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Inline Four Racing Crankshaft Balance

General engine tech -- Drag Racing to Circle Track

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Vintagewrench
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Inline Four Racing Crankshaft Balance

Post by Vintagewrench »

crank2.jpg
Crank1.jpg

This is a 1967 Volvo B18 1.8L crankshaft which is going to be used for vintage road racing on a variety of different tracks and hill climbs after it is ground and re-balanced.

The standard rod journal dia. is 2.135 but it is going to be ground approximately .246" to 1.889" and Carrello connecting rods with a Honda Clevite CB1663 (or possibly a 1.850 Clevite CB1798) rod bearing are going to be used. The grind will remove some weight plus the rod bottom end will be smaller and narrower and will have only about 380 grams of rotating weight.

Photos found of other Volvo racing cranks show counterweights that have been lightened considerably. I would like to cut down the counter weights here on our heavy-duty Monarch engine lathe to above the finished target weight before it goes out to be ground and balanced. This will end up leaving a lot less time spent at the balancing shop and in overall time it can be finished faster this way (spring is on the way). Also will round off or knife edge the weights in the milling machine.

Can anyone give some guidance on how to approach this project and would it be better to narrow the counterweights, which has been done in the past, or take it off of the OD of the weights assuming it can be balanced if it is done that way. Looking to have it run as smooth as possible in the 2500 to 7500 rpm range.
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Re: Inline Four Racing Crankshaft Balance

Post by BillK »

Unless you have another reason to cut the counterweights I would have it reground first and then checked. Cutting the counterweights is liable to require adding weight back to get it to balance.

Unlike a V-8 crankshaft yours has two rods directly opposite of the other two so I don't think your modifications are going to affect the balance much at all.

You don't use bobweights to balance it because you have two rods going up while two are coming down. The crankshaft is pretty much neutral balance.
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Re: Inline Four Racing Crankshaft Balance

Post by Vintagewrench »

BillK wrote: Fri Feb 14, 2020 2:54 pm Unless you have another reason to cut the counterweights I would have it reground first and then checked. Cutting the counterweights is liable to require adding weight back to get it to balance.

Unlike a V-8 crankshaft yours has two rods directly opposite of the other two so I don't think your modifications are going to affect the balance much at all.

You don't use bobweights to balance it because you have two rods going up while two are coming down. The crankshaft is pretty much neutral balance.
BillK
Thanks for your quick reply, we are aware of everything you mentioned about crankshaft balancing. Although we what to remove as much rotating weight a possible for faster acceleration. Have seen photos of a number of modified four inline racing cranks including Volvo cranks that have been lightened for this reason and are wondering how to go about it.
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Re: Inline Four Racing Crankshaft Balance

Post by BillK »

Cutting the od of the counterweight would seem to be most effective at removing weight and I suppose the same theory would be in effect since the counterweights are in pairs. Making them thinner would probably help windage but not sure what the trade off would be. Hopefully Dave will chime in, he has probably balanced more 4 cylinder cranks than I have ever seen :) The biggest issue I see from a balancing standpoint would be to try and do all four as close to the same as possible.
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Re: Inline Four Racing Crankshaft Balance

Post by modok »

it already has only half the counterweights.
Personally I would leave it alone. You can get a LOT more difference from a light flywheel. 1 gram of material at the edge of the flywheel probably is the same as about four at the counterweight, or more.
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Re: Inline Four Racing Crankshaft Balance

Post by Dave Koehler »

You will only be able to take as much off of the CW as the the rod journal (stroke) will allow.
You will likely find you can't take as much off as you think. Put it in the lathe to test this out.
While you may gain some acceleration you may find that bearing life is affected if you get carried away with CW reduction.
I have had some road race guys go down that ultralite path and the outcome was not life changing.
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Re: Inline Four Racing Crankshaft Balance

Post by MadBill »

A guy I knew got a crank made by a big name for his 4 cyl. 1,500 cc Coventry Climax-powered Lotus 15 because of frequent failures. It proved to be bulletproof, but now the engines were ripping out main webs. The new shaft had virtually no counterweights... #-o
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Re: Inline Four Racing Crankshaft Balance

Post by SchmidtMotorWorks »

I recommend chamfering the main side of the CWs and that's it.
Unless you have a spare crank to cut into pieces and test the balance.
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Re: Inline Four Racing Crankshaft Balance

Post by Vintagewrench »

Thanks to all of you for your responses. We will only bull nose it and vee other end and remove as little material as possible, even though some do not believe in it.

The flywheel is going to be changed from the original cast iron 22 lb lump to an 8 pound Fidanza piece we have and leave it at that.
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Re: Inline Four Racing Crankshaft Balance

Post by SchmidtMotorWorks »

Vintagewrench wrote: Sat Feb 15, 2020 3:32 pm Thanks to all of you for your responses. We will only bull nose it and vee other end and remove as little material as possible, even though some do not believe in it.

The flywheel is going to be changed from the original cast iron 22 lb lump to an 8 pound Fidanza piece we have and leave it at that.
Just looking by eye, it appears the the crank is already under counter weighted.
You can easily check this if you have a junk crank that you can cut into pieces and simply static balance it.
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Re: Inline Four Racing Crankshaft Balance

Post by Vintagewrench »

SchmidtMotorWorks wrote: Sat Feb 15, 2020 4:30 pm
Vintagewrench wrote: Sat Feb 15, 2020 3:32 pm Thanks to all of you for your responses. We will only bull nose it and vee other end and remove as little material as possible, even though some do not believe in it.

The flywheel is going to be changed from the original cast iron 22 lb lump to an 8 pound Fidanza piece we have and leave it at that.
Just looking by eye, it appears the the crank is already under counter weighted.
You can easily check this if you have a junk crank that you can cut into pieces and simply static balance it.
Yes, modok also noticed this earlier so the plan for now is to leave it the way it is.
The Old Motor
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Re: Inline Four Racing Crankshaft Balance

Post by Dan Timberlake »

MadBill wrote: Fri Feb 14, 2020 10:53 pm A guy I knew got a crank made by a big name for his 4 cyl. 1,500 cc Coventry Climax-powered Lotus 15 because of frequent failures. It proved to be bulletproof, but now the engines were ripping out main webs. The new shaft had virtually no counterweights... #-o
I think those Coventry Climaxes may be 3 man bearings, in an aluminum block. Originally a light weight fire pump engine.

The hot set-up for block center main life as of several a few years ago over 6xxx rpm was to make a crank that looked like a 5 man bearing crank, to incorporate plenty o' counterweight beside each rod journal.
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Re: Inline Four Racing Crankshaft Balance

Post by MadBill »

Yes, I can see that would help a ton. I suppose the designers didn't contemplate 8,000+ RPM.... #-o
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