LocostBuilders - powered by XMB

Pro's & Con's of a rear/mid engine compared to a 7 shape

Antnicuk - 15/8/18 at 07:47 PM

I'm looking at getting another kit car, I have had a couple of 7 style cars and a Sylva Stylus. Im considering a MEV Rocket style car as something different. I wondered what they are like compared to a 7 shape car in relation to performance and handling, it will mainly be a track day car.

russbost - 16/8/18 at 07:26 AM

The basic concept of having a (rear) mid engined car as opposed to a (front) mid engined car is that you'll finish up with a similar weight distribution of around 60/40 but with the weight biased towards the driving wheels instead of to the front - my personal view is that that simply has to be a better way of doing things (you don't see many front engined race cars winning races unless all the others are similarly hampered by the rule book!).

It also has the advantage that the front of the car can be more streamlined, tho' in the kitcar game that is not always taken advantage of

Against that is that you have development of over 60 years of similar 7 style, front engined rwd cars, & anyone that says a Caterham is slow is just being daft

One of our club members is thinking of selling his Honda 2.2 engined Rocket, I can get further details if it's of interest, he's based near North Weald in Essex

I've never driven a 7 so can only comment on having driven my own car which is around 62/38 rear biased & handles beautifully, I feel the biggest difference is in getting the power on the floor from a standing start & acceleration out of slow corners, once at speed I doubt there is a huge difference

Sam_68 - 16/8/18 at 08:33 AM

I've owned several examples of both, and have driven many more.

The answer is not as totally straightforward as you might expect: mid-engined cars come in a number of flavours.

Some (notably BEC mid-engined racers) can be more nose heavy than front-engined Seven types, once you've put a driver aboard. Also, there can be quite a difference between longitudinal and transverse mid-engined installations.

In my experience, the longitudinal mid-engined cars tend to be a lot more benign at the limit than transverse mid-engined. Transverse middies concentrate a lot of their weight (engine and driver upper body) not merely close to the back axle, but high up. They tend to be snappy little bastards at the limit, unless you do what Lotus did with the S2 Elise and kill a massive amount of front grip to compensate, which leads to fairly heavy understeer through most of the performance envelope.

The S1 Elise (of which I have owned two) is pretty typical of the breed: you can get the back end to step out predictably, and hold it there, if you are very precise and confident with your control inputs - I used to do it regularly on a favourite roundabout near where I lived - but you'd need to be very skillful and/or very lucky to get away with it regularly on faster bends, or on roads where you didn't know the surface intimately. The slightest mistake, and you end up pointing the wrong way.

On the positive side; the steering can be superbly light and precise (again, the S1 Elise is a prime example).

One big difference that is seldom discussed is driver perception, though:

You sense 'oversteer' and 'understeer' using the balance mechanisms that are located in your inner ear. Mid-engined cars place the driver's head in a very different position relative to the axis around which the car rotates as it under- or over-steers, compared to a front-engined car. With a 'Seven', your head is well behind that axis, magnifying the movements and making it easy to sense what's going on. With a mid-engined car, it is much closer (with 'cab forward' longitudinal mid-engine designs, it can even be on or in front of the axis), so they can feel different - less predictable - even if the slip angles being generated by the tyres are pretty similar.

Best answer, as always, it to drive one and see for yourself. Perhaps the Lotus Driving Academy might be a good and relatively cheap way to get a perspective?

Antnicuk - 16/8/18 at 09:45 AM

Thanks both, really useful
Russ, that's the car I'm looking at. 😉

Sam, you make a lot a valid points that hadn't occurred to me, particularly in the last paragraph. I love the feeling of the back end stepping out so I may miss that sensation in a mid engine set up

It's a shame seller's take offence if you test drive their cars sideways ☺️

russbost - 16/8/18 at 12:55 PM

I would expect a huge difference between driving something like an Elise & the Rocket, you are certainly significantly further back within the wheelbase of the Rocket (so more similar to the 7) plus it has a massively higher power/weight ratio, plus you are out in the open air - in fact about the only significant similarity I can think of is that the engine is at the same end!

Sam_68 - 16/8/18 at 01:41 PM

quote:
Originally posted by russbost
I would expect a huge difference between driving something like an Elise & the Rocket, you are certainly significantly further back within the wheelbase of the Rocket (so more similar to the 7)...

Actually, you're not.

I'm not sure how easy it will be to read this, but here's a quick composite picture of the CAD drawing of an S2 Elise overlaid onto a photo of the MEV Rocket, matched to their wheel positions.

As you can see, seating position within the wheelbase is pretty much identical - though in fact, if anything, the Elise places the head of its driver ever-so-slightly further to the rear than the Rocket. And it is blatantly obvious that in both cases the driver is a lot further forward from the rear axle line in both of them, than he would be in a 'Seven' style car (but I can do another overlay to prove it, if you wish).

The yellow line roughly indicates the mass centroid axis (the vertical axis the car will pivot around in a 'freebody' situation), assuming a 60:40 weight distribution. Again, it should be pretty obvious how much further rearwards the driver's inner ear would be, in a 'Seven' style front engined car.

ettore bugatti - 16/8/18 at 06:30 PM

If you just are looking for the quickest lap time.

Then you should look for the car with the best power to weight ratio, lowest center of gravity, the widest track, best brakes and the lowest drag.
But that's just the basic ingredients, you will need handling to get the most out of it.

So there is more to it than just front engine vs rear engine.

Is it still possible to get the Sylva Riot? That would be my choice over a Rocket.

Sam_68 - 16/8/18 at 07:45 PM

quote:
Originally posted by ettore bugatti
Then you should look for the car with the best power to weight ratio, lowest center of gravity, the widest track, best brakes and the lowest drag.
But that's just the basic ingredients....

Although to be fair, you should add a rearward weight distribution to that list: it gives you inherently better grip (assuming RWD) for acceleration, and inherently more even braking effort, when the weight transfers forward under braking.

Certainly agree about the R1ot, though - I'm not a fan of MEV at all; they are the Dutton/Robin Hoods of the current market - and I'd take a J15 (basically a R1ot with a streamlined body) over either of them.

Antnicuk - 16/8/18 at 08:37 PM

sounds harsh, I thought the cars that didnt use the MX5 subframes were ok, The rocket certainly looks ok to my novice eyes. What is it about them that is cheap or poor quality?

Sam_68 - 16/8/18 at 09:07 PM

Perhaps I am being excessively harsh - I am not a fan of exoskeleton cars to start off with - but the chassis design is pretty average at best, with lack of triangulation etc.

Even MEV themselves admitted that that the Rocket " ...was designed for a 100 ft lb of torque from the 1.6 Focus engine", that it "...suffered from straight line sensitivity, even worse with a quick rack" and that a C-max diesel version had "...so much torque it started to tear the rear wishbone mounts off!"

If the car's own designer/manufacturer is making comments like those, I'd want to be treading very carefully with one that's fitted with a 2.2 Honda engine, capable of a LOT more horsepower than the design was intended for.

Maybe it's great... but it wouldn't be an obvious first choice for me, I'm afraid.

russbost - 17/8/18 at 08:50 AM

I can't comment on the pro's & con's of the Rocket, I know nothing of the original build, tho' I would have thought stiffening up a chassis bracket or adding a bit of flitching etc. was beyond most people on here, I've seen the Rocket in question & it certainly hadn't got anything out of shape when I saw it - pretty substantial construction I would have said, very different car to the Riot which surely is much smaller - the exoskeletal thing you either like or you don't, it's personal choice

I don't understand the wheelbase/head position thing, I was basing it on the 2 pics below & the position of the headrest (ergo, position of driver's head, which, last time I looked was where the inner ear was), it's very clear in the pics that the Elise seat back is further forward (not by loads, but enough to see fairly easily) in the wheelbase, I don't understand why that would come out differently in Sam's overlay - very weird???

[img]
[/img]

[img]
[/img]

Sam_68 - 17/8/18 at 09:37 AM

quote:
Originally posted by russbost
I would {sic} have thought stiffening up a chassis bracket or adding a bit of flitching etc. was beyond most people on here...pretty substantial construction I would have said...

Here's a photo of a MEV Rocket chassis. Note the complete lack of triangulation across the top and bottom. You've got triangulated trellises at either side, but connection between the two is pretty tenuous, to say the least. Note also that even the side trellises aren't completely triangulated; the sides of the engine bay are completely untriangulated too (which may help explain why the monumental torque (!) of a C-max diesel caused enough distortion to start tearing the rear wishbones loose). We'd talking about a fundamental re-engineering of the chassis - not just adding the odd bracket or flitch plate - to make it acceptable.

quote:
Originally posted by russbost I don't understand why that would come out differently in Sam's overlay - very weird???

It doesn't come out differently. I can only imagine that you're being deceived by the different appearance between a car with bodywork and one without. If you actually put a scale rule on those two images of yours (remembering to adjust for the skew on the MEV photo, which isn't quite side on - look at the rollover bar), then calculate the percentage of wheelbase that the driver's head position is ahead of the rear axle line, you'll find it tells exactly the same story as the overlay photo I posted above. Hardly surprising, as the mechanical packaging is virtually identical.

[Edited on 17/8/18 by Sam_68]

russbost - 17/8/18 at 01:50 PM

Not unusual for me to disagree with you Sam, but in this instance I think you are talking utter bollocks. Measuring those 2 pics, I get wheelbase identical as 10.1cm, with the front of the headrest at 7.1cm from the front wheels on the Rocket & 6.5/6.6 on the Elise, I don't know what you've done with that overlay, but the pics don't appear to back it up.

It's fairly irrelevant anyway as re the business about not being able to sense oversteer if you're well forward in the car, well take a look at my driving position in the Furore, I would say it's probably just forward of halfway down the wheelbase, I sense oversteer waaaaay b4 it gets out of hand & have never even come close to getting the car dangerously out of line, I'd say if you need to sit over the rear wheels to be able to sense oversteer it's time to give up driving & take up something like millinery!

[img]
2 Furores [/img]

With regard to the Rocket chassis, the whole front of the frame is basically a triangle with the point cut off, you obviously can't triangulate the cockpit area, the frame itself is made from tube of around (I'm guessing) 50mm dia as opposed to 25mm square box, it's just possible as an engineer you may have come across the phenomenon that a round tube is somewhat stronger than a square one, & you're welding around double the area at the point any moment will bear about, the sides are well triangulated, with the exception of the rear rectangle where the engine/gearbox pokes thro' the sideframe, impossible to triangulate that (tho' it could be flitched), I've just googled Rocket chassis damage & come up with nothing, it sounds as tho' you are basing your opinion of the chassis on one piece of hearsay, I don't particularly like exo cars either, I don't see the point, but no need to slate something just because you don't like it

I would say to the OP, go take a look at the car, examine it thoroughly, particularly with respect to the rear chassis & suspension mounts, if it's built properly & you like it, then buy it, if it looks like it's made from baked bean cans leave it well alone ...............

Sam_68 - 17/8/18 at 02:04 PM

quote:
Originally posted by russbost
Not unusual for me to disagree with you Sam, but in this instance I think you are talking utter bollocks...

Ok, whatever. You and I are obviously never going to see eye to eye.

I am happy to leave others to judge the issue of driver packaging, based on the overlay I did above... but do bear in mind that the view of the Rocket is skewed, so that anything behind the near wheels looks further rearwards than it really is (as I said, look closely at the roll bar). I can explain perspective for you, but I can't understand it for you.

I find it slightly worrying that as a manufacturer of spaceframe cars, you fail to appreciate that by introducing transverse tubes at the scuttle, you're wrecking what semblance of triangulation there was... you're turning the 'almost triangle' into an 'almost triangle' and an untriangulated rectangle. Similarly with your previous comment about adding gussets: adding gussets to a properly triangulated structure actually worsens the situation, because you're introducing stress raisers. They're a crutch for the incompetent.

[Edited on 17/8/18 by Sam_68]

johnH20 - 17/8/18 at 06:55 PM

This is a fascinating subject to which there is not a right answer. Like Russ I have a static weight distribution of 37F/63R ( without driver ) with my Cyclone. I have extensive experience of an S2 Elise with professional set up with distribution 40F/60R ( with driver ). Almost anything can be made to work cf. BTCC FWD car and a Porsche RS3. Key considerations are power/ traction and braking. I nicely balanced Caterham/ Westfield at 150 - 180 bhp is a thing of joy, over 200 bhp traction becomes an increasing issue. I was never 100% confident in my Elise despite the pro set up. I could spin this ( when under instruction ) and not realise why, when it goes it goes very fast. I have not 'managed ' this in my Cyclone primarily due to cowardice but also my 'fail safe' understeer set up. If I really want to rag it I am more confident in my mildly modified ( pro set up ) MX5 with 50/50 weight distribution but not much power. At the end of the day it is what you want to do and how good you are. ( Russ knows I am a crap driver in Karts at least so take all of the above with a pinch of salt! )