, and the wheel/tyre combo is pretty robust, dealing with bumps and potholes well. They do allow for a super low car and they provide way more grip than we really ever need for hard cornering.

We run them with pressures really high (I used to run 8-10psi cold pressures when I raced) to try and reduce rolling resistance. The tyres still provide plenty of grip at 55psi - a figure we use by converting the maximum 4 bar pressures the sidewall marking state should be the highest pressure to run in them.

As with any choice in Electrathon/eV Challenge (our event over here) there are pros and cons. We've won plenty of races with cars fitted with 16" recumbent trike front wheels and tyres, and haven't yet won anything with our kart wheeled cars. But as I mentioned before, I'm confident we will perform well enough to give ourselves a shot in the future with those cars.

Not sure they'd be great on bigger ovals in the US, but a short track oval that sees a lot of hard cornering and road/street/carpark/kart tracks I think they'd be competitive. Our cars from the past 2/3 years have looked like the front end of a lay down/enduro kart with the back end of an electrathon car bolted to it. Worth a shot for you guys?

Cheers

Hi Clay,

I love the way the smaller tires can fit under the bodywork minimizing aero drag. I notice that in all the recent pictures you posted, that everyone in your series has bodywork over the tires.

In our series, bodywork over the wheels is the exception, rather than the rule. I wonder if that is due to the type of tracks we race on. The majority of our races are set up in parking lots. The tracks have many tight turns and are usually bumpy. On our car, we have found that widening the front track helps the vehicle pivot. Understeer seems to use up more energy in the turn. Covering our wide front track will create a large frontal area. So we run a slick driver compartment and the wheels way out there.

What are your tracks like? What average speeds are you running?

The wider tire having lower rolling resistance is counter intuitive but it makes sense to me. Most rolling resistance comes from the tire deforming to create the contact patch-going from a round tire to a flat spot on the road. The more the rubber has to move, the greater the energy loss. This is why a larger diameter tire should have less rolling resistance than a smaller diameter tire. The greater arc of the larger diameter tire is closer to the flat line of the road and changes less.

The wider vs. thinner tire is due to the shape of the contact patch and geometry. The skinny tire contact patch is long and thin while the wider tire contact patch is short and fat. If everything else is equal, than the rubber in the center of the arc of the skinny tire has to move further than the rubber in the middle of the wider tire.

The key, of course, is 'if everything else is equal'. Different tire construction such as thicker rubber or inability to run higher pressures are going to weigh in too. We get our lowest rolling resistance just before the tire wears through. This makes sense since the rubber is very thin and it takes less energy to deform it. Not ideal for finishing the race though.

Our current plan is to use tires from stand up scooters. They are about the same diameter as karting tires but are better suited for the leaning suspension we are going to try. We will use a scooter hub motor as well.

G'day again,

The whole 'skinny track, wide body' design ideas came from vehicles a bunch of schools in Australia do called pedal prix. They are velomobiles - image weird looking pedal powered electrathon cars - and in conversation with one of the designers of the most successful cars (who we bought parts from) he mentioned they had looked at either wide track designs or narrow and found the aero benefits of narrow track well outweighed the slight drop in cornering speeds that design philosophy compromised on.

I was very skeptical however after trying it I agree with his conclusions. We race on kart tracks and they have a number of tight hairpins and our narrow track cars handle them fine. If anything, the narrower track make the width of the road we are using wider - in a weird sort of way. The other benefits of the design - keeping everything inside the bodywork makes damage from contact less likely, it gives bigger drivers more ****pit space as the bodywork is a little bit wider, even making the cars easier to pack and transport as they are slimmer - have made it useful for us, but the main thing for us is the cars are simply faster, not only over 1 lap but also over an hour.

I'd imagine that for carpark tracks they would still work pretty well, maybe just with a little more ground clearance than we run as the surface might be a little rougher than a kart track.

And to answer your question we used to run on carpark tracks but now with insurance being what it is we race on kart tracks. Our average speeds are a bit lower than yours, but we are also running only 630Wh as opposed to roughly 1kWh in terms of battery capacity. Fastest lap average speeds are around 40-45km/h (25-28mph) depending on the track layout, with maximum distances of around 35km (22 miles). We also have 2 pit stops for driver changes through our races with minimum pit stop times of 1 minute stationary which eats into the distance a little.

Back to the design of the cars, the key for us was reducing the wheelbase. The pedal prix cars had a really short wheelbase to go along with their narrow track - again I thought that was all wrong. But by shortening the wheelbase the cars become really agile. You don't need a huge amount of turning angle on the front wheels for a tight turning circle. It does make them look a bit strange with these long overhangs on the nose but again as in my previous comments if you have a look at enduro/lay down karts they also have a pretty long nosecone, and they look ok.

But I guess that's the joy of events like Electrathon - you can basically try all sorts of stuff - there's no one right way to design anything.

Hi Clay,

It is always interesting to hear what solutions other teams are coming up with. There are lots of different ways to solve the same problems. So far, we have spent our time optimizing an existing vehicle, so we are somewhat limited in what we can try. I am excited about working on our clean sheet design.

I agree a shorter wheelbase is another way to allow the vehicle to pivot more readily. We moved our rear wheel as far forward as possible. 

<If anything, the narrower track make the width of the road we are using wider - in a weird sort of way.>

Also agree, the wider the arc the vehicle can take through the corner, the faster the speed possible. A narrower vehicle is further out at the turn in point and can go further in at the apex and thus a wider arc.

Your point about the bodywork protecting the other components is valid as well.

There are a lot of good reasons to try full bodywork.

Clean sheet design huh Cliff?

Well please keep us informed of your progress. 

I for one am very interested to see what you come up with