15 inch low profile ???

[Readit]

A question for you techies out there. Original UK SE wheels are 15 inch normal profile, S2 and US are 16 inch normal profile, many enthusiasts after-fit 17 inch low profile to maintain circumference and therefore gear ratios, speedo accuracy etc.

I had considered that by fitting 15 inch low profile wheels and tyres, the top speed of the car would be reduced, but 139 mph is not often used. Gear ratios would be changed and acceleration improved. The only downside I have come up with is speedo inaccuracy. Anybody have experience of this, or reasons to be careful?

[Brit-Car-Nut]

Tire circumference = 2 x pi x the tire's radius or easier, pi x the tire's diameter. pi=3.14159

Complicated analysis: The wheel turns (X) times per mile. Each turn is the tire's circumference. If you divide a mile (5280 feet or 63360 inches) by the tire's circumference you get the multiplier (X).

Then multiply the NEW tire's circumference times the multiplier and you will get the discrepency in distance traveled by the same number of rotations.

Easy analysis: divide the original tire's circumference by the new tire's circumference and get the percent deviation.

It doesn't take a huge difference in tire diameter to affect the speedometer:

Shorter tires will show higher speeds than traveling and more miles traveled; Taller tires will show lower speeds than traveling and less miles traveled.

[Rob P]

Not sure if this is of use to you but the link below will take you to a tool which will allow you to compare various tyres sizes to see how it affects the speedo reading

http://www.kouki.co.uk/utilities/visual ... calculator

[Readit]

Thanks for that Rob and John, From the tyre size calculator they suggest that the percentage difference between the new and old circumference would be the increase or decrease of power to the wheels, thus the acceleration. I wonder if it is as simple as that.

[Brit-Car-Nut]

It really depends on the power band where the car is most efficient. The final gearing should put the car in that power band as much as possible.

[theelanman]

Thanks for that Rob and John, From the tyre size calculator they suggest that the percentage difference between the new and old circumference would be the increase or decrease of power to the wheels, thus the acceleration. I wonder if it is as simple as that.

?????......mmmmm....I thought the % difference is the % that your speedo will now be out....(so would need recalibrating to read correctly)
I doubt it will increase or decrease power.....
if you have a smaller tyre then you will need it to go round faster to achieve the same speed as with 'normal' tyres....
if you have a bigger tyre then you will need it to go round slower........
the speed at which the drive shafts rotate in comparison with the revs will not change......
you may need more torque to turn larger wheels but thats a whole different animal

??????

[dapinky]

Readit,

Please don't take this the wrong way, it isn't meant to be a derogartory post......

....but what exactly is the question, and more importantly, is there a desired outcome, or just a collection of thoughts?

If it is purely for the look of the car there are different consideration to if you wish to change the performance.

My first thought is to the 'ride' - if you lower the profile further than what is already fitted, then there will be less flex in the tyre sidewalls, and a harsher ride (you'll notice road defects far more).

The least of your worries is the speedo calibration - that is easily put right, the odometer, on the other hand will always be out of true, and can't easily be sorted (probably not out enough to worry about, but a consideration all the same).

Basically, the speedo & odo rely on the corect combination of gears between the gearbox output shaft and the clock on the dashboard (some at the gearbox/drive addaptor, and more inside the clock unit) - this isn't an exact science as the ratios are not infinately adjustable - the 'best' ones are selected from those availlable. As such, if you were to leave the gearbox ones the same, and fit a speedo unit from a Calibra or Astra Mk 3 it will be wildly out (albeit consistently so). The ratios are selected to give an accurate correlation between final drive rotation speed and wheel rotation speed - so are basically dependant on the final drive ratio in the differential unit. An Astra has a clock which reads a maximum of 120/130mph, a Calibra will read 140 (or 160 for the V6/turbo), the elan is 140 (all of them have the maximum at full scale deflection of the needle, so clearly they aren't simply interchangable).

A smaller diameter circumference on the wheel will give a theoretical improvement in accelleration, and a lower speed for any given engine revs - but it probably wouldn't be noticable.

If you consider that a new tyre will have 8mm of tread, and a bald tyre will have 0mm - the odometer cannot remain accurate during the life of a tyre anyway as the circumference will diminish over the life of the tyre (so bald tyres accellerate faster than new ones - does anyone notice this!) - changing the profile just exagerates this slight difference one way or the other. A few years ago i did all the maths for new vs old tyres on different wheel sizes/profiles - it's on here somewhere - not enough difference to worry about.

The 'standard' profiles of 50% for a 15" wheel, 45% for a 16" & 40% for a 17" all give pretty much the same rolling circumference, but having used 17" and 15" wheels, I much prefer the ride on the 15" with the higher profile.

The lowest profile you would be able to fit on a standard 205mm width tyre is 40% (you may be able to find 35% if you look hard enough), other than that, you would need to go narrower.

The maths (assuming that 15" wheel = 381mm, and there are no losses)....

15" - 50% profile - - - - tyre width = 205, so tyre height = 102.5mm - - - therefore diameter = 381+102.5+102.5 = 586mm - - - - - circumference = 3.14x586 = 1840mm

15" - 40% profile - - - -tyre width = 205, so tyre height = 82.0mm - - - therefore diameter = 381+82+8s= 545mm - - - - - circumference = 3.14x545 = 1711mm

This shows a 7% difference......

but bear in mind that the 8mm of tread on a tyre would increase the total diameter by 16mm, and you see that there would be a 4% difference between a new tyre and a bald one anyway - which we don't notice - so will 7% show up??? (it might do, as it would be an instant change instead of a slow adaptation).

The speedo will read lower, but if the odometer is geared to allow for 869 revolutions of the tyre/mile (assuming 1600m per mile), you will only actually travel 1487m for every mile you clock up - meaning that the mileage will increase on the clock faster than it should. (okay, it's a small [7] %age, but maybe a consideration to you) - as we have already said, there is a 4% difference during tyre life anyway.

If you are trying to improve accelleration, then there are other options - either improve the torque output of the engine, or change the efficiency - one way is to fit the diff unit from an Elan NA which has a lower final drive ratio (not a quick, simple job) - another is to change your tyres/driving style/power delivery to maximise the traction between the car and the road.....

....sticky tyres with good clutch/throttle control will give far more noticable differences (IMHO).

The biggest problem with all of this, as John says, is keeping the car in the power band at the useful times - if you make any of the changes discussed, the overall gearing will change (during gear shifts) and you may find that there is a greater gap between gears, and the car drops out of the power band each gear-change, and you end up slower than you started!!!

[Readit]

Thank you for that detailed explanation Dave. The thought was a theoretical one at present, having considered the purchase of some aftermarket wheels and looking into the performance increase of an Everest chip which is roughly 20% increase. The largest 15 inch x 205mm tyre circumference or the smallest gave a percentage distance covered difference of nearly 20% which would theoretically increase the power to weight ratio by 20%. As an Elan has a top speed of nearly 140mph, this is not usually very useful and my consideration was to reduce the top speed in order to improve acceleration, which I find is the most exciting part of driving any Lotus.

The speedo, although a problem was not of primary consideration, ride height may be more problematic.

[rip]

The largest 15 inch x 205mm tyre circumference or the smallest gave a percentage distance covered difference of nearly 20% which would theoretically increase the power to weight ratio by 20%.

I know what you mean, but you're using the wrong terms.The dimensions of the wheels & tyre will have no effect on power at all.
They will affect torque (& therefore forward force) which is what you are hoping to achieve.

Power = force x speed.
Decrease the speed & you'll increase the force...which is what you are aiming to achieve.

Bear this in mind though:
Forgetting about top speed, Lotus geared the Elan for driveability. Your engine will be revving harder & consuming more fuel at a cruising speed of 70mph. Do you really want that>
At this speed, your turbo is only just on standby & reacts very quickly when you touch the accelerator. This is not a coincidence.

[theelanman]

The largest 15 inch x 205mm tyre circumference or the smallest gave a percentage distance covered difference of nearly 20% which would theoretically increase the power to weight ratio by 20%.

I know what you mean, but you're using the wrong terms.

the only way youre going to increase your power to weight ratio is lose weight or gain power......
forget the wheel going round bit......

[Readit]

Rip, I agree I am probably not using the correct terms, but if Lotus geared a car to do nearly 140 MPH in top gear I do not think it is economically geared for 70 MPH. I love the Elan's acceleration between 70 and 90 MPH, it seems that no time elapses between the speedo changing from one speed to the other. but we are talking about exceeding UK speed limits even at those relatively low speeds.

[Readit]

A further thought, fitting a Mountain Chip to increase performance must increase fuel consumption and strain on the engine, I would be interested in your views on where the difference is on the driveability.

[Fredjohn]

Another consideration is ride/handling. Lotus spent a lot it time and money on getting these right. Even though original tyre is no longer available, some may say replacement tyres don't replicate lotus's intentions, let alone changing profile.

Also similar and worth comparing is that the old Elan had 2 different diff ratios available: one for acceleration one for top speed. Putting in a different diff would have similar effect as changing tyre profile. But guess this is either not possible or very costly??

Edit: just re-read thread and seen dapinky's thorough post which covered my 2 thoughts. Need to read lec on pc instead of tiny mobile phone.

[theelanman]

Putting in a different diff would have similar effect as changing tyre profile.

????
surely if that was the case then why invent another diff when all you needed to do was change the tyres....
is the diff not 'geared' basically?.....3.55 or 3.7...convert the input from the prop into an output from the diff to the drive shaft......
the speed at which he drive shaft rotates doesnt change

[Fredjohn]

Putting in a different diff would have similar effect as changing tyre profile.

????
surely if that was the case then why invent another diff when all you needed to do was change the tyres....
is the diff not 'geared' basically?.....3.55 or 3.7...convert the input from the prop into an output from the diff to the drive shaft......
the speed at which he drive shaft rotates doesnt change

Yes it does: assuming a constant prop speed, the drive shaft speed will change if you put in a different ratio diff. Or viewed from a constant road speed you would need more or fewer revs for different ratio diffs. Thus altering the acceleration profile.

The overall wheel diameter changing also effectively changes the gearing. Hence the original question. , which dapinky thoroughly answered before I chipped in without having read his post!

[theelanman]

Putting in a different diff would have similar effect as changing tyre profile.

????
surely if that was the case then why invent another diff when all you needed to do was change the tyres....
is the diff not 'geared' basically?.....3.55 or 3.7...convert the input from the prop into an output from the diff to the drive shaft......
the speed at which he drive shaft rotates doesnt change

Yes it does: assuming a constant prop speed, the drive shaft speed will change if you put in a different ratio diff. Or viewed from a constant road speed you would need more or fewer revs for different ratio diffs. Thus altering the acceleration profile.

The overall wheel diameter changing also effectively changes the gearing. Hence the original question. , which dapinky thoroughly answered before I chipped in without having read his post!

my point exactly.....input to diff one speed...output a diff(erent) speed
I agree you need different revs to achiev the same speed depending on the diff.....
and also agree that the diff alters the acceleration profile
my point is....so why invent a different diff.....if when changing the wheels does it according to your explanation
the effect would be negligable by changing the wheels we are alking about.....which is why you have diferent diffs
if you were changing to tractor tyres then I agree it would make a difference.....and you could keep your diff
but increasing/decreasing the profile by a couple of % will do nothing noticable.....
other than knock your speedo off.....although it wasnt that acurate anyway....

[theelanman]

is the diff not 'geared' basically?.....3.55 or 3.7...convert the input from the prop into an output from the diff to the drive shaft......the speed at which he drive shaft rotates doesnt change

sorry just re-read myself.....I didnt express myself well at all.....what I meant was there is one speed in and another one out.....with a constant 'in' you get a constant 'out'.........and yes the output speed is different depending on the diff.....
if you have a constant input x through a 3.55 diff then you get output y
if you have a constant input x through a 3.7 diff then you get output z

[Fredjohn]

After this long discussion I'm glad we agree! Would hate to have upset lec members before I'd even met them. Wish the world's problems could be solved so easily.
I phrased it badly, but meant a different diff would have a bigger impact than lower profile, but would have its own problems.

Consensus seems to say stick with original, I think!!?

[rip]

Rip, I agree I am probably not using the correct terms, but if Lotus geared a car to do nearly 140 MPH in top gear I do not think it is economically geared for 70 MPH. I love the Elan's acceleration between 70 and 90 MPH, it seems that no time elapses between the speedo changing from one speed to the other. but we are talking about exceeding UK speed limits even at those relatively low speeds.

Engines usually have 3 different important speeds:

Max power (typically around 6000rpm for a road car)
Max torque (lower down, around 3000-4000rpm)
Max economy (lower still, 2000-2500?).

I think 3000rpm for cruising at 70 is enough. Any more would be noisier & consume more fuel.

A further thought, fitting a Mountain Chip to increase performance must increase fuel consumption and strain on the engine, I would be interested in your views on where the difference is on the driveability.

Engine & transmisison stresses would definintely increase when on-boost. many on here would argue that these can easily cope with the extra stresses.
Economy only suffers when on boost, which is not very often (apart from the first few weeks of fitting when the extra power is a little addictive )