different profile tyres front and rear

Joe_Horner

Colino, before you get too heavy on the misplaced sarcasm:

(1) As an ex aircraft engineer, with over 35 years experience of working on cars as a hobby including 10 years of restoring classics, trust me - I have plenty of engineering knowledge

(2) jacking (or lowering) suspension by playing with the springs is an entirely different thing to (effectively) jacking the bottom of the wheel by fitting a higher tyre. Changing springs will affect suspension angles because you're altering the suspension and its relationship to the rest of the chassis.

Jacking under the wheel (in this case by adding a little extra tyre height) doesn't affect that relationship at all - it just lifts the whole car up a little bit, with the suspension remaining in exactly the same position relative to the body. It's absolutely no different to driving onto a 10mm thick plank of wood.

If you can't visualise the complete and total difference between those two scenarios then I'm afraid it's you who has no engineering ability

Incidentally, you still haven't explained how such a small a change of any sort at the back is going to affect a front suspension setting. I can only assume your descent into failed sarcasm is to cover the fact that you haven't got a clue

marlot

Joe_Horner wrote: »

... you still haven't explained how such a small a change of any sort at the back is going to affect a front suspension setting.

Oh that's easy. Here's an example of extreme caster, which makes it easy to visualise how raising or lowering the rear could make a small difference.

http://www.motorcyclespecs.co.za/Custom%20Bikes/Big%20Bear%20Sled%20Chopper.jpg

It's a big issue in the bike world. With most bike having a relatively short wheelbase, raising or lowering the rear makes a noticeable change to the handling.

marlot

Joe_Horner wrote: »

... jacking (or lowering) suspension by playing with the springs is an entirely different thing to (effectively) jacking the bottom of the wheel by fitting a higher tyre...

Errr, no. In terms of their effect on caster angle, they amount to pretty much the same thing.

If you think of the tyre and spring as being one unit, they hold up that corner of the car. You could fit a taller tyre, and a shorter spring to balance each other out, so that the rear sits at the same height.

I still think the real-world effect of raising the rear of a fiesta by 10mm is tiny, but if we're going to do this from an engineering perspective, we should do it right.

marlot

I think I've just found the caster angle settings for a fiesta. This is for an ST, so may not be appropriate to the OP's car.

Seems the correct setting is 3 degrees 47 minutes, +/- 1 degree.

If we take a wheelbase of 2500mm, and raise the rear by 10mm, that makes a change in caster angle of about 0.23 degrees.

So Colino is right about it having a measurable effect. But because Ford specify a pretty wide tolerance (a fiesta is not designed as a high performance machine!) there's a good chance it'll still be within tolerance. The only way to be sure would be to measure it, of course.

Joe_Horner

Thanks Marlot, after his little unprompted outburst above (not to mention his "which is most funny" comment on the last page) I was really looking forward to seeing Colino's engineering expertise explain but you've given him the answer - funnily enough, with about the same angular difference I calculated and posted earlier.

Only, it still doesn't answer the question of how it would make a meaningful change.

Bear in mind that the spring rate on the rear of a Fiesta will be (at an educated guess) around 3kg/mm, so a 60kg load in the boot will make that same 10mm difference to rear ride height. And that's ignoring the increased deflection of the tyre sidewalls from the extra load, which will add an extra few mm of movement.

In fact, with a 42 litre tank, just filling the fuel tank from empty will make half that difference. So going to Tesco, putting 30kg of shopping in the boot and filling your tank at the same time will, according to Colino, make potentiall dangerous changes to the suspension angles. Shouldn't someone point this out to Ford?

Spicy_McHaggis

Joe_Horner wrote: »

Thanks Marlot, after his little unprompted outburst above (not to mention his "which is most funny" comment on the last page) I was really looking forward to seeing Colino's engineering expertise explain but you've given him the answer - funnily enough, with about the same angular difference I calculated and posted earlier.

Only, it still doesn't answer the question of how it would make a meaningful change.

Bear in mind that the spring rate on the rear of a Fiesta will be (at an educated guess) around 3kg/mm, so a 60kg load in the boot will make that same 10mm difference to rear ride height. And that's ignoring the increased deflection of the tyre sidewalls from the extra load, which will add an extra few mm of movement.

In fact, with a 42 litre tank, just filling the fuel tank from empty will make half that difference. So going to Tesco, putting 30kg of shopping in the boot and filling your tank at the same time will, according to Colino, make potentiall dangerous changes to the suspension angles. Shouldn't someone point this out to Ford?

Ford have designed the car to carry a full tank of fuel and 30kg of shopping have they not?

But I can't recall any Fiestas leaving their factory with two different profile of tyres on the same size rims.

I wonder why not, must be a good money saving thing for them to throw any odd tyres on a car.

Joe_Horner

Yes, they have designed it to carry that, which is my point.

In fact, (at a guess, without the spec sheets) they've designed it to carry a couple of hundred kg in the boot and a full tank. Using my best guestimate spring rate above, that would make nearly 40mm difference to the rear ride height.

They've also designed it to carry no shopping and an empty tank.

Those two situations, which it has been designed for, will make considerably more difference to the rear ride height than an extra 10mm of rubber will, yet will be safe. Ergo, the extra rubber will also be safe because the theoretical change it will make is considerably less than the range of conditions the chassis was designed for.

As for not selling them like that - why on earth would they? (a) They'd have to procure 2 sizes of tyre in half the quantity each, which would inevitably cost more and (b) traditionally, customers would have been a little unhappy about having to carry 2 spare wheels. Less of a problem now that spares seem to be optional, I grant you, but tradition counts for a lot in selling stuff.

JustinR1979

My old cavalier cdi had pump up rear shocks, bet a few of them were a few mm higher at the back than they should have been; don't recall any headlines about all them crashing.

Iceweasel

JustinR1979 wrote: »

My old cavalier cdi had pump up rear shocks, bet a few of them were a few mm higher at the back than they should have been; don't recall any headlines about all them crashing.

Now if someone was to fit 100 cars with totally bald tyres and only three of them crashed, then the remaining 97 of the them would consider that bald tyres were not a concern, and anyone who said they were, was a scare-monger.

Such overwhelming proof can't be argued against.

In the USA of A, New Mexico, North Dakota, South Carolina and West Virginia have no tyre minimum depth requirement and we don't have any headlines about mass fatalities, do we?

So all this scare-mongering about tyre tread depth required in other states and countries must therefore be a load of tosh.

Some capitalist conspiracy between governments and tyre companies perhaps - to get us to buy tyres we don't really need.

JustinR1979

I'd suggest bald tyres were more dangerous than rear shocks set a few mm higher than ideal.
New shocks/springs will often see the ride height altered.