Reasons not to build a 4×4 Kit Car

Pin It

You’d think, now I can see the ‘light at the end of the tunnel’, I would be pro 4×4. I’m not…. Well sort of….. Maybe…. Several people have asked me to help me to build a 4×4 Haynes Roadster. 2 people in the last week alone.

Reasons not build 4×4 Kit Cars

There are advantages to four wheel drive, but for a lightweight, low slung car that already has 50/50 weight distribution adding 4×4 makes no sense at all.

Extra Traction

Lets face it, a Super 7 style car isn’t short on grip. If ultimate cornering was your goal, you’d go with a super light weight motorbike engine, stripped out interior and modern sticky tyres. The goal of a super light track day car is a pure, uncomplicated driving experience where ‘standard’ driving techniques can be used under all conditions. Adding unsophisticated 4×4 would destroy that. If I tried to chase a track day Locost or Caterham through a bend with my nose heavy, fat 4×4, I’d under-steer off into the tyre wall.

Available Technology

There’s a few common types of 4×4 systems out there:

  • Off Road Luxury Cars
  • Pick-ups
  • Saloon Cars

The top 2 options often have longitudinal engines (good) but the front differentials tend to be similar size or bigger than the popular alloy bodied Sierra rear differential. 95% are too big and heavy for this situation. The 3rd option is where you’ll be searching. Unfortunately most modern cars have transverse engines with both differentials built into the gearbox. Many cars you’ll be looking at will be older, maybe from the 1980’s or 1990’s. Your chosen front diff must have the same ratio as the rear and a suitable ratio for you’re choice of gearbox and wheels. The sort of 4×4 system that will fit a tiny car such as a Locost or Haynes Roadster isn’t going to be that sophisticated and probably from a car 20 or more years old. In my case, a 23 year old, 1991 Ford Sierra Cosworth 4×4. With these early systems, when driven on a slippery surfaces, the power from the engine will go to the wheel with the least amount of traction. Which ever wheel is slipping the most, gets the most power. Therefore, unless you drive off-road or permanently on the limit of traction, any advantage will be small.

Weight

Adding 4×4 to my car has probably added over 20% in weight. Percentage wise that’s huge compared with a production car. A 2wd Sierra Cosworth weighed 1207Kg and the 4×4 1305Kg (just 8% increase). It’s a balancing act. Is the percentage weight increase worth the traction advantage of an early 4×4 system? In the 1990s, unless the conditions were particularly slippery, the 2wd Cosworths were more competitive than their 4wd cousins. At 20% weight increase, my car will be slower around corners than it’s 2×2 equivalent under virtually all conditions, probably with massive under-steer. On a standard car like the Sierra, the front differential is mounted to the side of the sump. With my car it is 18″ in front of the engine, half way up the engine block. Not only have I added weight, I have shifted the centre of gravity forward and upwards (very bad). There are advantages to keeping the unsprung mass as low as possible. A 4×4 system has an off centre differential and heavy weight drive-shafts and hubs. The unsprung mass at the front is several times that of a 2wd car. With the extra weight and additional rotating mass, bigger, heavier brakes are needed increasing the unsprung mass even further. The more weight you add, means the more power, braking and grip you need to get the same corner speeds.

Software Analysis

When you are looking at a 2wd system in a suspension analysis package, chances are, if you search hard enough, you’ll find corner weights, centre of gravities, wheelbase info etc for similar cars. My Kangaloosh even came with a Caterham as an example. However, due to lack of published data, my 4×4 set-up was totally made up. A Caterham has virtually perfect corner weights, great centre of gravity etc. My 4×4 hasn’t, therefore getting the plotted curves and graphs anywhere near that of the ‘near perfect’ Caterham was almost a waste of time. It’s easy to see why modern super-cars have ultra sophisticated, computer controlled, adaptive suspension and braking systems. Similar problems arise when you ask a ‘professional’ company to set-up your system. Watch their faces drop when you say 4×4. All of a sudden the people that will set up your car has halved.

Solving Problems

There have been endless issues with this build. I’ve had to cut up many ‘off the shelf’ items that would simply bolted up to a 2wd drive car. I’ve also had to find countless un-proven solutions.

  • Water Pump
  • Exhaust manifolds
  • Steering racks

Latest Problem
Ackermann Angle
Because of the height of the differential, the route the steering shaft takes becomes a real issue. I’ve had to swap to a rear mounted rack, which has meant swapping my uprights from one side of the car to the other. The other week, I finished the steering rack extensions, the next night I looked at the uprights and thought, ‘Oh nuts! I forgot I had to modify them!’ – the whole build has been like this. The issue of Ackermann angle needs to be cured. Chances are I will machine my own 4×4 front uprights. I can assure you, that will be challenging. Essentially, the steering arm needs to be ‘bent’ so that the track rod end pivot is about 36mm inboard. Often you need to finish an item to visualise that what you have made is either wrong or occupies space required by something else. Another Problem… I thought the 2 piece propshaft would be easy. I’d just use bits of a 1960’s Volvo, bits from a Sierra and a custom shaft. I’d seen a similar system on another 4×4 Locost. I showed my 3D CAD drawing and some photos to an expert recently, he said “it’s not that simple”. He said he’d sort something out for me and get back. That was 5 weeks ago. This happens a lot with unique ‘one off’ set-ups. I’m still confident he’ll get back, but timescales get really dragged out.

Timescales

There is usually a much better selection of companies selling products for 2wd kit cars than there are for 4×4. The competition between them is greater and prices are lower. Finding suitable parts will mean rummaging at autojumbles and hours surfing on line. Visualising how something will finally look is usually impossible until you find the ‘missing link’. You will make many parts only to throw them out at the next build stage. It’s Beauleiu auto jumble next weekend, so I’m going there looking for parts and inspiration. The part I’m looking for might not even exist! An example I have Granada 4×4 5 stud hubs. I’ve had a ebay ‘watch’ set on these for the last 3 years and I have only ever seen the 2 I have on the car. They were both from different cars and sellers. Luckily they are in excellent condition, but that’s not always the case.

Unsprung Mass

To reduce the unsprung mass and to clear the heavy and bulky front drive shafts, I’ve had to swap to inboard suspension. Adding push-rods and cantilevers adds to the overall weight, so advantages in one area are disadvantages in others. There is no room left for a conventional anti-roll bar, so I have gone for a rising spring rate cantilever set-up. To add a non conventional push rod roll bar set-up would mean more sprung mass. Because of the 20% weight increase, I’ve fitted bigger brakes. I’ve swapped my cast iron brake calipers for expensive billet aluminium items to reduce unsprung weight but I’ve also fitted the heavier vented discs from a Granada. Huge cast iron vented discs such as fitted to a 4×4 Porsche Cayenne weigh many times that of an original Super 7. Light weight carbon fibre discs could cost the same as the whole build budget and bring other issues with them.

Familiarity

With a 2wd car, most people builders are far more familiar with what is right and wrong and how to perform most assembly jobs. Many tasks on a 2wd vehicle can be managed in isolation. You can read up on one small subject, head out to the garage and head back inside an hour later with a large smile. With a tightly packed 4×4 the technical data is harder to find and experts are thing on the ground. To complete one task, you often have to read up on 10 subjects in parallel before a solution will even become obvious. Even then, you’ll find yourself working on 10 mini projects at once. Parts will get bolted on, only to get thrown away a week later. With a 2wd build there is a reasonable chance that at the point you decide to build a car, you’ll be able to go online and order 80% of the parts, with many of them brand new. With a 4wd thats going to be more like 1%.

‘Q’ Plate

The parts for a 4×4 Haynes Roadster will need to come from many makes of cars e.g. Citroen BX, Volvo Amazon and Ford and Rover. There are parts in this 4×4 system from 1962 through to 1996. Getting ‘New Off the Shelf’ (NOS) items for rare items is next to impossible, therefore everything on the car is going to be second-hand. Obtaining proper receipts for items at Auto-jumbles and some scrap yards can be a real issue. The IVA inspector will see the lack of receipts as ‘second-hand’ and of ‘unknown origin’. Because of this random parts bin, this car will be ‘Q’ plated.

Fuel Economy

The fuel economy of a 4×4 system is going to be much worse than the 2wd equivalent. Physics states it takes a finite amount of energy to move a given object a set distance. If that object is heavier or has more friction, it takes more energy. A 4×4 system is going to sap power. Therefore you have less power to move more weight. You’ll get there slower and you’ll use more fuel. Often, with a petrol engine, driving slower means the car is outside it’s peak efficiency band. If you crawl along in a traffic jam at 5mph, you’ll use more fuel than if you were going a constant 56mph.

Emissions

If your choice of engine is rather large, ‘smoky’ or close to the IVA emissions requirements, e.g. a V8 designed in the 1960s, then 4×4 is not smart. Basically 4×4 is a dumb option for fuel economy or emissions.

Abandonment

Many kit cars builds are abandoned part way through the build. Some for many years. A 4×4 build will throw up a new excuse for abandonment every time you get within 10 feet. If you are a newbie or likely to have limited time, go 2wd!


This all sounds like I didn’t plan anything. WRONG! If you are familiar with this build you’ll see everything was modelled very accurately in 3D CAD before anything was cut from steel. Yet, still I have endless issues.

So why did I go for 4×4?

The answer is simple. I wanted a challenge and there are very few home built 4×4 Haynes Roadster’s out there. I don’t like to copy anything and creating anything bespoke will hold my interest – even if it is only fit for the scrap man. Logic did not play a part. 20 years a Boxing and Martial Arts Instructor gave me brain damage, I blame that….. I’m not into racing and when this car is finished, no doubt I will instantly sell it. The passion for me is learning and building, but not the driving. I’ve been struggling to think of a good reason and I haven’t really thought of any. Knowing what I have learnt, would I build a 4×4 again. Maybe I would…HHmmmm…. Nah probably not!

Pin It

Web Design by Go Web Solutions