Changing Final Drive any downsides?

Changing Final Drive any downsides?

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Toastfrenzy

Original Poster:

24 posts

137 months

Wednesday 7th November 2012
quotequote all
Kozy said:
Going down a wheel size would have similar effects yes. The effects on RPM and acceleration are probably not as pronounced as a ratio change, but there are more benefits, less unsprung and rotating mass (by far and away the best place to lose weight), smaller, lighter, cheaper tyres etc.

My point is though that personally I would not spend money on gearing that could be spent on improving the engines output unless the car was significantly over or undergeared, which I don't believe a Clio is. Mods like this are great at making the car feel faster, but it's largely placebo with a near stock engine.
I have a set of catcams 438 - which has a gain of 15-20bhp - so near 190-195 and 875kg dry.

Krikkit

26,500 posts

180 months

Wednesday 7th November 2012
quotequote all
Toastfrenzy said:
Krikkit said:
Also worth thinking about what kind of diff you're using if it's getting pulled to bits - a plate is a significant upgrade from an ATB/Quaife type.
The only thing putting me off the plate diff is the maintenance - I can't get the box out myself (would have to pay to get it removed) and not too sure of the cost of the yearly strip & re-plating. Guessing that would be at least 2-£300 plus the £150 to remove - refit the box.

Not sold on the ATB - yet, though I've got an option on a 1 year old one, been used in 4 races, in the pipe line. This may be the deciding factor (cost and maintenance) for an LSD...It'll be an improvement over the open STD one fitted at the moment.
The annual/two year maintenance shouldn't be too costly, a chap I know runs a gripper plate in his Peugeot and a service was about £150 with some upgraded pins and bits of pieces to bring it up to current spec. But the costs spiral if you can't get the box out yourself etc.

If you don't want a plate, then you at least need an ATB. The difference in lap-times just from adding that will be significant, and a plate is almost the same amount again...

Toastfrenzy

Original Poster:

24 posts

137 months

Wednesday 7th November 2012
quotequote all
Krikkit said:
The annual/two year maintenance shouldn't be too costly, a chap I know runs a gripper plate in his Peugeot and a service was about £150 with some upgraded pins and bits of pieces to bring it up to current spec. But the costs spiral if you can't get the box out yourself etc.

If you don't want a plate, then you at least need an ATB. The difference in lap-times just from adding that will be significant, and a plate is almost the same amount again...
Bought the quaife today - a lot cheaper than new. Also after working out the time\expense of a gripper decided to go for the ATB - still to decide on the FD though.

Pixelpeep

8,600 posts

141 months

Wednesday 7th November 2012
quotequote all
always wondered this for a road car.

Lower RPM's = less fuel being used

what would be the downsides of say in my mapped Astra VXR which has plenty of poke in gears 1-5 changing 6th to something really long so you can only change up to it when you are doing 70 and so the revs @ 70mph would be 1500rpm.

Or does it not work like that?

Pumaracing

2,089 posts

206 months

Wednesday 7th November 2012
quotequote all
One serious error in your calculations is that the rolling radius of most tyres is NOT what the simple calculation from width, aspect ratio and wheel diameter would suggest. It is usually about 97% of that with full tread depth and 96% when half worn. That's as much as a 5mph error in what you think your rev limited top speed might be.

I suggest you check with the tyre manufacturer for an exact rolling radius.

As to gearing it makes very little difference in overall vehicle performance compared to what you might think. Engine bhp per unit time determines how much energy can be turned into motion and gearing only juggles this about slightly. Fitting a higher (numerical) final drive means each gear accelerates faster but has to be changed up sooner. Swings and roundabouts I'm afraid.

However the 4.91 does best match your potential top speed and would make a marginal improvement in overall performance.

Kozy

3,169 posts

217 months

Wednesday 7th November 2012
quotequote all
Pixelpeep said:
always wondered this for a road car.

Lower RPM's = less fuel being used

what would be the downsides of say in my mapped Astra VXR which has plenty of poke in gears 1-5 changing 6th to something really long so you can only change up to it when you are doing 70 and so the revs @ 70mph would be 1500rpm.

Or does it not work like that?
It does kind of work like that. You need the same power to maintain speed, regardless of whether you are at 4000rpm or 1500rpm, the difference is to generate the 30 or so BHP you need at 1500rpm you'll need a lot more throttle than you do at 4000rpm. The bonus is that using more throttle at lower RPM means reduced pumping and friction losses, and a higher effective compression ratio resulting in a higher thermal efficiency.

Not sure how well it works on a turbo car due to the amount you have to richen the mixture mind...

Kozy

3,169 posts

217 months

Wednesday 7th November 2012
quotequote all
Toastfrenzy said:
Any idea how to estimate how quicker the 7400 limit would be reached with a shorter FD..
In what gear, from what speed, going uphill or downhill? It wouldn't really mean much TBH.

If, as you say, further power upgrades are prohibitively expensive then the FD might be worthwhile, but it really is down to luck as to whether it benefits you more than it hinders you. If I couldn't find anything better to spend the money on I would probably just stick it in the pot towards consumables/testing/entry fees etc.

davepoth

29,395 posts

198 months

Thursday 8th November 2012
quotequote all
Given that you aren't using 1st gear at all for racing, it might be more worthwhile looking at a closer ratio gearset, although I have no idea of the relative prices.

Toastfrenzy

Original Poster:

24 posts

137 months

Thursday 8th November 2012
quotequote all
Received a reply from the FD manufacturer this morning - the 25% discount offered brings the price down to £579 (inc 20% vat), Not sure of the fitment of the crown wheel now that they have offered two choices and I have a Quaife to fit - can any one offer advice in this respect?

The price quoted earlier £644 wasn't including VAT either so quite a saving I've been offered.

Dear Peter,
the final drive ratios can be supplied in about 10 days from the order.
If you couple it with the original gearbox, the pinion is fully compatible with the original one (same layout).

Normally the crown wheel is supplied bolted-type (for our DA016 LSD) for racing use and you can adapt the original differential with it with easy modifications.
But if you need a pressure-type crown wheel (as the original layout), we can produce it for you in the same time production of the pinion.

25% discount was a July and August offer.
35% discount is for orders placed in the Koeln Iternational Expo (next week).

But if you confirm me the order and the payment is placed before the end of november, i can give to you the 25% offer in any case.

Thanks and regards,
Giulio Dialti



Original Message ----- From:

> - Your message: Hi, I'm looking at a final drive ratio of either 12/56 to 12\59, for my Renault clio 172 Cup with a JC5 gearbox...
>
> Will your final drive fit this gear box with no other modification? The bearings\needle rollers are the same/
>
> Also you had a price discount of 25% -35% (got an email earlier) can i also have a discount applied too?
>
> Thanks

Toastfrenzy

Original Poster:

24 posts

137 months

Thursday 8th November 2012
quotequote all
davepoth said:
Given that you aren't using 1st gear at all for racing, it might be more worthwhile looking at a closer ratio gearset, although I have no idea of the relative prices.
I did look into this, dog, straight cut and to replace the gear set would be around 2-3k. Take it you mean something like the top 2 gear sets on this page? http://www.bacciromano.com/index.php?p=prodotti&am...

Not sure If I want to spend that amount though would be of great benefit...

Toastfrenzy

Original Poster:

24 posts

137 months

Thursday 8th November 2012
quotequote all
Kozy said:
In what gear, from what speed, going uphill or downhill? It wouldn't really mean much TBH.

If, as you say, further power upgrades are prohibitively expensive then the FD might be worthwhile, but it really is down to luck as to whether it benefits you more than it hinders you. If I couldn't find anything better to spend the money on I would probably just stick it in the pot towards consumables/testing/entry fees etc.
Yes good advice!! - changing the FD isn't as straight forward as I'd thought - will still try and work out, in a rough form the new gear changes on track first.

Kozy

3,169 posts

217 months

Thursday 8th November 2012
quotequote all
Toastfrenzy said:
I did look into this, dog, straight cut and to replace the gear set would be around 2-3k. Take it you mean something like the top 2 gear sets on this page? http://www.bacciromano.com/index.php?p=prodotti&am...

Not sure If I want to spend that amount though would be of great benefit...
I think that would possibly represent even less VFM than the FD would. Those sets are great for the balls out, big budget race cars that have been tuned to produce ALL of their power between 7500rpm and 9000rpm and nothing much below that, as they really need to keep the engine on the boil. On a fairly standard engine with a nice wide powerband not that much altered from stock, you do not stand to gain a lot from cutting out large chunks of your power band. Again, it would be small gains for certain tracks and possibly hindrance on others.

Tonight I'll see if I can model up a thrust curve for your car so you can see the real difference in wheel torque deployed over the full speed range.

It will look something like this which I done for my car showing the effects of a shorter FD:



You can see how the in gear gains get pretty much cancelled out when you need to change up!


Pumaracing

2,089 posts

206 months

Thursday 8th November 2012
quotequote all
Well I ran your car through my vehicle performance simulation program with the actual tyre rolling radius and gearing on both diffs. The performance is almost identical on both. Because of where gear change points occur the car is actually slower to 60 and 100 mph on the 4.91 diff than stock by the odd tenth of a second but a tad faster to hit 120 mph. Essentially a pointless expenditure of money.

Kozy

3,169 posts

217 months

Thursday 8th November 2012
quotequote all
Pumaracing said:
Well I ran your car through my vehicle performance simulation program with the actual tyre rolling radius and gearing on both diffs. The performance is almost identical on both. Because of where gear change points occur the car is actually slower to 60 and 100 mph on the 4.91 diff than stock by the odd tenth of a second but a tad faster to hit 120 mph. Essentially a pointless expenditure of money.
My results show the same thing. Save your money. smile

zybertran

2 posts

112 months

Friday 5th December 2014
quotequote all
I did not run through the simulation, but had a look at the excel tool you used, it seems to be giving me different values? you might want to look at the data again? the rolling circumference from your input does not match with mine.. probably the SLR is calculated different??

I am 2 year late though!!