New pedal cart (velomobile) design – help with spec

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New pedal cart (velomobile) design – help with spec

Postby madmarsupial » Sat Mar 23, 2013 2:03 pm

My sporting club competed for many years in a local 24hr pedal cart charity event, but have not done so for about 7 years and I'd like to get us back in.

We used to have two carts ( ... directlink) that had their origins in the 1980s, but were constantly rebuilt and remodeled over the years. Sadly, these carts were in a considerable state of disrepair and with no one willing to either do or fund the work, they were passed on to others.

At the moment I could easily get a team of competitors – if only I had a cart!

My decision is that if we are to get back into the competition, we have to do it with a new build clean sheet design…and I’d like your help to narrow down the design specification a bit! I’m a keen road cyclist, but I know nothing about velomobiles, recumbent trikes etc. I’ve done plenty of research on line and looked a lot of designs, but experience counts for so much more.

Rules/basic spec:
The event is for four wheel pedal driven carts, front steering.
The ‘box’ formula is as follows:
- Length max 2030mm
- Width max 1125mm
- Wheel track (width) min 650mm
- Wheel size max 560mm (26” MTB)
- The minimum internal radius of the track is 6m, so a turning circle of 6m is needed for best performance.
This makes for a fairly short a dumpy design (as shown in the pictures).

The Old Carts – Champion formula
The old carts were proven performers and the club won the event many times and still holds a couple of the records.
The ‘old guys’ tell me that this is because our carts included:
- Ackermann steering before everyone else
- Front wheel drive
- One had a limited slip differential
- The LSD cart also had centre hub steering
- Overall the carts were solid
- Didn’t lose as much speed in the corners
The carts had a lot of very much hand made parts. The workshop at one of the highschools was the birthplace of those huge front hubs (for the centre hub steering) and the LSD unit. A sailing spinnaker pole is a rear axle. The marine repair guys did a lot of fiberglass work for those bodies. Even the big rear disk brakes were custom!

The NEW plan:
I’d like to build a completely new cart based on the following key features:
- Front wheel drive
- Limited slip diff (off the shelf Samagaga part)
- Standard cycling disk brakes
- Ackermann steering
- 20”/451 wheels (most teams are using these)
- More reclined seating position (lower, more efficient)
- Adjustable (fore-aft) seating for different rider height
- Carbon/nomex honeycomb tub chassis
- Light single skin (carbon/glass) fairing body
So much more is available as standard parts these days that I’m only going to custom make what I have to. This will probably mean king-pins and stub axles (depending how we design our front drive and steering). I’ve got access to a marine boat yard and have experience in rowing boat building, so the carbon/honeycomb work is not going to be the hard part.

Centre hub or centre point steering? Which would make a difference?
The old guys say the centre hub steering was the ‘magic’ touch, but is it any better than a well designed centre point king pin based design? With so many recumbent trike designs out there you would think that someone had worked out the geometry by now.
The custom hubs required for the centre hub design are a complication, but one of our club members is in engineering and manufacturing so we could get these designed on CAD and CNC milled if needed.

Other geometry
Do we want to angle (from vertical) the line of the steering if we do centre hub? The king pins on most recumbent trikes angle inwards to achieve the centre point steering, but most also angle backwards to give varied camber during steering. Are there any advantages or disadvantages?

Wheel base and track.
I’d like to maximize my length to get a more stable cart (see how close the wheels are in some of the photos on the official sites). The location of the front wheels will be governed by the drivetrain and the forward projection of the cranks, but the rear wheels can be moved around the more reclined rider (even if the rider has their shoulders between the wheels).
The width of the cart is something I’m not sure about. The max width in the rules (1125mm) is a lot wider than most recumbent trikes (Greenspeed all about 800-860mm) so I’m not sure if I need to go too wide. At what point do I stop making serious gains in stability and cornering?

Different front and rear track?
I had thought that making the rear track narrower might assist with aerodynamics – trying to get as much of a Kamm tailed shape as possible within the box rules. Worth bothering with?

Brake layout – in theory the braking is always best done at the front, but with a complex and crowded front drive and hub layout would it be best to stick them on the back and suck it in? Has anyone seen a neat front drive and brake set up we could look at?

The short length makes it a little hard, but what are the key things I want to do?
- all over tear drop covering every thing including all wheels and cockpit (Kamm tailed of course)
- all over tear drop covering all wheels, but with open cockpit
- tear drop fuselage with all wheels outboard (like a 1930s race car), faired or unfaired wheels?

With the many changes of rider a full covered cockpit might be a nuisance for the value of the aerodynamic returns. Also the HK climate might make it less than enjoyable (even if I install some NACA vents for airflow).

There you have it!
Quite a long post and so many questions. Hopefully some of you can help refine and improve my research.

I've got the prospect of some small seed-funding so I'm hoping to get us into the 2014 event.

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Re: New pedal cart (velomobile) design – help with spec

Postby Aushiker » Sun Mar 24, 2013 9:11 pm

Sorry cannot help with the technical stuff but thanks for sharing the links to the Grand Prix. Looks like a load of fun!


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Re: New pedal cart (velomobile) design – help with spec

Postby Roinik » Mon Mar 25, 2013 7:39 am

Having a look at the track it is nice and flat with hairpins at each end. I guess there are a few things to consider with your design.

1. The weight of the machine will be a telling factor with your speed out of the corners.
2. The smaller the frontal projection, the faster the machine. The aerodynamic drag on the machine will be proportional to the area multiplied by the cube of the speed.
3. Front wheel drive is typically heavier than the rear wheel drive variants due to the complex differential required to reduce the amount of torque steer and scrubbing of the wheels. The easiest method is to make the front light and put the power through the rear wheel/s.
4. Check out the Trilsed Aquila for one of the fastest velos on the pedal prix circuit
5. As for steering geometry, positive caster of circa 2 deg (i.e. top of kingpin is laid back slightly) is good. The biggest help will be to camber the wheels. Typically, the best cornering velos have the wheels laid in by 8 - 11 deg. This will allow you to go faster through the corners without tipping over. BTW I'm surprised that the pedal cars aren't required to have a roll bar.
6. The type of steering is not really an issue and is personal preference. Under seat, beside seat or tiller is personal preference. Consideration should be given to driver changes and the adjustments required to fit each driver. A block of foam is generally used in the pedal prix to 'adjust' for different rider heights.
7. If you place your rear wheels close together and imitate a single rear wheel then you will be able to make the tail as narrow and as close to the teardrop shape as possible. Couple this with a full length floor, hood and wheel covers then you will be faster again. You may want to camber your rear wheels slightly to get even better cornering. This doesn't need to be as dramatic as the front wheels because an angle of 3-4 deg will be enough to stop sliding.
8. Ventilation is always an issue. Most velos have closed wheel arches and vent holes in the front or foot holes. You could get around this with the back of the wheel arches being open and a breeze hole at the rear of the shell at the narrow point in the tail. The reason that most other velomobiles are vented at the front on through the foot wells is that venting in the wheel arches allows water into the cabin during wet weather.
9. There are several methods of construction that could be adopted. Another consideration would be to make a space frame (this could be out of carbon fibre tubes if you can be bothered moulding the tubes etc or out of Cr-Mo tubing. The very lightweight skin can then be formed over the spaceframe chassis. This is typically what Trisled do for their racing velomobiles and it can produce a very light and fast machine.
10. The paint job should be RED because red velomobiles go faster.

You do realise that you will be introducing something very fast and unique to this whole event. Don't be surprised if the rules are changed for the next event to preclude the changes that have been mentioned (aka 1934 and the UCI).

Anyone other opinions are welcomed.

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Re: New pedal cart (velomobile) design – help with spec

Postby madmarsupial » Tue Mar 26, 2013 12:59 am

Thanks for your reply.

I've discovered (the British Pedal Cart Championships) and their rules are the same so I'm finding a great wealth of information there.

Everyone is saying to avoid front wheel drive - not worth the effort and complexity. I'm also getting a few good tips on the steering and wheel base set up.

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Re: New pedal cart (velomobile) design – help with spec

Postby Roinik » Tue Mar 26, 2013 7:59 am

In this case FWD's main advantage is for those cases where you are likely to lose traction often. My small amount of involvement in speedway found that FWD would eat the RWD opponents due to the ability to drive the car through the corners under power rather than using the power slide method. This isn't really an issue with a pedal car and the amount of power lost in the long chain doesn't really make up for the difficulties and power losses in the differential, let alone the torque steer and wheel scrubbing issues. RWD also allows you to optimise your wheel base so that you can get a longer and more balanced machine.

Also check out the design of the Greenspeed Glyde, Trisled Rotovelo/Aquila, Milan, WAW and Quest velomobiles for inspiration. Your contacts from the British site will also be able to assist as well due to their experience. Greenspeed also have a hub design that enables the use of a disk brake and a quick release wheel on the 406 size. I'm sure that they will sell you the parts, Magura brakes and 406 wheels if you ask nicely. Just remember to adjust the camber on the kingpins so you can corner faster. Adoption of this ability will speed up puncture repairs on the machine and wheel replacement if there is any wheel damage.

BTW, if the HK race is as brutal as the Pedal Prix then the machine should be capable of coping with a few bumps. Some 100 mile an hour tape is essential.

Good luck as it looks like it could be fun. BTW if it was me, a hood may be useful for the night riders due to the cooler temps.

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Re: New pedal cart (velomobile) design – help with spec

Postby coln72 » Thu Mar 28, 2013 10:08 am

Nice looking vehicles you had back in the day. Some advanced engineering going on there.

I am involved in managing a school team that competes in a local 24 hour race an after constructing a few trikes of my own or helping with the other trikes that the school runs over the last 10 years, I'd thought I would throw my 2 cents into the ring.

1. FWD - personally I don't think it is an advantage. What you gain in running a shorter chain etc. you loose in adding complexity and packaging issues. If you want to run FWD, is there really any need to drive both wheels? In the past our school has run FWD trikes with only one wheel being driven and had no issues with torque steer. The trike tracked straight at all times and very rarely lost drive. Saved a lot of messing about with diffs. I would rather run RWD.

2. Construction - while a CrMo spaceframe my not be super sexy, they are strong, require simple equipment to build and are easily repaired/modified. A recent study at the event we complete in, where every vehicle was weighed by a university group, found that a CrMo frame with a body made from coreflute was the lightest construction method. We could easily build trikes less than 30kg race ready. Built an aluminium frame once and it ended up under 25kg ready to race.

3. Definitely run with your rear wheels close together if you can package it. Allows a nice aero shape. If you have to, run a 16" (349) rear wheels to help packaging.

4. Wheel size - I'd go for 349 rims all round. Allows you to build a narrower vehicle and still maintain clearance to the front wheels while turning. Tyres are a little harder to come by, but you can get them. I am researching using a 305 rim atm.

5. CofG - make it as low as possible. Saved around 20sec per lap by dropping the rider around 40mm the first year I started managing a team.

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Re: New pedal cart (velomobile) design – help with spec

Postby madmarsupial » Mon Apr 01, 2013 1:18 pm

Drive train - just about everyone is telling me to skip the FWD. Looked at a few interesting designs that skip a conventional LSD and use sprague (spragg - spelling varies!) clutches/bearings as a less complicated but equally efficient system.

I can probably arrange CrMo or alloy, depending who I ask to help with that. I ride with a cycling club sponsored by a frame builder - perhaps I can get it done in XCr or Reynolds 953! ; ) Body with carbon/glass is easy as I've got access to that stuff.

Wheels? I'm not sure that going as small as a 16" Brompton size wheel would be any advantage over a 406 or 451. The width of the seat, rider shoulders and steering (if I go with an underseat yoke) will make more of a difference to the body than anything else. Wheels might all end up outboard of the body anyway.

CoG - I've now seen a couple of chassis designs that do drop the seat lower than the axles by a significant (3"?) amount. Will look at that obviously.

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