Second gear from standstill

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timberstone
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Second gear from standstill

Post by timberstone » Mon Dec 16, 2019 4:56 pm

We posted the earlier Chat about tire diameter and top speed, but now wish to question the opposite end of the speed range:
With the original rear differential gear ratio of 5.125 to 1 and Dunlop original tires of 27" to 27-1/2" diameter, wondering whether any excessive wear of gears or clutch will result due to starting, from level standstill, in second gear ??
First gear seems too low, as only a few mph are gained before needing to shift at between 2000 to 2500 rpm.
Second gear seems like an ordinary first gear, at least on level pavement.

Does mounting larger diameter tires -- about 30-1/2" -- gain much speed advantage in first gear - on level ground?
Or is it better to use rear differential gearing of 4.875 to 1 ??
If so, in what ways.
There is a great difference in the expense between the cost of larger tires as opposed to cost of lower differential ratio gears, so would appreciate real data input about either choice.

Octagonally yours,
Boxley (Robert and MGTC0820)

Tom Lange, MGT Repair
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Re: Second gear from standstill

Post by Tom Lange, MGT Repair » Tue Dec 17, 2019 5:00 am

A TC box is a strong box - that's why TD racers used to retro-fit TC gearboxes - they are such better units than TD gearboxes. TC boxes hardly ever break, or wear out (other than the flimsy rear transmission mounts, which break all the time, and need to be reinforced). To my mind, first gear is designed to get you underway, and should be used. I tell people with TD cars to barely get moving, then shift into second and goose the throttle. That saves much wear on the very weak TD first gear.

But not with a TC. Perhaps the problem is that you don't stay in first long enough! Shifting at 2,000RPM is way too early; you have a lot of power left in your engine - I go much closer to 3,500 (or more) in my TCs. I would also say that you are putting more strain on the drivetrain - especially the clutch - when you start in second.

Finally, I question whether doing the differential swap for a Morris Minor pumpkin is the same cost as larger tires - I see stock Morris Minor pumpkins (actually HIGHER ratio) for $100-150, and Bob Grunau's conversion is not that expensive. Larger tires will cause fender-fit problems, steering-lock problems, and will look peculiar. People tried it in the days when model A tires from J. C. Whitney were all that could be had. Not so today, when there are plenty of 4.50x19 tire suppliers out there.

Tom Lange
MGT Repair

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timberstone
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Re: Second gear from standstill

Post by timberstone » Tue Dec 17, 2019 6:22 pm

Tom, thanks for some pointers.
Question where to get rear differential gears so reasonably that will fit properly. Noted a few suppliers in UK for the Morris Minor and Spridget differentials, but seems a welter world to investigate.
Found a posting by Robert Grunau in Ttalk.info for the following:
"TA/TC new high ratio crown wheel and pinion gear set, 4.625 ratio $750.00." -- but this was dated in 2014. So sent an email to GrunauMGatgmail.com (Replace at with @)for more information.
Then found "TA differential" in Ttypes.org -- dated December 2019 -- of "7/37" ratio (which figures out at 5.28 ratio)in the UK for 400GBP. This seems like the wrong way to go, plus my recollection was that the TA had 8/39 pinion and gears which would be 4.875 ratio.
MGTC0820 has the stock original rear differential gear ratio of 5.125 (marked 8/41 on the housing)that has been recently rebuilt with the bearing upgrades as outlined by Roger Furneaux. Maybe should have opted, at the time, for the ratio of 8/39 that is commercially available as ring and pinion sets of 4.875 -- or the even the 4.625 -- ratio for $800 to $1000.

I am understandably reluctant to go with these, with total costs winding up around $2,000, when with just slightly larger tires I could accomplish a similar result for a few hundred dollars.
And we would take off with first gear being more frequently usable.

Octagonally yours,
Boxley (Robert and MGTC0820)

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timberstone
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Location: Richmond VA

Re: Second gear from standstill

Post by timberstone » Tue Dec 17, 2019 6:22 pm

Tom, thanks for some pointers.
Question where to get rear differential gears so reasonably that will fit properly. Noted a few suppliers in UK for the Morris Minor and Spridget differentials, but seems a welter world to investigate.
Found a posting by Robert Grunau in Ttalk.info for the following:
"TA/TC new high ratio crown wheel and pinion gear set, 4.625 ratio $750.00." -- but this was dated in 2014. So sent an email to GrunauMGatgmail.com (Replace at with @)for more information.
Then found "TA differential" in Ttypes.org -- dated December 2019 -- of "7/37" ratio (which figures out at 5.28 ratio)in the UK for 400GBP. This seems like the wrong way to go, plus my recollection was that the TA had 8/39 pinion and gears which would be 4.875 ratio.
MGTC0820 has the stock original rear differential gear ratio of 5.125 (marked 8/41 on the housing)that has been recently rebuilt with the bearing upgrades as outlined by Roger Furneaux. Maybe should have opted, at the time, for the ratio of 8/39 that is commercially available as ring and pinion sets of 4.875 -- or the even the 4.625 -- ratio for $800 to $1000.

I am understandably reluctant to go with these, with total costs winding up around $2,000, when with just slightly larger tires I could accomplish a similar result for a few hundred dollars.
And we would take off with first gear being more frequently usable.

Octagonally yours,
Boxley (Robert and MGTC0820)

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Steve Simmons
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Re: Second gear from standstill

Post by Steve Simmons » Wed Dec 18, 2019 4:51 pm

Seems to me the only thing you'll hurt by starting in 2nd gear is the clutch.

The TA used 8/39 (4.875) and these could be found for around $600 new last time I looked. They also pop up used now and then on eBay. Bob G and Roger F both sell new 8/37 (4.625). I've run TA gears for several years now and am just about to install a set of 4.625 with the hopes that I can still climb hills.

You could also install a slightly larger tire and used TA gears. A friend of mine has that setup and he runs a few hundred RPM lower than I do. I believe his tires are 4.75/5.00 Firestones.
1949 TC8975 / XPAG 9609
1948 TC6011 / XPEG1182 (XPAG6472)
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timberstone
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Re: Second gear from standstill

Post by timberstone » Wed Dec 18, 2019 5:38 pm

Yes, Steve, I sure hope your wished for combination of tires and differential will not decrease hill climbing ability. Maybe you have increased the horsepower of your XPAG?
Those Firestone tires are close in listed diameter (at 29.8") and circumference (at 93.62") to the Excelsior model that I am favoring -- at listed measurements of 30.5" and 95.82", respectively, but those do not have the tread similar to the original Dunlops like the Excelsiors do.

Tom Lange rightfully points out that it is probably not good for the clutch and drive train to use Second (2nd) gear to start from standstill, unless on a slight downward incline or already barely moving. We just tried it occasionally over the years, but now also have new drive train parts. My reluctance is with always having to rev up to 3,500 revs in order to get a good start in modern traffic.

What to do for Boxley (Robert and MGTC0820) -- who have close to the stock horsepower when everything is "healthy" -- is to get the 30.5" diameter Excelsior tires and try to swap in the 8/39 (4.875 ratio of the TA) into this TC. The cruising speed will be at a more relaxed rpm, without too much decrease in acceleration and climbing ability. Not sure yet about going as far as the 4.625 rear end -- and whether that will be an all around improvement.

Just wondering why MG went with the gear ratio of 5.125 with the much improved XPAG engine?
Maybe it helped with acceleration, but sure not in top end speed.

Octagonally yours,
Boxley (Robert and MGTC0820)

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Steve Simmons
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Re: Second gear from standstill

Post by Steve Simmons » Wed Dec 18, 2019 6:06 pm

Remember what roads were like in England in the 1940's, and also that speed limits within cities were set to 30 MPH maximum. So the low gearing gave sporty acceleration and plenty of top speed for what most people were using the cars for.

My TC has increased HP but not a tremendous amount. I tour with TC Motoring Guild regularly, and my car is always one of the faster ones in the pack but not always the fastest. I will be increasing it further soon, but that's a story for another day.

I feel that the added stress of starting in 2nd will only be felt by the clutch. The rest of the drive train will probably see less stress, since the torque applied from a standing start will come on much more gradually than it would in 1st gear. Starting in 2nd, you have to let the clutch out much slower and it will take longer to reach maximum torque. So the rest of the car might thank you, even though the clutch will have to be replaced sooner.

Obviously there is no magic formula or we would all be using it. Every TC seems to be set up a bit differently than all others.
1949 TC8975 / XPAG 9609
1948 TC6011 / XPEG1182 (XPAG6472)
http://www.mgnuts.com

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