LocostUSA.com

Learning how to build Lotus Seven replicas...together!
It is currently June 24, 2019, 4:22 pm

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 17 posts ]  Go to page Previous  1, 2
Author Message
PostPosted: May 2, 2016, 3:05 pm 
Offline
User avatar

Joined: August 27, 2005, 1:04 am
Posts: 1372
Location: Kamloops, BC, Canada
Posthumane wrote:
turbo_bird wrote:
Your current 106" wheelbase and 58% rear bias gives the balance point as 61.48" ahead of the rear tire. So now with the new 96" wheelbase, you would be down to about 56% on the rear if the cg stays in the same spot.

I think you might have that backwards. 58% rear weight bias means the CG is 42% of the wheelbase from the rear tire, or 44.52" from the centre of the rear axle. If there was no CG movement, moving the front wheels back puts the CG 46.3% from the rear axle, for a 53.7% rear bias. However, as jack points out, the amount of unsprung weight in the front wheels/brakes/suspension is significant.

Wrightcomputing, do you have the overall weight of your car and the unsprung weight on the front? I'm guessing you have light wheels and such, but let's assume an average 100 lb unsprung weight on a 1500 lb car for example, using method of moments.
A car weighing 1500 lbs with a 58% rear bias has the CG 44.52" from the rear axle, which puts the moment about the rear axle at 66780 in-lbs (1500*44.52). Subtract the moment produced by your unsprung weight at 106" (100 lb * 106" = 10600) and add back the moment of your unsprung weight at 96" (100lb * 96" = 9600 in-lb), or alternatively simply subtract the change in moment (100 lb * 10in = 1000 in-lb). The new moment is 65780 in-lbs, for a new CG of 65780/1500 = 43.85" from the rear axle. This corresponds to 45.7% of your 96" wheelbase, which puts you at a 54.3% rear bias.


Wow, yes I did. Thanks for the correction. I'm going to blame it on being in a hurry before I finished my coffee. I corrected the second part when 5he numbers went the wrong way, but didn't think through why that had happened.
Kristian

_________________
V6 in a book frame build. Now registered.
viewtopic.php?f=35&t=7587
viewtopic.php?f=33&t=18172


Top
 Profile  
Reply with quote  
PostPosted: May 2, 2016, 6:06 pm 
Offline
User avatar

Joined: December 7, 2012, 8:28 am
Posts: 1410
Location: Sarasota
Posthumane wrote:
turbo_bird wrote:
Your current 106" wheelbase and 58% rear bias gives the balance point as 61.48" ahead of the rear tire. So now with the new 96" wheelbase, you would be down to about 56% on the rear if the cg stays in the same spot.

I think you might have that backwards. 58% rear weight bias means the CG is 42% of the wheelbase from the rear tire, or 44.52" from the centre of the rear axle. If there was no CG movement, moving the front wheels back puts the CG 46.3% from the rear axle, for a 53.7% rear bias. However, as jack points out, the amount of unsprung weight in the front wheels/brakes/suspension is significant.

Wrightcomputing, do you have the overall weight of your car and the unsprung weight on the front? I'm guessing you have light wheels and such, but let's assume an average 100 lb unsprung weight on a 1500 lb car for example, using method of moments.
A car weighing 1500 lbs with a 58% rear bias has the CG 44.52" from the rear axle, which puts the moment about the rear axle at 66780 in-lbs (1500*44.52). Subtract the moment produced by your unsprung weight at 106" (100 lb * 106" = 10600) and add back the moment of your unsprung weight at 96" (100lb * 96" = 9600 in-lb), or alternatively simply subtract the change in moment (100 lb * 10in = 1000 in-lb). The new moment is 65780 in-lbs, for a new CG of 65780/1500 = 43.85" from the rear axle. This corresponds to 45.7% of your 96" wheelbase, which puts you at a 54.3% rear bias.

Perfect thank you the car weight with driver is 1760lbs (10 over minimum weight). Front unsprung mass is about 160lbs
The CG should still be at 44.52
Moment 1760 x 44.52 = 78355.2
Unsprung moment = 106 x 160 = 16960
New moment = 96 x 160 = 15360
Difference 1600
New moment = 76755.2/1760 = 43.61"
New front weight 45.43%
or 54.6% rear bias.

Looking at the numbers I really think shortening the chassis is a good move. Thanks for all the help everyone.

_________________
2015 & 2016 EMod Florida State Autocross Champion
2013 & 2014 DSP Florida State Autocross Champion

Scrap Metal Build Log viewtopic.php?f=35&t=14558


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 17 posts ]  Go to page Previous  1, 2

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
POWERED_BY