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MG MGB Technical - replacing 4th gear for highratio

Is there any information on replacing 4th gear for higher ratio in lieu of overdrive.
Sanders

4th gear doesn't involve any gearing, it is a direct drive from the input to the output.
paulh4

Here's an interesting video which explains how a 4 (or 5) speed manual gearbox works.

https://www.youtube.com/watch?v=wCu9W9xNwtI
Dave O'Neill 2

A common mod is to fit a 5 speed from another vehicle. Datsun boxes were popular.

Search and check out info on the MGEXP.com site.

Herb
H J Adler

A few places here doing Mazda MX5 5-speed conversions, expensive though at about 3k. It would be a fraction of that to get an OD box.
paulh4

Couple of the guys here have gone with 3.3? final drives and are really happy with them
MGBs pull them real easy-
William Revit

That change of axle ratio is comparable with an overdrive, or 5th gear ratio. I can see why they chose it and how it works. Seems to be a personal choice thing whether you like a long legged car or one that's revs, personally for me the torquey B series with an overdrive works well.
Stan Best

It is, but on all gears of course. 2nd to 3rd is a bit of a struggle anyway if taking off on a hill.

The MGC and V8 had a lower ratio because they have the torque. With Sanders being in the USA and probably a low-compression engine unless he boosts the power it would be worse.
paulh4

How does one categorise a higher/lower ratio in cwp terms? I always thought that a higher number eg 3.9:1 meant a lower ratio and a lower number, say, 3.7:1 meant higher. In other words, a higher ratio meant lower engine revs for a given speed in a given gear.
Peter Allen

I take it - as do many Google explanations - that a higher number relative to '1' denotes a higher RATIO. It's a lower GEAR in terms of selecting a lower gear to descend a steep hill, maybe that's where the confusion comes from.

This http://www.mossmotoring.com/the-highs-and-lows-of-differential-gear-ratios/ starts off well enough with:

"The lower the number, the faster the car will go with the same number of engine revolutions. The higher the number the better the car will accelerate, but at the expense of high speed cruising."

And goes on to say:

"A high numerical gear ratio is called a low gear or low rear end, and vice versa. Low gears give fast acceleration, high gears give better cruising."

But finishes up confusing the issue by saying:

"So if you install a bigger, more powerful engine you probably want to change to a higher gear ratio from stock."

Higher gear, lower ratio.
paulh4

Peter --spot on
Paul--confusing explanation


Low is low like low gear -1st gear
High is high like top gear

That's why they call low gear(1st gear) low gear cos it's lower geared

5:1-diff ratio --low geared
2.5:1-diff ratio --high geared

Driving the car--
start off in a low gear--then into 2nd (higher ratio)--into 3rd(higher again)
then 4th (top gear)
Top would mean high ----------?

And just for the record, one of the guys here with the taller 3.3? axle and 4spd all synchro box is a us import low compression car and he reckons that it pulls the taller ratio with ease and feels heaps better on the road than the original 3.9 ratio
William Revit

"That's why they call low gear(1st gear) low gear cos it's lower geared "

But a higher ratio, as I said, which isn't what Peter said.

paulh4

No it's not a higher ratio at all--That's what I mean by your explanation is confusing
----------
"That's why they call low gear(1st gear) low gear cos it's lower geared "
---lower gear ratio=more revs,less speed
---higher gear ratio=less revs,more speed
-as Peter said ,and I agree with him and his understanding of ratios
eg
--A Ferrarri is higher geared than a dunny truck
William Revit

Wow, you are getting your knickers in a twist over this. A higher numerical value means lower gearing. For example a 3.3 ratio means that the wheels are turning at the propshaft speed divided by 3.3. The torque is also multiplied by 3.3. As Paul says in fourth (non overdrive) the propshaft is turning at the same speed as the engine.
Paul Hollingworth

"A higher numerical value means lower gearing"

Correct. But

"I always thought that a higher number eg 3.9:1 meant a lower ratio and a lower number, say, 3.7:1 meant higher."

"start off in a low gear--then into 2nd (higher ratio)--into 3rd(higher again) "

Not correct. Both say a higher ratio is a higher gear but it isn't.

A higher number is a higher ratio, but gives lower gearing. 'Ratio' refers to the numerical relationship, but the effect on gearing is reversed, which is why I said select a lower gear for descending a steep hill (or pulling away from a standstill), which has a higher numerical ratio.

We are heading towards a situation where two people speaking the same language will be unintelligible to each other. Statements along the lines of one thing being 'ten times smaller' than something else drive me up the wall :o)
paulh4

Instead of expressing the gear ratio as e.g. 3.9:1, divide 1 by 3.9 ie approx 0.256 then low ratios have low numbers and vice versa .... simples!
Chris at Octarine Services

Yeah, it's easy - I can't really believe what's going on here
Just for you Paul I did the Google thing and typed in --Diff ratios explained-- and the answer --

Differential Gear Ratio determines the number of times the drive shaft (or pinion) will rotate for each turn of the wheels (or ring gear). ... With a lower gear ratio the drive shaft (and thus the engine) turns more for each revolution of the wheel, delivering more power and torque to the wheel for any given speed.
Differentials Questions West Coast Differentials

And that's how I see it
willy

Gearing has always amused me right from back in the pushbike racing days
You could ride a gear like eighty one for example and change to say a ninety one eight and cadence is reduced for any given speed
(higher geared dare I say it) but for some weird reason there are some gears that don't work --A straight ninety for example is avoided like the plague by bike riders, it's lower geared than a ninety one eight by a tiddle and you'd think it would be easier to spin but no, it's just a dead gear that doesn't work-------why
William Revit

I don’t think anyone is disputing how gearing works, just that the terminology is confusing - where low/high ratio and low/high gearing mean the opposite of each other.
Dave O'Neill 2

Eggsactly.

ITV forecaster last night said "We've had no daylight today ..."

Last week the BBC reported that the SNP were "aiming to reduce emissions 100% by 2040"
paulh4

It's all bullsh&t Dave---- How can anyone say a higher geared diff is a lower geared diff because the number is lower----wtf

and Chris--so MGBs have got 0.256 axles now

I don't want to do this anymore so I'm not going to---

Back to the original question of this thread
Stick a 3.3 c/w and pinion in it--you won't be disapointed and it would be the cheapest easiest option in my opinion

willy
William Revit

You keep missing the point that numerical ratio and how a gear is described have an inverse relationship.
paulh4

Apologies. I feel it was me who caused all this sturm und drang.

All correspondents clearly have an understanding of how gearing works, me originally from my childhood Meccano days. When I later got my first bicycle it threw me at first that the cog on the front was bigger than that at the back on a single speed bike, but my understanding was restored when I came across derailleur gears. It's all down to the required effort to move something efficiently.

What I think it comes down to is the use of the English. My conclusion is that when folk commonly use the term higher or lower ratio rear axle (as I do) it's a kind of shorthand for the vehicle being higher or lower geared. Thus my MGB with a 3.9 cwp has overall higher gearing than my Riley with a 4.89 equivalent, which I suppose is what Paul is saying about the the inverse relationship.
Peter Allen

Meccano for me too, and Lego Technical for our son. The rolling chassis with rack and pinion steering, coil-over suspension, gearbox and engine with moving pistons was brilliant, and has subsequently been built by his son.
paulh4

Gear ratio Driving Driven

https://www.sae.org/binaries/content/assets/cm/content/learn/education/motortoycar-samplelessonplan.pdf

My $0.02 worth. LOL.
Mike
Mike Ellsmore

Mike -LOL--
I found page 117 interesting--
"A colon is often used to show a gear ratio"
Ha ha ha ha ha
For what it's worth, if anyone wants to go the 3.3 axle conversion -Brown & Gammons have a specially machined set that fits an MGB carrier
It won't fit the MGC carrier , it is specially offset for the B carrier
If you need one for you C Chris it would mean finding a B carrier to do it ,or a suitable spacer ring between the crownwheel and carrier might work ok
Peter- Don't feel bad, it's all good fun,it's just taking Paul a while to realise I'm winding him up, or is that down-----------lol
William Revit

That this debate occurs only with diffs always interests me. Never occurs with gearboxes, where first gear (the one with the highest number) is referred to as low gear, we change down to a ration with a higher number, change up to a ratio with a lower number, and the gear with the lowest number is called top.
Paul Walbran

"where first gear (the one with the highest number) is referred to as low gear"

"the gear with the lowest number is called top"

There must be something about being upside down ...
paulh4

Nope, typical first gear ratio 3.5:1, typical top gear 1:1
Paul Walbran

It is a quiet time on the MGA/Mgb forums these days - maybe everyone’s queries have been resolved?
Mike
Mike Ellsmore

How's your racer been going Mike
I've been meaning to ask if you ever refitted your panhard bar in a different spot or decided better without it---Can't actually remember what was going on there
willy

panhard bars and gear ratios on the same thread, this will be good------------
William Revit

I think considering gear name and gearing ratio, in the same breath, is the confusing thing.
In gear names low means low speed, high means high speed.

Gear ratios are a simple way of defining the number of input turns to one turn of the output shaft.

Personally, I can't recall having heard low /high gear used in ages, just 1st, 2nd, etc

Herb
H J Adler

But we do say things like changing up and changing down
Paul Walbran

My post was intended to be a simple observation that it's fascinating that the higher/lower confusion exists when talking diff ratios, but not when talking gearbox ratios where the maths is similar.
Paul Walbran

On gearboxs, if you run out to full revs then you go up a gear--not down a gear
---- My point all along is that, for example
If i go to the track and select the diff ratio that i think will be the one and go out and find the tacho hitting 8 and the gong going nuts, then that tells me that i need to pull the revs back a few hundred so the diff needs bumping up to get the revs down---same as going up a gear in the gearbox---not down

yes--no

William Revit

No! If you're running out of revs, then you want to LOWER your diff ratio from, say, 3.9:1 to 3.3:1. Gear ratios are inverse to going up a gear or down at gear. When your are going down or to a lower gear you are increasing the gear ratio. If your are going up or to a higher gear you are lowering the gear ratio.
bhall1

Oh no--I'm shattered--
If I put a lower diff in it's going to rev harder--lucky we run rev limiters then eh--
And there I was thinking I'd have to put a taller diff in
Lower geared to me(using your examples) is the 3.9 with 3.3 being a taller ratio with less revs, you'd have to agree with that-----

3.9 will be lower geared than a 3.3 every day of the week
William Revit

You have to laugh ...

paulh4

...or cry.
Dave O'Neill 2

I can't believe how long this ridiculous conversation has been going on. We all understand how gear ratios work in practice. What does it matter if the nomenclature seems confusing?
Mike Howlett

May I echo your sentiments
g a f carr

It's not really a laughing thing'-- it's different opinions as to how ratios are expressed-
Here's part of a Jaguar writeup from a Northern hemisphere(US) source-----
-----------------------------------
Why 5 speeds?

The question is different for a road car vs. a race car. On a road car, it is sometimes desirable to have a low rear axle ratio to give better off-line performance. This is why Jaguar fitted 3.54's to many US delivery cars. On the other hand, a low ratio rear limits top speed, so the cars so equipped had no shot at the magic 150 number. Furthermore, running a 3.54 means a fair amount of cabin noise at highway speeds, due to high rev levels. Finally, a low ratio hurts gas mileage.

To drop the revs and gain back what's lost with a 3.54, two options are available: go to a higher axle ratio, or add an overdrive gear. Although more historically sensitive, higher axle ratios are not attractive, since off the line performance is still what counts here in the USA. A five speed (with an overdrive fifth) offers a way to get performance from takeoff to 150. It will also drop the revs at highway speed, giving a quiet ride and better gas mileage. The question is how to do it without dramatically changing the nature of the car: in other words, to add a fifth speed in a way that Sir William might have approved of.

On a race car, the requirement is different. For high speed racing, the rear axle ratio is often very high....LeMans Jaguars had axle ratios at least as high as 3.07, in fact, the highest ratio in the "pit kit" was 2.64! This allowed the cars to achieve high top speeds...170+ was typical for the LeMans E-Types. But this limits performance from a dead stop, so an extra low first gear could be used to compensate. If the gear ranges were evenly spaced, then the driver would find that there wasn't enough gearing to cope with a challenging road course. One possible solution is to add an extra gear, but top gear would remain 1:1. This would allow the use of evenly spaced ratios, and provide a good selection for the driver. The ZF was such a close-ratio box. Note that close ratio doesn't imply that the top gear has to be 1:1, although overdrive close ratio boxes tend to be six speeds. A disadvantage is that more gears add drag, reducing the effective output of the engine.
William Revit

Hi Willie,
I removed the Panhard rod a couple of years ago after the discussion on line here. I reckon it handles better now than before (anyway my time up Rob Roy hillclimb has improved and I have gotten older!)
Cheers
Mike
Mike Ellsmore

Hi Mike--thanks for that--as long as it's fast it's good, You might be like a good wine and get better and better and better-
I've been helping a mate build a MGA coupe, what a project, should have bought a goer and rebuilt it really but he bought a rolling chassis with a bare body, no trim or engine/gearbox, It's just starting to come together now but has been an endless search for bits and pieces--
Cheers
willy
William Revit

Interesting Mike's experience with his Panhard Rod. I made one for by '72 B and can't say I really noticed the difference on the road. However, on a trackday I was getting snap oversteer on a sharp corner and couldn't make out whether that was down to the rod or the budget tyres I using at the time. Since removed it and have put better quality tyres on (Uniroyal Rain Expert). Certainly the tyres made a huge improvement in the rain, but, perhaps more surprisingly, in the dry too.
Peter Allen

peter
Yeah, it all comes down to where the bar is fitted, most are wrong
Can't remember exactly what was up with Mike's but it was fighting the natural suspension flow
William Revit

This thread was discussed between 21/11/2019 and 02/12/2019

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