How much will lowering unsprung weight by 100lbs affect performance?

TL:DR Samurai on 31" tires that weight 60lbs a piece to tires that weight 35lbs a piece. Performance benefit?

I have been working on my samurais inability to go highway speeds and I have figured out the solution that isnt adding a turbo. I will be going to a "smaller" tire(31x9.50 to a 30x9.50) that also weigh 20-30lbs less per tire. I know I will be gaining performance just on the size difference as it gets the gearing back to stockish.(4.16:1 tcase which is a 12% high range reduction with 3.73 gears in the axles) How much will just the 100lbs+ of unsprung weight reduction help performance?

Losing 100 lbs can affect performance depending on where you loose that weight. In your case since the weight is rotational you will feel a major effect since your also changing the cars gearing as you already know. Taking 100 lbs out of the trunk, if you had that much there, wouldn't be nearly as noticeable.

what's holding it back from highway speeds? is it running out of "breath" or is it running out of gearing? Even heavy tyres should let it get up to 65mph.. eventually

In reply to mad_machine:

breath and gearing since it has the early transmission with the .795 5th instead of the .865 5th. it will do 60mph on flat ground in 5th any incline and 5th is useless.

mad_machine wrote: what's holding it back from highway speeds? is it running out of "breath" or is it running out of gearing? Even heavy tyres should let it get up to 65mph.. eventually

I'm guessing it runs out of power with 65 hp pushing a brick through the air.

The smaller tires will help acceleration. The fact that they are slightly smaller will help lower the wind resistance. Still won't win a drag race against a fit toddler on a big wheel but improved certainly.

In reply to KyAllroad:

hey it has a 1.6 block with a 1.3 head so it might have 85hp.

If you want the appearance of a huge tire on a small vehicle, you will have to accept the trade off. You want a much more pleasant vehicle, in probably every way, go back to a 215/70 15.

Rotational mass will affect acceleration, but will not limit ultimate top speed.

MrChaos wrote: In reply to mad_machine: breath and gearing since it has the early transmission with the .795 5th instead of the .865 5th. it will do 60mph on flat ground in 5th any incline and 5th is useless.

not quite the same thing there.... gearing would indicate you are hitting the redline at 65nph... your engine is simply running out of breath

mad_machine wrote:

MrChaos wrote: In reply to mad_machine: breath and gearing since it has the early transmission with the .795 5th instead of the .865 5th. it will do 60mph on flat ground in 5th any incline and 5th is useless.

not quite the same thing there.... gearing would indicate you are hitting the redline at 65nph... your engine is simply running out of breath

Gearing can be the cause of it running out of power though. If you're too far below the powerband, you won't have the guts to push it through the air. In this case, top speed is likely higher in 4th than 5th as you'll have more power available.

The 100 + lbs being lost are rotating as well as unsprung. Though still ginormous the shorter and skinnier giant hunks of rubber are 'quite a bit' of an improvement when spinning. Since the power you have is so marginal and it takes so little power to maintain freeway speeds everything you gain will be gravy over the minimum required to move your odd assortment of constraints. So color me optimistic. My answer to your question is "substantial".

Iirc every one lb of unsprung weight is equivalent to 4 lbs unsprung. Let's just say it is a HELL of a difference. Expecially if you only have sub 100hp on the first place.

Mass that is rotating and translating is effectively 1.5x sprung mass. As stated, top speed shouldnt be affected by the weight of the tires but the smaller diameter and profile will make a huge difference. Rotating tires are not very aerodynamic. The lower weight will help in every other category though: acceleration, braking, MPG, etc...

It would be interesting to experiment with ecomodder style aero modifications and see how much they help. You have a unique situation where you can actually measure max speed before and after mods without ending up in jail.

I'm confused. Aren't transfer cases 1-1 in high ? If so, then 65 mph should come in around 2600 rpm. oops, forgot the transmission od ratio. that would lower the rpm more.

Now about 2100.

In simple terms loosing one pound of unsprung weight is equal to loosing four pounds of sprung weight.
So, gearing and other factors aside, loosing this 100 pounds will be like taking 400 pounds out of the interior.

JohnRW1621 wrote: In simple terms loosing one pound of unsprung weight is equal to loosing four pounds of sprung weight. So, gearing and other factors aside, loosing this 100 pounds will be like taking 400 pounds out of the interior.

Sort of. Unsprung weight affects suspension behavior and ride quality far more than the same amount of sprung weight. Acceleration wise, they're the same. Rotating weight is the one that matters far more to acceleration than static weight (whether sprung or unsprung). In this case, the tires just happen to be both rotating and unsprung, although only the rotating part really matters for this issue.

In reply to iceracer:

the 4.19:1 gears in the tcase in the samurai lower high range by 12%

They were geared to just barely do the speed limit from stock, and you screwed with the gearing. It came with 26 or maybe 27 inch tires so you changed the gearing by 14 percent. So it would follow that there has to be a trade off. They are very popular wheeling trucks around here and I don't think even a stock looking one has ever passed me on the highway. Usually they are tooling along in the slow lane looking very uncomfortable.

bearmtnmartin wrote: They were geared to just barely do the speed limit from stock, and you screwed with the gearing. It came with 26 or maybe 27 inch tires so you changed the gearing by 14 percent. So it would follow that there has to be a trade off. They are very popular wheeling trucks around here and I don't think even a stock looking one has ever passed me on the highway. Usually they are tooling along in the slow lane looking very uncomfortable.

For 31's I realistically need the 4.9:1 gears to get slightly better than factory gearing. I just ordered a ZOR MySide2 and a 44mm Screaming Eagle CV carb so once these tires are done or it has bothered me enough it is getting 30's, but going to jeep yj springs will come before that.

MrChaos wrote: In reply to iceracer: the 4.19:1 gears in the tcase in the samurai lower high range by 12% But we don't drive on the highway in low range.

In reply to iceracer:

It is still a 12% reduction in high gear thats why its 2h, 4h, N, 4lo. It is an 85% reduction in low range.