Weight distribution

[AMMO]

When setting up a racing motorcycle the weight distribution front to rear is deemed to be very important. In the old days a 50/50 weight distribution was thought to be best. Nowadays you would be looking to have 51/52% on the front. These figures are for the bike without the rider.

 

The front end bias is to stop the front end "washing out" amongst other things.

 

When I used to ride motorcycles I always found they handled better with a full tank of fuel or if you sat right up on the tank. The road bikes I rode generally had a 52% rear wight bias.

 

Anyone have the front to rear weight distributions of various engined Caterhams? Is the car generally very front end biased? How do the different engine weights affect front end grip?

 

Just curious.

 

AMMO

[Eugene]

Interesting question Ammo.

I do not have the answers but know that my Seven bites into corners better when the tank is low on fuel, making the car front end heavy(er). When I set my corner weights I have also seen the difference a tank of petrol makes!

 

BTW. have eventually met Paolo as he is only just back in the office!

 

Small Boy - with Loud Toy!

See Eugene here

and

Lotus@Herts hereid=green>

[Pierre Gillet]

Sorry to highjack this thread, but can I weigh my Seven by adding the weights of each corner measured one after the other with the same bathroom scale?

Cheers,

 

Pierre

[R2D2]

I think so providing the height of all the corners are the same.

[AMMO]

Eugene

 

Glad you finally caught up with Paolo. I've heard Rome is quite cold at the moment. How are you settling in to your pasta, pizza and red wine diet? Got the Seven over yet?

 

Can you remember from your corner weight setting roughly what your weight distribution was?

 

AMMO

[Morls]

On the bike thing, loading up the front definatly seems to help get it on it's side quicker and shifting weight backwards on corner exit helps give better traction for acceleraion.

 

I'm not good enough to really tell on a road or race bike but on a super moto/hillclimber where all the movements seem exagerated this definatly applies, unless you have wheelie problems.

 

The theory...

 

I think the corner entry theory on a bike is to do with the displacement of the front tyre centreline from the direction of travel when turning at medium or high speed.

 

Imagine 1. A line drawn from this to the rear contact patch...The contact line.

 

Remember you are, deliberatly or unconsciously, turning the wheel left to bank right at anything over about 20mph.

 

Imagine 2. Where, (front-to-rear) the CoG is. Say it's 600mm back from the front contact patch on bike x and 800mm on bike y.

 

When the bike is leaning right, the CoG is to the right of the contact line.

 

Importantly, it's further from the line in bike X than bike y.

 

The distance of the CoG from the contact line is what develops most of the high speed cornering force, hence why we hang off to make it more.

 

And why 125s with narrow tyres, (whose banked over contact line is nearer the tyre centreline), are quicker mid corner than bikes with big wide tyres.

 

The other bike issue is camber thrust.........

 

 

 

Mark

[Morls]

Sorry, had to get the pizzas out the oven.

 

Camber thrust:

 

Imagine two cones with the thick ends glued togather then rolled on a table, this is a tyre.

 

As soon as it falls to one side it starts to turn, the seveverity of the turn dependant on the gradient of each cone.

 

Modern big bikes have a severe cone on the front and a gentle cone on the rear which gives them an oversteery feeling as the front trys to follow a tighter arc than the rear.

 

By putting more weight through the front the bike will behave more like the front tyre charicteristic than the rear and hence accelerate towards the theoretical centre of the circle you're riding around quicker.

 

I hope the above makes sense and apologies if I've been teaching granny etc.

 

My point is... The reasons a bike needs weight on the front to turn does not apply to a car. In fact when I've had similar cars with different engines the lighter engined ones have all understeered less.

 

I always thought the quickest turning cars were those with the least polar moment of inertia, like an Ice ballerina; when they want to spin faster, bringing all their weight, (their arms), towards the centre.

 

Eugine's better turning with a low fuel load would support this theory.

 

Might be wrong tho.

 

 

 

Mark

 

Edited by - Mark Jackson on 18 Jan 2002 20:11:42

[Morls]

And one other thought.

The Caterham fireblade has a rear heavy CoG. The reports all mention how quickly it turns in, anyone finished building one yet? Or driven one?

 

Mark

[AMMO]

Mark

 

I've always thought it's so cool when you can stick "polar moment of inertia" into a sentence.

 

Incredible how many motorcyclists do not realise they are actually turning right to go left and vice versa.

 

I've always thought that the 125 bikes going around corners faster was mainly a question of weight. Put simply they are easier to control and correct.

 

We hear stories all the time of Caterhams on road tyres going around the outside of slick-shod Porsches, probably for the same reason.

 

Very interesting your experience of lighter engined cars having less understeer. I would have thought it would have been the opposite. Maybe something to do with the ability to brake later and having the front end loaded up more? I dunno. Just guessing.

 

AMMO

[Eugene]

Ammo,

Getting to like Roma a lot. I live in the Castelli Romani hills in Grottaferrata, a stones throw from Frascati. Lovely places both. The wine, pasta and pizza is great and so are the people.

 

From what I can recall my corner weights were 310ish front and 280ish rear without driver and low on petrol, with driver the rear offside increased to 340ish with some addition to the other corners (a reduction to front nearside I think). I have all the figures at home (UK)...

 

 

 

Small Boy - with Loud Toy!

See Eugene here

and

Lotus@Herts hereid=green>

[DonkeyDave]

unloaded a 1.6K engined widetrack 6 speed 7 gave a total weight of 537 (15 litres of fuel) front:rear 49.3/50.7

Loaded with driver 45.9:54.1

 

However the front to rear ratio isn't very important in 4 wheel vehicles. The cross weight percentage is of much more interest, this is LR+RF/total. The nearer you can get this to 50 the better. After much fiddling I've left it at 50.1!

[AMMO]

Eugene

 

Glad Rome is to your liking. Try to eat some gnocchi (traditionally served on a Thursday for some obscure reason). This should help put weight on the back wheels for better traction.

 

DonkeyDave

 

Thanks for that bit of info.

 

 

 

AMMO

[Morls]

I've been thinking about the front heavy weight distribution/understeer thing and have 3 theories, either or all of which could be completely wrong.

 

Assuming; A constant radius curve, A freewheeling car.

 

1. A simple theory. When you drive the same type of car with different engines, such as the XF/Pinto RWD escorts of my youth, the Pinto engined one has harder front springs to compensate for the weight.

The harder front springs could promote the understeer.

 

Or...the practical, "Blue Peter" method.

 

2. This one really needs a diagram but....

You take a 2 foot long stick with a hole drilled through each end. At the far end you attach a 6" stick with a hole in the middle, (using a finger tight nut and bolt), and a lump of blue tack at each end.

Draw an arrow along the 6" stick and pretend it's a car going round a corner, with the respective mass of the "front" and "rear" blue tacks giving the weight distribution.

 

Put a loose bolt through the other end and spin it round one's head in the direction of the arrow.

 

If the blue tack at the front of the "car" is the bigger, the front will rotate on a greater radius than the rear, and vise-versa.

 

Using this theory, a front heavy car will tend to run wide once the limits of adhesion are reached, a rear heavy car would tend to tighten it's line or ultimatly spin.

 

Or....The trying to remember college method.

 

3. The force keeping the car on line is the friction generated between tyre and road.

The force opposing this friction is equal to the lateral acceleration, (toward the centre of a roundabout), multiplied by the car's mass.

 

AS the lateral acceleration is equal across the length of a car, the lateral forces generated at each end will be proportional to the ratio of masses.

 

Given this, and assuming equal tyre compounds and sizes at both ends, a car with 60% of it's weight through the front would overcome friction force at this end first, and understeer.

 

I've assumed a fee wheeling car as what complicates it all is the driving wheels, where applying too much power breaks the grip before the centripetal force does.

Which, in a seven, is the fun bit.smile.gif

 

 

 

Mark

 

Edited by - Mark Jackson on 20 Jan 2002 22:02:04