Straight Cut LT77 5th Gear?

V8Wedgehead · Postby **V8Wedgehead** » 14 Jun 2012 19:50

Anyone ever seen or used a set of STR stright cut 5th gears? I acquired a set and I know from the Mincie 4 Speeds with SC gears they are not quiet but require little effort for gear changes. The set is .93.

Michael
1980 RALLY TR8 FHC #0020

john 215 · Postby **john 215** » 15 Jun 2012 05:02

Hi Michael,

Never used a set on one of our cars, but as with all straight cut gears they are noisier, but personaly think a fair price to pay for extra strenght. Not indestructable though, was with Bert Smeet when he broke a couple of gears at Goodwood rally a couple of years ago in his ex works car

Be nice to see a picture of such a rare item

Cheers John

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Marko · Postby **Marko** » 15 Jun 2012 07:59

V8Wedgehead what would you use a straight cut gearbox for?

Straight cut gears are weaker than helical gears (with all other parameters the same,material, widht, etc...). Due to backlash and interuption in power delivery due to their design, you get of and on mesh of 2 teeth and 1 tooth at a time, hence the noise and lower strenght.

Straight cut gears have less mechanical losses in power transmision, cheaper to make and dont load gearbox bearings axialy, thats why they are used in racing.

Gear change effort has nothing to do with the gear teeth since they are constantly meshed together.

Cobber · Postby **Cobber** » 15 Jun 2012 08:46

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Marko

Straight cut gears have less mechanical losses in power transmision, cheaper to make and dont load gearbox bearings axialy, thats why they are used in racing.

<hr height="1" noshade id="quote"></blockquote id="quote">
As always our learned friend Marko is correct,
A better but much more expensive way of reducing power losses and preventing axial loading of gearbox bearings is to use double cut helical (herring bone patten) gears. These are seldom seen these days, in fact the last time i saw any was on the drive of large banbury extruder (Made in @ 1964) at the old Goodyear tyre plant in Thomastown.
Apart from the fact that double helically cut gears are hideously expensive to manufacture, the other disadvantages to using them is weight and bulk. 

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Marko · Postby **Marko** » 15 Jun 2012 12:57

thats overkill [:D]

new materials are readily available that can increase performance of regular gearboxes by fitting straight cut gears, with or without dog cluctch engagement.

The biggest problem with pushing more power trough a transmision is the gearbox shaft distance (gear pair center distance), and shaft diameter.
Because you can have a gear pair of best possible gear design, made out of best possible material, and they are going to tear each other appart if the gearbox shafts start to bend each other causing a wrong gear mesh.

Thats why on proper racing gearboxes you got:
bearing , 2 gears, bearing, 2 gears, etc...

Its not the gear design that kills the the gears its usualy shaft deflection

bmcecosse · Postby **bmcecosse** » 15 Jun 2012 21:50

Oooohh yes - I've seen the Banbury gears in action - I used to work for UniRoyal tyres. And the Citroen trademark symbol is based on such gearing - originally they were gear manufacturers. To get back to the str cut gears - the main reason for them is COST. Relatively easy to make different ratios in small quantities. Much noisier (but a Mini with str cut gears is a marvelous thing!). And as others have pointed out -the gear is not usually the weakest link in the 'box........

V8Wedgehead · Postby **V8Wedgehead** » 16 Jun 2012 01:32

John

Here you go as requested

I got two sets from a factory sponsered TR8 racer. He never had an issue and it complemented the close ratio set. I want to retain a LT77 in my Rally project since so much of it is vintage.

Michael
1980 RALLY TR8 FHC #0020

Marko · Postby **Marko** » 16 Jun 2012 08:18

Cost of the straight cut gear and helical gears are the same.
Gear cutting/grinding machines are built to make helical gears, cutting straight gears is just one of the settings on the machine.

Helical gear is the base gear from whitch the straight gear is derived from.

Are all gears in the gearbox straight cut? or just 5. gear?

What would be the price of a straigh gear pair for that gearbox?

V8Wedgehead · Postby **V8Wedgehead** » 16 Jun 2012 13:38

The rest of the gears are standard non-straight cut gears factory close ratio set, on right. Standard gears on left.

From what I was told STR/BL Motorsport only made about 10 to 20 sets of the 5th gear straight cut gears. I got a deal of the century on the CR and straight gears considering how rare they are. Bert snatched up the last two sets of CR gears after I bought mine. He really needed them so they went to a good home. I think I am going to a gear manufacturer to see how much they would cost to be made.

Michael
1980 RALLY TR8 FHC #0020

Chris Turner · Postby **Chris Turner** » 16 Jun 2012 17:12

A very good rally driver told me a couple of years ago that there would be little or no advantage on a stage with either the close ratio, or straight cut gearboxes for a driver such as myself with limited capabilities. He said put your ex works close ratio box away safe and use the easuily replaced standard LT77 gearbox.

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FI Spyder · Postby **FI Spyder** » 16 Jun 2012 23:19

I don't know what Tony Pond et al raced but John Buffum raced his without a 5th gear suffering a little on top end but making up for it in reliability saying 5th was the weak point of the gear box. Couldn't have been far wrong with Championships and lots of wins.

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

Workshop Help · Postby **Workshop Help** » 17 Jun 2012 00:24

Yes, my friend, I read that too. Let us remember tho he was racing full out in that hot rod 1976 white TR7 with a power output nearing the design limits of the LT77 gearbox. Plus it was on a race track dicing around with all the rest of the worlds hot exotic cars. My point is that driving environment is far different from our more or less 100 horsepower street cars scampering about in a speed limit and driving behavior controlled world.

Is 5th gear a weak gear for us? I don't think so. Not in our as defined environment. Oh, sure, slip into fifth under 40mph and it'll groan a bit. But, it's common sense to stay in fourth till the engine torque is ready for the shift, which is about 40mph on level-ish ground.

I like our LT77s. They are strong, have a good mechanical feel thru the shifter, and place the gear ratios at the right spots in the power curve. I like it just as it is.

Mildred Hargis

Marko · Postby **Marko** » 17 Jun 2012 11:16

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by V8Wedgehead

I think I am going to a gear manufacturer to see how much they would cost to be made.

<hr height="1" noshade id="quote"></blockquote id="quote">

Could you tell me what will be the quote, just to compare with prices here.

FI Spyder · Postby **FI Spyder** » 17 Jun 2012 13:03

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Mildred

Is 5th gear a weak gear for us? I don't think so. Not in our as defined environment.
I like our LT77s. They are strong, have a good mechanical feel thru the shifter, and place the gear ratios at the right spots in the power curve. I like it just as it is.
<hr height="1" noshade id="quote"></blockquote id="quote">

I agree with you but we were talking about straight cut and gears for racing here. He was racing with 220 HP for the 4 cylinder and somewhat more for the eight. Can't recall what he had with the 8 using carbs (for reliability in the rough world of rally) but Tullius had about 330hp for longer endurance races and 360HP for the shorter races with FI. I think he left 5th gear out as well.

Regardless, a LT77 that is not abused should last 180,000 miles.

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

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Straight Cut LT77 5th Gear?

Straight Cut LT77 5th Gear?

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