# Mathews 70% to 80% Let-off Swap (Straight-line Max Cams)



## Jollyarcher (Feb 8, 2010)

*** Please feel free to read on if you ever wanted to make your 70% let off Mathews bow equipped with a Straight-line Max cam, (like that of the 1st series SL cams used on the MQ1 and MQ32, as well as the 2nd series SL cams equipped on the Q2, the Q2XL and the SQ2, and even the 3rd series SL cam made exclusively for the Legacy)... into an 80% let off bow. This cam will work in each application.

We will be using a stock 80% cam from the MQ1, the "A" 30 inch cam for this example. Any draw length, (letter designation) Straight-line Max cam with the HL stamping works equally as well.

Here's the why & how it works on the other Mathews bows named above that were FACTORY equipped with the Straight Line Max Cam.

The MQ1 was the first Straight-line Max cam available in both 70 and 80% let off. The only difference between the two let off options was that the 80% cam, having a larger control lobe, required a 2.5 inch longer string and a 1/8 shorter cable. Both were available as a stock option from Mathews when using that 80% let off cam.

The Straight-line Max cam was virtually unchanged in series 2, and the final series 3, used exclusively on the Legacy. With the proper string and cable lengths as outlined above, the first generation Straight-line Max cam in the HL offering allows your MQ32, Q2, Q2XL, SQ2 and even the Legacy to be converted into an 80% let off bow.

I recently completed this for my taxidermist, who's been a long time shooter of the Mathews Q2. His shoulder problems and the 70% let off was becoming a real issue... so much that he considered purchasing a crossbow.

This solution was an small undertaking in both time and expense compared to the cost of an unwanted X-bow. Its success allows him to keep hunting the way he prefers... and with 80% let off!

PLEASE NOTE: There is a slight drop in poundage when installing the high let off 80% cam on a bow that was originally designed for 70%. We found on two separate bows, the loss in draw weight ranges between 4 and 6 pounds.

This is the only, "issue" (if you want to call it that), that comes with the gain in let off when the 80% cam is installed on any of the bows listed with the exception of the MQ1.

Install is straight forward and in both recent swaps, only a new 2.5 inch string was purchased to make the change. The cable only required a few twists to shorten it the required 1/8 inch, thus making the purchase of a new cable unnecessary.

The local bow shop said it couldn't be done... we did two bows for friends in this fashion last month and both paper tuned bullet holes in under 5 arrows. (1 in 3 shots and 1 in 1 shot.) Even our friends employed at the local Mathews dealer were impressed. Just swap the cam, add the 2.5" longer string, dial in the bows advertised axle the axle, (twist the cable 1/8"), set the cam rotation as Mathews designed and you're golden... and lighter about 5 pounds overall.

So, if you have basic bow tuner skills and wondered if you could do this... yes, you CAN!

These are just my findings and both bows function extremely well. I don't recommend that you swap anything on any equipment unless you have a thorough understanding of it's function and how the modification will effect your particular implement.

Likewise, I assume no liability for harm to your person or property should you choose similar experimentation. I am simply sharing my findings for those that may have wondered about such a let off change.


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## DS1 (Jul 18, 2010)

Hello Jollyarcher,
I saw your post about converting a Mathews bow from 70% letoff to an 80% letoff using a 30" cam.
I have an SQ2--27"(SL2DR) 70# bow(70% letoff) and was wanting to know if I was to get a 27" SLDRHL Cam and add 2 1/2" to string and 1/8" shorter cable if I would be able to get the same results? Would everything work O.K., and it give me an 80% letoff without damaging the bow?
I have it set at 62# right now due to a damaged shoulder, arm, & neck, so I would have room to tighten the limbs,to compensate for the draw weight loss.
I can't afford a new bow right now and when I came across your post, it was
like a dream come true.
If you know of any safe way I can get it to work I would really appreciate it.


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## Jollyarcher (Feb 8, 2010)

*I'm on it...*

Hey D... I'm certain we can on the SQ2 and I'm happy to help! 

I'll do the numbers, (for the proper string / cable combo at your draw length), and then post 'em up here before the day is over.


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## Jollyarcher (Feb 8, 2010)

*Ok...*

Day over yet? Ok, good then... let's see what we have in the numbers department.

I see that the SQ2 swap string length will be a wee bit different than the MQ1, due to dimension variances in the short draw cam lobe size and the the longer ATA example bow.

You are 100% correct, in that to achieve 80% let off in your 27" draw, a D letter cam with the HL designation is required.

To our benefit, the specs of your bow, (31" ATA / 8.5 brace), are so very close to that of its SL cam equipped cousin, the MQ32. It's advertised 32" ATA / 8.5 brace makes your swap easy peezy lemon squeezy.

Helping greater still is the factory servings on the MQ32 replacement string are already in the proper location for cam engagement on your SQ2. No special string construction or modification from those available at your local archery shop retail counter are required. Very nice indeed.

Since I don't have a SLDHL cam to measure, we will use the published data, knowing both bows in this example use the same SL variant cam and idler wheel configuration.

Focused on the 1" longer MQ32, (brace height spot on with your SQ2), we discover a 27" draw using an 80% let off cam requires an 88 1/8 inch string.

Using their chart, we see that the same MQ32 using a 70% cam calls for an 85 1/2 inch string.

Now, looking to your SQ2 in it's 70% offering, we see that it calls for an 84 1/2 inch string.

An inch different. Limb deflection is changed, due to the SQ2 sporting a 31 inch ATA. The MQ32 was built to be tuned at 32 inches ATA. Shortening the string adds preload to the limbs and shortens the ATA, while bumping the draw weight slightly.

The cable variance between the two bows (SQ2 @ 33 11/16 vs. the MQ32 at 33 7/8) is only 3/16 of one inch.

I would tune the SQ2 stock cable by twisting as opposed to buying a new one made for the MQ32.

Now, all things known about these two bows, you could easily run the stock string and cable set from the MQ32 on the SQ2 for the 70 to 80 % cam swap. However, you must keep in mind that the string will be approximately one inch longer and will allow the limbs to relax as much. This will likely result in a bit more than 5 pounds of peek draw weight loss from your SQ2's maximum specification.

So, remembering that the 70% to 70% string and cable length comparison between these two bows only differed by one inch string to string, (due to their inch difference ATA's), you could run a slightly shorter string (.5 to 1 inch) to prevent your SQ2 bow limbs from the tension loss.

Personally, I would run the factory MQ32 string with the SLDHL cam, and then add twists to that string in order to achieve my desired draw weight.

I would be equally happy with either cable, be it from the SQ2 or the MQ32, knowing that the minimal difference in their length (just 3/16 inch) would not pose a problem for tuning.

I'm confident that these suggestions will work for you... and the end result will be one that you are very pleased with.

Live nearby? Bring it over... I'll assemble it and tune 'er up for ya... for free.

Good shootin' / huntin' to ya bud.


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## DS1 (Jul 18, 2010)

Thanks Jolly

 I've gotten a bit confused on what to do. 
I'm going to have to replace my string & cable any way because they're just worn out. I saw your post & thought about buying another cam to be able to get 80% letoff and then buy what ever size custom string & cable was needed to make the bow as exact & perfect as possible, with an 80% letoff. The reason I said I can't afford a new bow right now is because the bow I want cost over $1000.00 , so I have to make due for now.

I guess it's not that bad though considering I walked outside with it while ago
while I was thinking things over and shot at my target twice and almost 
pulled a Robin Hood. The arrow just didn't stay stuck in the other one but
busted the nock & split the shaft about 1 1/2" down.
Add more arrows & nocks to the list.

I'm not giving up on the idea but going to think things over and maybe get a SLDRHL Cam & try to come up with some exact measurements from it.

I sure do appreciate everything you've done.
You will probably be hearing from me before long.

 THANKS


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## DS1 (Jul 18, 2010)

*Unsure*

JOLLY How about if I were to have a string made that was 1" shorter(87 1/8") than the factory MQ32(32" ATA) 80% letoff string(88 1/8") and leave factory servings in the same place except take 1/2" off of each end of the center serving,(where arrow nock attaches), and use an SQ2(31" ATA) cable(33 11/16"), and an SLDRHL (27") cam.

Do you think that this will work or not on my SQ2 to achieve 80% letoff and not affect draw weight or limb preload?


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## Jollyarcher (Feb 8, 2010)

DS1 said:


> How about if I were to have a string made that was 1" shorter (87 1/8") than the factory MQ32 (32" ATA) 80% letoff string (88 1/8") and leave factory servings in the same place except take 1/2" off of each end of the center serving,(where arrow nock attaches), and use an SQ2 (31" ATA) cable (33 11/16"), and an SLDRHL (27") cam.


That's exactly what I would do to preserve your factory limb deflection and retain the draw weight rating of your bow.



DS1 said:


> Do you think that this will work or not on my SQ2 to achieve 80% letoff and not affect draw weight or limb preload?


I agree 100%.

Use the factory SQ2 cable. It's 3/16" of extra length can be easily consumed by adding twists when you fine tune the optimal cam rotation, (at rest) setting.

Using the shorter MQ32 cable will not allow you to gain length if needed to attain the proper cam at rest location. I have a chart around here somewhere that outlines this setting, based on the lower cam perimeter weighted lobe distance from the cable at rest to yield maximum performance of your SL Max Cam.

Shout if you need it and I'll get it for you bud.


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## DS1 (Jul 18, 2010)

Jollyarcher,

To be perfectly honest with you I do not know what limb deflection is or a lot of other technical stuff about bows but I am trying to learn so I don't have to keep carrying my bow by the shop when something needs to be done to it.

You had said to {Use the factory SQ2 cable. It's 3/16" of extra length can be easily consumed by adding twists when you fine tune the optimal cam rotation, (at rest) setting} {Using the shorter MQ32 cable will not allow you to gain length if needed to attain the proper cam at rest location}

I thought the MQ32 cable at 33 7/8"[33 14/16"] was longer than the SQ2 cable[33 11/16"] and if you added twists to the already shorter cable that it would make it even shorter. It seems to me that the MQ32 cable is the one that has the 3/16" of extra length.


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## eddie_tobler (Jun 20, 2008)

I've done something similar to this before, and you can get the poundage back to what it originally was. 

In the limb pocket just place a 3/32 thickness shim inside the back part of the limb pocket, by doing that you will increase the deflection and poundage. Typically you can get about 5lbs back this way.


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## Jollyarcher (Feb 8, 2010)

DS1 said:


> I thought the MQ32 cable at 33 7/8"[33 14/16"] was longer than the SQ2 cable[33 11/16"] and if you added twists to the already shorter cable that it would make it even shorter. It seems to me that the MQ32 cable is the one that has the 3/16" of extra length.


Correct... looking back, I see now that they were accidentally reversed in my post. Sorry about that.



eddie_tobler said:


> I've done something similar to this before, and you can get the poundage back to what it originally was.
> 
> In the limb pocket just place a 3/32 thickness shim inside the back part of the limb pocket, by doing that you will increase the deflection and poundage. Typically you can get about 5lbs back this way.


A super idea! Similar shims were available for the Hoyt Defiant back in the day. Thanks for sharing that eddie!


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## stxbrewer (Jul 28, 2011)

I have a Mathews Legacy with a 27 inch draw length, what cam would I need to convert ly Legacy to 80% letoff?


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## Ricky San (Sep 4, 2018)

Hello Jolly，
Thanks for your information.
I have a 1999 MQ1 and just ordered a StraightLine MaxCam for it. Am going to use it for my 70's next year. Will keep you posted.
Rick


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