MXR Spring weight and Sag


synics

Active member
Location
New Jersey
Hey Guys,
So on my 2018 MXR I have been bottom out on the rear, and not even on jumps, but hitting whoops hard and such. I don't know what spring rate is on the bike as I can't see any markings on the spring. I ran my info through the race tech calculator and it says stock is 5.8kg, and that is what it recommends for my weight.

Should I get a stiffer spring anyway? How would I even know what spring rate?
Or, which setting should I mess with on the shock to see if that will reduce the bottoming?

Although I'm not sure that would do anything I suppose. Here is my RaceTech info:

Modifiers:
Riding Type: Enduro
Skill Level: Intermediate/B Class
Stiffness Preference: Standard Stiffness
Age: 30-44 Years Old
Height: Standard Rider Height
Gas Tank: Standard Gas Tank
Spring Conversion: No Spring Conversion
Lowering: Not Lowered
Rider Weight: 195
Sag: 107 mm

REAR SHOCK SPRINGS
Recommended Spring Rate:5.87 kg/mm
Stock Spring Rate:5.8 kg/mm (stock)
 

Fod

Well-known member
Location
CA
I'm guessing there are some my18 mxr bikes that have a very light valving in the shock vs some that were better and had what was probably tested and supposed to be in it. That said, mine was extremely soft. I believe stock rate was 6.0 kg... either way I am 185 and I went to 6.2, then 6.4 but for motocross. It helped, but valving was the main fix. Not talking minor valving... we did 3 revisions of going stiffer till it was good. Brian at N2 dirt did a fantastic job but they are west coast.
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
There is a suspension sub-forum that has a lot of this info. Suspension

The stock 2018 MXR has a 60N/mm spring. That’s 6.1 kg/mm. It should have a marking on it like the one below. I upgraded to a 64N/mm spring on my bike, which helped. But I’m 240 lbs. My 150 lbs son rides it just fine without complaint. I find the stiffer spring actually balances the forks a bit better. But that might just be me.
1652161062084.jpeg
 

synics

Active member
Location
New Jersey
There is a suspension sub-forum that has a lot of this info. Suspension

The stock 2018 MXR has a 60N/mm spring. That’s 6.1 kg/mm. It should have a marking on it like the one below. I upgraded to a 64N/mm spring on my bike, which helped. But I’m 240 lbs. My 150 lbs son rides it just fine without complaint. I find the stiffer spring actually balances the forks a bit better. But that might just be me.
View attachment 8548
Thanks man. One thing that may be the problem actually is that my tire is too far up in the swing arm because I switched to a 13 tooth sprocket up front and then put a 120 tire on the back. I should probably get a longer chanlin and start there first which is kind of annoying because those pro taper slim chains are like 130 bucks. Ugh.
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Yes, I also added a link to the chain on both of my bikes. It made fitting wider tires easier. It also moved the tire back a bit so it doesn’t rub on that little protruding section under the rear fender when it does bottom. Otherwise the bikes tend to get this skid-mark under that white piece under the fender. After moving it back it doesn’t look like it hits anymore.
1652191245905.jpeg
 

synics

Active member
Location
New Jersey
Yes that's exactly what I'm taking about! But doesn't adding a link to the chain compromise the strength? So you're saying you have two Master links on one chain?
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
No. You just buy a longer chain. I bought a 120 link chain, and then cut it down to 118 links instead of the stock 116. At least I think that was the number. Basically just the next link up that you can put the master link. I put the old chain next to the new one and marked the next link to cut it.

I went with o-ring, as it is much quieter than the stock non o-ring. But that is personal preference.
 

Philip

Administrator
Staff member
Location
Ann Arbor, MI
You can also keep your chain, re-install the stock 12-tooth front sprocket, and buy a new 49-tooth rear sprocket. This will move the rear wheel back by 1 chain link. The new 12/49 ratio will be almost the same as your current 13/53 ratio.
 

Philip

Administrator
Staff member
Location
Ann Arbor, MI
Here is my RaceTech info:

Modifiers:
Riding Type: Enduro
Skill Level: Intermediate/B Class
Stiffness Preference: Standard Stiffness
Age: 30-44 Years Old
Height: Standard Rider Height
Gas Tank: Standard Gas Tank
Spring Conversion: No Spring Conversion
Lowering: Not Lowered
Rider Weight: 195
Sag: 107 mm

REAR SHOCK SPRINGS
Recommended Spring Rate:5.87 kg/mm
Stock Spring Rate:5.8 kg/mm (stock)

This is all incorrect. RaceTech is notoriously clueless about N/mm and kg/mm. The stock MXR rear spring rate is 60 N/mm (6.1 kg/mm), and it should be pretty good for your weight and skill level.

Also, many Alta riders have found that smaller race sags work better.
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Don’t o-ring chains have extra drag = reduced range?
Possibly, but I think it is super negligible.

Frankly, having an extra donut for breakfast probably affects the range more. I love the guys that will replace all the bolts on their bike with titanium to reduce weight, and then pack a toolkit, chainsaw, change of underwear, and a boombox on the back and expect to get better performance. The biggest variable on the bike is your own fat butt and how you ride.

Basically, I like the quieter, smoother sound of the o-ring. Makes it feel like a different bike. I could care less about negligible parasitic drag. O-ring chains tend to last longer as well.
 

synics

Active member
Location
New Jersey
I have the pro taper cold forged slim o-ring chain and I love it as it has like half the chain noise as the stock chain had. It's also 2 mm narrower than a regular o-ring
 

synics

Active member
Location
New Jersey
You can also keep your chain, re-install the stock 12-tooth front sprocket, and buy a new 49-tooth rear sprocket. This will move the rear wheel back by 1 chain link. The new 12/49 ratio will be almost the same as your current 13/53 ratio.
I have a 52t sprocket... If I switch that out with the 53, would
I'd be losing a lot of low end torque given that I have a 13 tooth on the front?
 

Philip

Administrator
Staff member
Location
Ann Arbor, MI
I have a 52t sprocket... If I switch that out with the 53, would
I'd be losing a lot of low end torque given that I have a 13 tooth on the front?
You'd gain a negligible amount of low-speed torque. The stock MXR gearing is 12/53, but many run 14/53 or 13/53. And I run 12/45.

By going with taller gearing, you are losing low-end torque but gaining torque and power at speeds above 35mph. See the magic explained in the charts in the thread below.

 

Matt

Spaceship Jockey
Also it is worth asking when the last time you had the shock rebuilt/oil changed. I definitely notice mine start to fall off after 20-30 hours of hard ridding. I am too cheap to get the oil changed often but I typically try to get it down around 40-50 hours. After a fresh oil change I feel invincible for a while ha. Huge hits it just soaks up like nothing, small chop almost doesn't exist and it's always very planted. Also I messed up a chain once and ended up with two master links on it for about 20-25 hrs and never had an issue. It was a renthal o ring chain.
 

synics

Active member
Location
New Jersey
I just got 55 hours.... But the thing is on an alta I don't see the hours being the same as they are a gas bike. It's not the same equivalent of riding because I never turn the key off and I think when the key is on it's counting hours but half of the time it's just sitting there doing nothing with the key on. I'm still at like 600 miles total.
 

Matt

Spaceship Jockey
I just got 55 hours.... But the thing is on an alta I don't see the hours being the same as they are a gas bike. It's not the same equivalent of riding because I never turn the key off and I think when the key is on it's counting hours but half of the time it's just sitting there doing nothing with the key on. I'm still at like 600 miles total.
If you know your average moving speed at least roughly you can get an idea of where you at timewise to compare to the 40hr oil change mark.
 

Fod

Well-known member
Location
CA
When your rear axle gets adjusted back for a longer wheelbase, the shock and spring will need to work harder to hold up the bike. I thought it was negligible for my low level riding but it was noticeably softer.
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
When your rear axle gets adjusted back for a longer wheelbase, the shock and spring will need to work harder to hold up the bike. I thought it was negligible for my low level riding but it was noticeably softer.
Definitely moving the axle back makes the swingarm a longer lever. But two links moves the axle about half an inch farther back (two links is more like an inch in total chain length, but divide by two since there are two sides to the chain.) So percentage wise there is not a whole lot of difference. I did end up flipping the stock chain adjuster blocks so that the long end was forward though. Gives me more adjustment room.

It also theoretically makes it more difficult to wheelie. But it doesn’t really matter to me since Mother Nature did not see fit to bequeath the wheelie gene upon my pitiful self.
 

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