9point8 Slack-R IS41 Charlie Kit 1.3º

Charlie Slack-R Kit IS41 110 - 118mm Headtube Length 1.3º

9point8 Slack-R kits allow you to change the head tube angle on bikes with IS headsets.

Slack-R Charlie Kit Specification:

  • Upper Cup IS41
  • Lower Cup IS52
  • Designed to fit Headtube lengths 110mm - 118mm
  • 1.3º of adjustment


If you've ever wanted to try a slacker or steeper head tube angle on your bike but have been restricted by your IS headset then the new Slack-R is for you.
Using a patent pending modular system, you can now adjust the head angle by 1.2° to 1.8° depending on your frames geometry and which Slack-R kit is installed.

Slack-R is now compatible with MOST Tapered IS headsets.
There are 3 different styles of upper cups. IS41, IS42 and IS42S now to go along with the IS52 lowers.

IS41/IS52 will fit a number of bikes from manufacturers such as Yeti, Santa Cruz, and Trek (eliminates the need for a Knock Block in most cases). Please note Cube bikes use a deeper upper bearing seat so the Slack-R kit does not fit Cube bikes with IS41 upper bearings.

IS42/IS52 will fit many bikes from Cannondale, Norco, Marin, Orbea, GT, Revel, Scott and many more including some Specialized with a standard depth IS42/IS52 headset. Standard bearing seat depth is 3mm

IS42S/IS52 will fit Specialized bikes that have the 6.8mm deep upper bearing seat. (some Specialized bearings sit deeper than the standard IS42 geometry so they require their own specific top cup to insure proper fitment)

Headset types do vary across the same manufacturer, make sure you are sure what kind of headset you have. (IS/EC/ZS or hybrids of these)

Some IS bearing seats in the frames have a smaller bearing seat inside diameter than others so in some cases a bit of sanding will be required to fit in the 34.5mm upper sleeve and 44.5mm lower sleeve. In most cases the frame allow the kits to drop right in but in some cases opening up the upper and/or lower bearing seat inside diameter(s) will be required so double check your frame clearance prior to ordering if unsure.

Follow the three checks below to determine which Slack-R kits are available for your frame:

1. Confirm your headset is IS standard. If you're not sure check your frame manufacturers specification.
Park Tools also have a useful headset guide here: https://www.parktool.com/blog/repair-help/standardized-headset-identification-system

2. Confirm you have enough steerer tube length to accomodate the extra height of the Slack-R kit. If you have at least 20mm of steerer above your stem, below or combined in your current set-up then you will have enough room to install a Slack-R.

3. Measure or check your frame manufacturers specification to get your head tube length. This is often listed as HTL in manufacturers geometry charts. It's important to make sure this measurement is correct so if you are measuring yourself please ensure you use an accurate tool such as a digital caliper.

When installing any Slack-R kit please be aware that the effect on your bikes geometry will be more than just slackening the head tube angle.
Generally you should expect a small change in bottom bracket height, a slight increase in stack, a small decrease in reach and a noticeable increase in the wheelbase.

Brand: 9point8
* Headset Check: